GnuTLS 3.2.13

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GnuTLS

This manual is last updated 1 January 2014 for version 3.2.13 of GnuTLS.

Copyright © 2001-2013 Free Software Foundation, Inc.\\ Copyright © 2001-2013 Nikos Mavrogiannopoulos

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”.


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1 Preface

This document demonstrates and explains the GnuTLS library API. A brief introduction to the protocols and the technology involved is also included so that an application programmer can better understand the GnuTLS purpose and actual offerings. Even if GnuTLS is a typical library software, it operates over several security and cryptographic protocols which require the programmer to make careful and correct usage of them. Otherwise it is likely to only obtain a false sense of security. The term of security is very broad even if restricted to computer software, and cannot be confined to a single cryptographic library. For that reason, do not consider any program secure just because it uses GnuTLS; there are several ways to compromise a program or a communication line and GnuTLS only helps with some of them.

Although this document tries to be self contained, basic network programming and public key infrastructure (PKI) knowledge is assumed in most of it. A good introduction to networking can be found in [STEVENS], to public key infrastructure in [GUTPKI] and to security engineering in [ANDERSON].

Updated versions of the GnuTLS software and this document will be available from http://www.gnutls.org/.


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2 Introduction to GnuTLS

In brief GnuTLS can be described as a library which offers an API to access secure communication protocols. These protocols provide privacy over insecure lines, and were designed to prevent eavesdropping, tampering, or message forgery.

Technically GnuTLS is a portable ANSI C based library which implements the protocols ranging from SSL 3.0 to TLS 1.2 (see Introduction to TLS, for a detailed description of the protocols), accompanied with the required framework for authentication and public key infrastructure. Important features of the GnuTLS library include:

The GnuTLS library consists of three independent parts, namely the “TLS protocol part”, the “Certificate part”, and the “Cryptographic back-end” part. The “TLS protocol part” is the actual protocol implementation, and is entirely implemented within the GnuTLS library. The “Certificate part” consists of the certificate parsing, and verification functions and it uses functionality from the libtasn1 library. The “Cryptographic back-end” is provided by the nettle and gmplib libraries.


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2.1 Downloading and installing

GnuTLS is available for download at: http://www.gnutls.org/download.html

GnuTLS uses a development cycle where even minor version numbers indicate a stable release and a odd minor version number indicate a development release. For example, GnuTLS 1.6.3 denote a stable release since 6 is even, and GnuTLS 1.7.11 denote a development release since 7 is odd.

GnuTLS depends on nettle and gmplib, and you will need to install it before installing GnuTLS. The nettle library is available from http://www.lysator.liu.se/~nisse/nettle/, while gmplib is available from http://www.gmplib.org/. Don’t forget to verify the cryptographic signature after downloading source code packages.

The package is then extracted, configured and built like many other packages that use Autoconf. For detailed information on configuring and building it, refer to the INSTALL file that is part of the distribution archive. Typically you invoke ./configure and then make check install. There are a number of compile-time parameters, as discussed below.

Several parts of GnuTLS require ASN.1 functionality, which is provided by a library called libtasn1. A copy of libtasn1 is included in GnuTLS. If you want to install it separately (e.g., to make it possibly to use libtasn1 in other programs), you can get it from http://www.gnu.org/software/libtasn1/.

The compression library, libz, the PKCS #11 helper library p11-kit, as well as the TPM library trousers, are optional dependencies. You may get libz from http://www.zlib.net/, p11-kit from http://p11-glue.freedesktop.org/ and trousers from http://trousers.sourceforge.net/.

A few configure options may be relevant, summarized below. They disable or enable particular features, to create a smaller library with only the required features. Note however, that although a smaller library is generated, the included programs are not guaranteed to compile if some of these options are given.

--disable-srp-authentication
--disable-psk-authentication
--disable-anon-authentication
--disable-openpgp-authentication
--disable-dhe
--disable-ecdhe
--disable-openssl-compatibility
--disable-dtls-srtp-support
--disable-alpn-support
--disable-heartbeat-support
--disable-libdane
--without-p11-kit
--without-tpm
--without-zlib

For the complete list, refer to the output from configure --help.


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2.2 Overview

In this document we present an overview of the supported security protocols in Introduction to TLS, and continue by providing more information on the certificate authentication in Certificate authentication, and shared-key as well anonymous authentication in Shared-key and anonymous authentication. We elaborate on certificate authentication by demonstrating advanced usage of the API in More on certificate authentication. The core of the TLS library is presented in How to use GnuTLS in applications and example applications are listed in GnuTLS application examples. In Other included programs the usage of few included programs that may assist debugging is presented. The last chapter is Internal architecture of GnuTLS that provides a short introduction to GnuTLS’ internal architecture.


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3 Introduction to TLS and DTLS

TLS stands for “Transport Layer Security” and is the successor of SSL, the Secure Sockets Layer protocol [SSL3] designed by Netscape. TLS is an Internet protocol, defined by IETF1, described in [RFC5246]. The protocol provides confidentiality, and authentication layers over any reliable transport layer. The description, above, refers to TLS 1.0 but applies to all other TLS versions as the differences between the protocols are not major.

The DTLS protocol, or “Datagram TLS” [RFC4347] is a protocol with identical goals as TLS, but can operate under unreliable transport layers such as UDP. The discussions below apply to this protocol as well, except when noted otherwise.


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3.1 TLS layers

TLS is a layered protocol, and consists of the record protocol, the handshake protocol and the alert protocol. The record protocol is to serve all other protocols and is above the transport layer. The record protocol offers symmetric encryption, data authenticity, and optionally compression. The alert protocol offers some signaling to the other protocols. It can help informing the peer for the cause of failures and other error conditions. See The Alert Protocol, for more information. The alert protocol is above the record protocol.

The handshake protocol is responsible for the security parameters’ negotiation, the initial key exchange and authentication. See The Handshake Protocol, for more information about the handshake protocol. The protocol layering in TLS is shown in Figure 3.1.

gnutls-layers

Figure 3.1: The TLS protocol layers.


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3.2 The transport layer

TLS is not limited to any transport layer and can be used above any transport layer, as long as it is a reliable one. DTLS can be used over reliable and unreliable transport layers. GnuTLS supports TCP and UDP layers transparently using the Berkeley sockets API. However, any transport layer can be used by providing callbacks for GnuTLS to access the transport layer (for details see Setting up the transport layer).


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3.3 The TLS record protocol

The record protocol is the secure communications provider. Its purpose is to encrypt, authenticate and —optionally— compress packets. The record layer functions can be called at any time after the handshake process is finished, when there is need to receive or send data. In DTLS however, due to re-transmission timers used in the handshake out-of-order handshake data might be received for some time (maximum 60 seconds) after the handshake process is finished.

The functions to access the record protocol are limited to send and receive functions, which might, given the importance of this protocol in TLS, seem awkward. This is because the record protocol’s parameters are all set by the handshake protocol. The record protocol initially starts with NULL parameters, which means no encryption, and no MAC is used. Encryption and authentication begin just after the handshake protocol has finished.


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3.3.1 Encryption algorithms used in the record layer

Confidentiality in the record layer is achieved by using symmetric block encryption algorithms like 3DES, AES or stream algorithms like ARCFOUR_128. Ciphers are encryption algorithms that use a single, secret, key to encrypt and decrypt data. Block algorithms in CBC mode also provide protection against statistical analysis of the data. Thus, if you’re using the TLS protocol, a random number of blocks will be appended to data, to prevent eavesdroppers from guessing the actual data size.

The supported in GnuTLS ciphers and MAC algorithms are shown in Table 3.1 and Table 3.2.

AlgorithmDescription
3DES_CBCThis is the DES block cipher algorithm used with triple encryption (EDE). Has 64 bits block size and is used in CBC mode.
ARCFOUR_128ARCFOUR_128 is a compatible algorithm with RSA’s RC4 algorithm, which is considered to be a trade secret. It is a fast cipher but considered weak today.
AES_CBCAES or RIJNDAEL is the block cipher algorithm that replaces the old DES algorithm. Has 128 bits block size and is used in CBC mode.
AES_GCMThis is the AES algorithm in the authenticated encryption GCM mode. This mode combines message authentication and encryption and can be extremely fast on CPUs that support hardware acceleration.
CAMELLIA_CBCThis is an 128-bit block cipher developed by Mitsubishi and NTT. It is one of the approved ciphers of the European NESSIE and Japanese CRYPTREC projects.
SALSA20_256SALSA20_256 is a fast stream cipher. This is currently a GnuTLS extension.
ESTREAM_SALSA20_256ESTREAM_SALSA20_256 is a faster variant of SALSA20, and is one of the selected ciphers of the ESTREAM competition. This is currently a GnuTLS extension.

Table 3.1: Supported ciphers.

AlgorithmDescription
MAC_MD5This is an HMAC based on MD5 a cryptographic hash algorithm designed by Ron Rivest. Outputs 128 bits of data.
MAC_SHA1An HMAC based on the SHA1 cryptographic hash algorithm designed by NSA. Outputs 160 bits of data.
MAC_SHA256An HMAC based on SHA256. Outputs 256 bits of data.
MAC_UMACThis is a very fast MAC algorithm based on universal hashing, described in [RFC4418]. This is currently a GnuTLS extension.
MAC_AEADThis indicates that an authenticated encryption algorithm, such as GCM, is in use.

Table 3.2: Supported MAC algorithms.


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3.3.2 Compression algorithms used in the record layer

The TLS record layer also supports compression. The algorithms implemented in GnuTLS can be found in the table below. The included algorithms perform really good when text, or other compressible data are to be transferred, but offer nothing on already compressed data, such as compressed images, zipped archives etc. These compression algorithms, may be useful in high bandwidth TLS tunnels, and in cases where network usage has to be minimized. It should be noted however that compression increases latency.

The record layer compression in GnuTLS is implemented based on [RFC3749]. The supported algorithms are shown below.

GNUTLS_COMP_UNKNOWN

Unknown compression method.

GNUTLS_COMP_NULL

The NULL compression method (no compression).

GNUTLS_COMP_DEFLATE

The DEFLATE compression method from zlib.

GNUTLS_COMP_ZLIB

Same as GNUTLS_COMP_DEFLATE .

Figure 3.2: Supported compression algorithms

Note that compression enables attacks such as traffic analysis, or even plaintext recovery under certain circumstances. To avoid some of these attacks GnuTLS allows each record to be compressed independently (i.e., stateless compression), by using the "%STATELESS_COMPRESSION" priority string, in order to be used in cases where the attacker controlled data are pt in separate records.


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3.3.3 Weaknesses and countermeasures

Some weaknesses that may affect the security of the record layer have been found in TLS 1.0 protocol. These weaknesses can be exploited by active attackers, and exploit the facts that

  1. TLS has separate alerts for “decryption_failed” and “bad_record_mac”
  2. The decryption failure reason can be detected by timing the response time.
  3. The IV for CBC encrypted packets is the last block of the previous encrypted packet.

Those weaknesses were solved in TLS 1.1 [RFC4346] which is implemented in GnuTLS. For this reason we suggest to always negotiate the highest supported TLS version with the peer2. For a detailed discussion of the issues see the archives of the TLS Working Group mailing list and [CBCATT].


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3.3.4 On record padding

The TLS protocol allows for extra padding of records in CBC ciphers, to prevent statistical analysis based on the length of exchanged messages (see [RFC5246] section 6.2.3.2). GnuTLS appears to be one of few implementations that take advantage of this feature: the user can provide some plaintext data with a range of lengths she wishes to hide, and GnuTLS adds extra padding to make sure the attacker cannot tell the real plaintext length is in a range smaller than the user-provided one. Use gnutls_record_send_range to send length-hidden messages and gnutls_record_can_use_length_hiding to check whether the current session supports length hiding. Using the standard gnutls_record_send will only add minimal padding.

The TLS implementation in the Symbian operating system, frequently used by Nokia and Sony-Ericsson mobile phones, cannot handle non-minimal record padding. What happens when one of these clients handshake with a GnuTLS server is that the client will fail to compute the correct MAC for the record. The client sends a TLS alert (bad_record_mac) and disconnects. Typically this will result in error messages such as ’A TLS fatal alert has been received’, ’Bad record MAC’, or both, on the GnuTLS server side.

If compatibility with such devices is a concern, not sending length-hidden messages solves the problem by using minimal padding.

If you implement an application that has a configuration file, we recommend that you make it possible for users or administrators to specify a GnuTLS protocol priority string, which is used by your application via gnutls_priority_set. To allow the best flexibility, make it possible to have a different priority string for different incoming IP addresses.


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3.4 The TLS alert protocol

The alert protocol is there to allow signals to be sent between peers. These signals are mostly used to inform the peer about the cause of a protocol failure. Some of these signals are used internally by the protocol and the application protocol does not have to cope with them (e.g. GNUTLS_A_CLOSE_NOTIFY), and others refer to the application protocol solely (e.g. GNUTLS_A_USER_CANCELLED). An alert signal includes a level indication which may be either fatal or warning. Fatal alerts always terminate the current connection, and prevent future re-negotiations using the current session ID. All alert messages are summarized in the table below.

The alert messages are protected by the record protocol, thus the information that is included does not leak. You must take extreme care for the alert information not to leak to a possible attacker, via public log files etc.

AlertIDDescription
GNUTLS_A_CLOSE_NOTIFY0Close notify
GNUTLS_A_UNEXPECTED_MESSAGE10Unexpected message
GNUTLS_A_BAD_RECORD_MAC20Bad record MAC
GNUTLS_A_DECRYPTION_FAILED21Decryption failed
GNUTLS_A_RECORD_OVERFLOW22Record overflow
GNUTLS_A_DECOMPRESSION_FAILURE30Decompression failed
GNUTLS_A_HANDSHAKE_FAILURE40Handshake failed
GNUTLS_A_SSL3_NO_CERTIFICATE41No certificate (SSL 3.0)
GNUTLS_A_BAD_CERTIFICATE42Certificate is bad
GNUTLS_A_UNSUPPORTED_CERTIFICATE43Certificate is not supported
GNUTLS_A_CERTIFICATE_REVOKED44Certificate was revoked
GNUTLS_A_CERTIFICATE_EXPIRED45Certificate is expired
GNUTLS_A_CERTIFICATE_UNKNOWN46Unknown certificate
GNUTLS_A_ILLEGAL_PARAMETER47Illegal parameter
GNUTLS_A_UNKNOWN_CA48CA is unknown
GNUTLS_A_ACCESS_DENIED49Access was denied
GNUTLS_A_DECODE_ERROR50Decode error
GNUTLS_A_DECRYPT_ERROR51Decrypt error
GNUTLS_A_EXPORT_RESTRICTION60Export restriction
GNUTLS_A_PROTOCOL_VERSION70Error in protocol version
GNUTLS_A_INSUFFICIENT_SECURITY71Insufficient security
GNUTLS_A_INTERNAL_ERROR80Internal error
GNUTLS_A_USER_CANCELED90User canceled
GNUTLS_A_NO_RENEGOTIATION100No renegotiation is allowed
GNUTLS_A_UNSUPPORTED_EXTENSION110An unsupported extension was sent
GNUTLS_A_CERTIFICATE_UNOBTAINABLE111Could not retrieve the specified certificate
GNUTLS_A_UNRECOGNIZED_NAME112The server name sent was not recognized
GNUTLS_A_UNKNOWN_PSK_IDENTITY115The SRP/PSK username is missing or not known
GNUTLS_A_NO_APPLICATION_PROTOCOL120No supported application protocol could be negotiated

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3.5 The TLS handshake protocol

The handshake protocol is responsible for the ciphersuite negotiation, the initial key exchange, and the authentication of the two peers. This is fully controlled by the application layer, thus your program has to set up the required parameters. The main handshake function is gnutls_handshake. In the next paragraphs we elaborate on the handshake protocol, i.e., the ciphersuite negotiation.


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3.5.1 TLS ciphersuites

The handshake protocol of TLS negotiates cipher suites of a special form illustrated by the TLS_DHE_RSA_WITH_3DES_CBC_SHA cipher suite name. A typical cipher suite contains these parameters:

The cipher suite negotiated in the handshake protocol will affect the record protocol, by enabling encryption and data authentication. Note that you should not over rely on TLS to negotiate the strongest available cipher suite. Do not enable ciphers and algorithms that you consider weak.

All the supported ciphersuites are listed in ciphersuites.


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3.5.2 Authentication

The key exchange algorithms of the TLS protocol offer authentication, which is a prerequisite for a secure connection. The available authentication methods in GnuTLS follow.


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3.5.3 Client authentication

In the case of ciphersuites that use certificate authentication, the authentication of the client is optional in TLS. A server may request a certificate from the client using the gnutls_certificate_server_set_request function. We elaborate in Certificate credentials.


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3.5.4 Resuming sessions

The TLS handshake process performs expensive calculations and a busy server might easily be put under load. To reduce the load, session resumption may be used. This is a feature of the TLS protocol which allows a client to connect to a server after a successful handshake, without the expensive calculations. This is achieved by re-using the previously established keys, meaning the server needs to store the state of established connections (unless session tickets are used – Session tickets).

Session resumption is an integral part of GnuTLS, and Session resumption, ex-resume-client illustrate typical uses of it.


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3.6 TLS extensions

A number of extensions to the TLS protocol have been proposed mainly in [TLSEXT]. The extensions supported in GnuTLS are:

and they will be discussed in the subsections that follow.


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3.6.1 Maximum fragment length negotiation

This extension allows a TLS implementation to negotiate a smaller value for record packet maximum length. This extension may be useful to clients with constrained capabilities. The functions shown below can be used to control this extension.

size_t gnutls_record_get_max_size (gnutls_session_t session)
ssize_t gnutls_record_set_max_size (gnutls_session_t session, size_t size)

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3.6.2 Server name indication

A common problem in HTTPS servers is the fact that the TLS protocol is not aware of the hostname that a client connects to, when the handshake procedure begins. For that reason the TLS server has no way to know which certificate to send.

This extension solves that problem within the TLS protocol, and allows a client to send the HTTP hostname before the handshake begins within the first handshake packet. The functions gnutls_server_name_set and gnutls_server_name_get can be used to enable this extension, or to retrieve the name sent by a client.

int gnutls_server_name_set (gnutls_session_t session, gnutls_server_name_type_t type, const void * name, size_t name_length)
int gnutls_server_name_get (gnutls_session_t session, void * data, size_t * data_length, unsigned int * type, unsigned int indx)

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3.6.3 Session tickets

To resume a TLS session, the server normally stores session parameters. This complicates deployment, and can be avoided by delegating the storage to the client. Because session parameters are sensitive they are encrypted and authenticated with a key only known to the server and then sent to the client. The Session Tickets extension is described in RFC 5077 [TLSTKT].

Since version 3.1.3 GnuTLS clients transparently support session tickets.


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3.6.4 HeartBeat

This is a TLS extension that allows to ping and receive confirmation from the peer, and is described in [RFC6520]. The extension is disabled by default and gnutls_heartbeat_enable can be used to enable it. A policy may be negotiated to only allow sending heartbeat messages or sending and receiving. The current session policy can be checked with gnutls_heartbeat_allowed. The requests coming from the peer result to GNUTLS_E_HERTBEAT_PING_RECEIVED being returned from the receive function. Ping requests to peer can be send via gnutls_heartbeat_ping.

int gnutls_heartbeat_allowed (gnutls_session_t session, unsigned int type)
void gnutls_heartbeat_enable (gnutls_session_t session, unsigned int type)
int gnutls_heartbeat_ping (gnutls_session_t session, size_t data_size, unsigned int max_tries, unsigned int flags)
int gnutls_heartbeat_pong (gnutls_session_t session, unsigned int flags)
void gnutls_heartbeat_set_timeouts (gnutls_session_t session, unsigned int retrans_timeout, unsigned int total_timeout)
unsigned int gnutls_heartbeat_get_timeout (gnutls_session_t session)

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3.6.5 Safe renegotiation

TLS gives the option to two communicating parties to renegotiate and update their security parameters. One useful example of this feature was for a client to initially connect using anonymous negotiation to a server, and the renegotiate using some authenticated ciphersuite. This occurred to avoid having the client sending its credentials in the clear.

However this renegotiation, as initially designed would not ensure that the party one is renegotiating is the same as the one in the initial negotiation. For example one server could forward all renegotiation traffic to an other server who will see this traffic as an initial negotiation attempt.

This might be seen as a valid design decision, but it seems it was not widely known or understood, thus today some application protocols use the TLS renegotiation feature in a manner that enables a malicious server to insert content of his choice in the beginning of a TLS session.

The most prominent vulnerability was with HTTPS. There servers request a renegotiation to enforce an anonymous user to use a certificate in order to access certain parts of a web site. The attack works by having the attacker simulate a client and connect to a server, with server-only authentication, and send some data intended to cause harm. The server will then require renegotiation from him in order to perform the request. When the proper client attempts to contact the server, the attacker hijacks that connection and forwards traffic to the initial server that requested renegotiation. The attacker will not be able to read the data exchanged between the client and the server. However, the server will (incorrectly) assume that the initial request sent by the attacker was sent by the now authenticated client. The result is a prefix plain-text injection attack.

The above is just one example. Other vulnerabilities exists that do not rely on the TLS renegotiation to change the client’s authenticated status (either TLS or application layer).

While fixing these application protocols and implementations would be one natural reaction, an extension to TLS has been designed that cryptographically binds together any renegotiated handshakes with the initial negotiation. When the extension is used, the attack is detected and the session can be terminated. The extension is specified in [RFC5746].

GnuTLS supports the safe renegotiation extension. The default behavior is as follows. Clients will attempt to negotiate the safe renegotiation extension when talking to servers. Servers will accept the extension when presented by clients. Clients and servers will permit an initial handshake to complete even when the other side does not support the safe renegotiation extension. Clients and servers will refuse renegotiation attempts when the extension has not been negotiated.

Note that permitting clients to connect to servers when the safe renegotiation extension is not enabled, is open up for attacks. Changing this default behavior would prevent interoperability against the majority of deployed servers out there. We will reconsider this default behavior in the future when more servers have been upgraded. Note that it is easy to configure clients to always require the safe renegotiation extension from servers.

To modify the default behavior, we have introduced some new priority strings (see Priority Strings). The %UNSAFE_RENEGOTIATION priority string permits (re-)handshakes even when the safe renegotiation extension was not negotiated. The default behavior is %PARTIAL_RENEGOTIATION that will prevent renegotiation with clients and servers not supporting the extension. This is secure for servers but leaves clients vulnerable to some attacks, but this is a trade-off between security and compatibility with old servers. The %SAFE_RENEGOTIATION priority string makes clients and servers require the extension for every handshake. The latter is the most secure option for clients, at the cost of not being able to connect to legacy servers. Servers will also deny clients that do not support the extension from connecting.

It is possible to disable use of the extension completely, in both clients and servers, by using the %DISABLE_SAFE_RENEGOTIATION priority string however we strongly recommend you to only do this for debugging and test purposes.

The default values if the flags above are not specified are:

Server:

%PARTIAL_RENEGOTIATION

Client:

%PARTIAL_RENEGOTIATION

For applications we have introduced a new API related to safe renegotiation. The gnutls_safe_renegotiation_status function is used to check if the extension has been negotiated on a session, and can be used both by clients and servers.


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3.6.6 OCSP status request

The Online Certificate Status Protocol (OCSP) is a protocol that allows the client to verify the server certificate for revocation without messing with certificate revocation lists. Its drawback is that it requires the client to connect to the server’s CA OCSP server and request the status of the certificate. This extension however, enables a TLS server to include its CA OCSP server response in the handshake. That is an HTTPS server may periodically run ocsptool (see ocsptool Invocation) to obtain its certificate revocation status and serve it to the clients. That way a client avoids an additional connection to the OCSP server.

void gnutls_certificate_set_ocsp_status_request_function (gnutls_certificate_credentials_t sc, gnutls_status_request_ocsp_func ocsp_func, void * ptr)
int gnutls_certificate_set_ocsp_status_request_file (gnutls_certificate_credentials_t sc, const char * response_file, unsigned int flags)
int gnutls_ocsp_status_request_enable_client (gnutls_session_t session, gnutls_datum_t * responder_id, size_t responder_id_size, gnutls_datum_t * extensions)
int gnutls_ocsp_status_request_is_checked (gnutls_session_t session, unsigned int flags)

A server is required to provide the OCSP server’s response using the gnutls_certificate_set_ocsp_status_request_file. The response may be obtained periodically using the following command.

ocsptool --ask --load-cert server_cert.pem --load-issuer the_issuer.pem
         --load-signer the_issuer.pem --outfile ocsp.response

Since version 3.1.3 GnuTLS clients transparently support the certificate status request.


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3.6.7 SRTP

The TLS protocol was extended in [RFC5764] to provide keying material to the Secure RTP (SRTP) protocol. The SRTP protocol provides an encapsulation of encrypted data that is optimized for voice data. With the SRTP TLS extension two peers can negotiate keys using TLS or DTLS and obtain keying material for use with SRTP. The available SRTP profiles are listed below.

GNUTLS_SRTP_AES128_CM_HMAC_SHA1_80

128 bit AES with a 80 bit HMAC-SHA1

GNUTLS_SRTP_AES128_CM_HMAC_SHA1_32

128 bit AES with a 32 bit HMAC-SHA1

GNUTLS_SRTP_NULL_HMAC_SHA1_80

NULL cipher with a 80 bit HMAC-SHA1

GNUTLS_SRTP_NULL_HMAC_SHA1_32

NULL cipher with a 32 bit HMAC-SHA1

Figure 3.3: Supported SRTP profiles

To enable use the following functions.

int gnutls_srtp_set_profile (gnutls_session_t session, gnutls_srtp_profile_t profile)
int gnutls_srtp_set_profile_direct (gnutls_session_t session, const char * profiles, const char ** err_pos)

To obtain the negotiated keys use the function below.

Function: int gnutls_srtp_get_keys (gnutls_session_t session, void * key_material, unsigned int key_material_size, gnutls_datum_t * client_key, gnutls_datum_t * client_salt, gnutls_datum_t * server_key, gnutls_datum_t * server_salt)

session: is a gnutls_session_t structure.

key_material: Space to hold the generated key material

key_material_size: The maximum size of the key material

client_key: The master client write key, pointing inside the key material

client_salt: The master client write salt, pointing inside the key material

server_key: The master server write key, pointing inside the key material

server_salt: The master server write salt, pointing inside the key material

This is a helper function to generate the keying material for SRTP. It requires the space of the key material to be pre-allocated (should be at least 2x the maximum key size and salt size). The client_key , client_salt , server_key and server_salt are convenience datums that point inside the key material. They may be NULL .

Returns: On success the size of the key material is returned, otherwise, GNUTLS_E_SHORT_MEMORY_BUFFER if the buffer given is not sufficient, or a negative error code.

Since 3.1.4

Other helper functions are listed below.

int gnutls_srtp_get_selected_profile (gnutls_session_t session, gnutls_srtp_profile_t * profile)
const char * gnutls_srtp_get_profile_name (gnutls_srtp_profile_t profile)
int gnutls_srtp_get_profile_id (const char * name, gnutls_srtp_profile_t * profile)

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3.6.8 Application Layer Protocol Negotiation (ALPN)

The TLS protocol was extended in draft-ietf-tls-applayerprotoneg-00 to provide the application layer a method of negotiating the application protocol version. This allows for negotiation of the application protocol during the TLS handshake, thus reducing round-trips. The application protocol is described by an opaque string. To enable, use the following functions.

int gnutls_alpn_set_protocols (gnutls_session_t session, const gnutls_datum_t * protocols, unsigned protocols_size, unsigned int flags)
int gnutls_alpn_get_selected_protocol (gnutls_session_t session, gnutls_datum_t * protocol)

Note that you can use these functions with protocols that are registered in the Application Layer Protocol Negotiation IANA registry. If you wish to use them for other custom protocols then prefix them with the 3 characters ‘exp’ (lower-case).


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3.7 How to use TLS in application protocols

This chapter is intended to provide some hints on how to use the TLS over simple custom made application protocols. The discussion below mainly refers to the TCP/IP transport layer but may be extended to other ones too.


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3.7.1 Separate ports

Traditionally SSL was used in application protocols by assigning a new port number for the secure services. That way two separate ports were assigned, one for the non secure sessions, and one for the secured ones. This has the benefit that if a user requests a secure session then the client will try to connect to the secure port and fail otherwise. The only possible attack with this method is a denial of service one. The most famous example of this method is the famous “HTTP over TLS” or HTTPS protocol [RFC2818].

Despite its wide use, this method is not as good as it seems. This approach starts the TLS Handshake procedure just after the client connects on the —so called— secure port. That way the TLS protocol does not know anything about the client, and popular methods like the host advertising in HTTP do not work4. There is no way for the client to say “I connected to YYY server” before the Handshake starts, so the server cannot possibly know which certificate to use.

Other than that it requires two separate ports to run a single service, which is unnecessary complication. Due to the fact that there is a limitation on the available privileged ports, this approach was soon obsoleted.


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3.7.2 Upward negotiation

Other application protocols5 use a different approach to enable the secure layer. They use something often called as the “TLS upgrade” method. This method is quite tricky but it is more flexible. The idea is to extend the application protocol to have a “STARTTLS” request, whose purpose it to start the TLS protocols just after the client requests it. This approach does not require any extra port to be reserved. There is even an extension to HTTP protocol to support that method [RFC2817].

The tricky part, in this method, is that the “STARTTLS” request is sent in the clear, thus is vulnerable to modifications. A typical attack is to modify the messages in a way that the client is fooled and thinks that the server does not have the “STARTTLS” capability. See a typical conversation of a hypothetical protocol:

(client connects to the server)

CLIENT: HELLO I’M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

SERVER: OK

*** TLS STARTS

CLIENT: HERE ARE SOME CONFIDENTIAL DATA

And see an example of a conversation where someone is acting in between:

(client connects to the server)

CLIENT: HELLO I’M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON’T HAVE THIS CAPABILITY

CLIENT: HERE ARE SOME CONFIDENTIAL DATA

As you can see above the client was fooled, and was dummy enough to send the confidential data in the clear.

How to avoid the above attack? As you may have already noticed this one is easy to avoid. The client has to ask the user before it connects whether the user requests TLS or not. If the user answered that he certainly wants the secure layer the last conversation should be:

(client connects to the server)

CLIENT: HELLO I’M MR. XXX

SERVER: NICE TO MEET YOU XXX

CLIENT: PLEASE START TLS

(here someone inserts this message)

SERVER: SORRY I DON’T HAVE THIS CAPABILITY

CLIENT: BYE

(the client notifies the user that the secure connection was not possible)

This method, if implemented properly, is far better than the traditional method, and the security properties remain the same, since only denial of service is possible. The benefit is that the server may request additional data before the TLS Handshake protocol starts, in order to send the correct certificate, use the correct password file, or anything else!


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3.8 On SSL 2 and older protocols

One of the initial decisions in the GnuTLS development was to implement the known security protocols for the transport layer. Initially TLS 1.0 was implemented since it was the latest at that time, and was considered to be the most advanced in security properties. Later the SSL 3.0 protocol was implemented since it is still the only protocol supported by several servers and there are no serious security vulnerabilities known.

One question that may arise is why we didn’t implement SSL 2.0 in the library. There are several reasons, most important being that it has serious security flaws, unacceptable for a modern security library. Other than that, this protocol is barely used by anyone these days since it has been deprecated since 1996. The security problems in SSL 2.0 include:

Other protocols such as Microsoft’s PCT 1 and PCT 2 were not implemented because they were also abandoned and deprecated by SSL 3.0 and later TLS 1.0.


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4 Authentication methods

The initial key exchange of the TLS protocol performs authentication of the peers. In typical scenarios the server is authenticated to the client, and optionally the client to the server.

While many associate TLS with X.509 certificates and public key authentication, the protocol supports various authentication methods, including pre-shared keys, and passwords. In this chapter a description of the existing authentication methods is provided, as well as some guidance on which use-cases each method can be used at.


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4.1 Certificate authentication

The most known authentication method of TLS are certificates. The PKIX [PKIX] public key infrastructure is daily used by anyone using a browser today. GnuTLS supports both X.509 certificates [PKIX] and OpenPGP certificates using a common API.

The key exchange algorithms supported by certificate authentication are shown in Table 4.1.

Key exchangeDescription
RSAThe RSA algorithm is used to encrypt a key and send it to the peer. The certificate must allow the key to be used for encryption.
DHE_RSAThe RSA algorithm is used to sign ephemeral Diffie-Hellman parameters which are sent to the peer. The key in the certificate must allow the key to be used for signing. Note that key exchange algorithms which use ephemeral Diffie-Hellman parameters, offer perfect forward secrecy. That means that even if the private key used for signing is compromised, it cannot be used to reveal past session data.
ECDHE_RSAThe RSA algorithm is used to sign ephemeral elliptic curve Diffie-Hellman parameters which are sent to the peer. The key in the certificate must allow the key to be used for signing. It also offers perfect forward secrecy. That means that even if the private key used for signing is compromised, it cannot be used to reveal past session data.
DHE_DSSThe DSA algorithm is used to sign ephemeral Diffie-Hellman parameters which are sent to the peer. The certificate must contain DSA parameters to use this key exchange algorithm. DSA is the algorithm of the Digital Signature Standard (DSS).
ECDHE_ECDSAThe Elliptic curve DSA algorithm is used to sign ephemeral elliptic curve Diffie-Hellman parameters which are sent to the peer. The certificate must contain ECDSA parameters (i.e., EC and marked for signing) to use this key exchange algorithm.

Table 4.1: Supported key exchange algorithms.


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4.1.1 X.509 certificates

The X.509 protocols rely on a hierarchical trust model. In this trust model Certification Authorities (CAs) are used to certify entities. Usually more than one certification authorities exist, and certification authorities may certify other authorities to issue certificates as well, following a hierarchical model.

gnutls-x509

Figure 4.1: An example of the X.509 hierarchical trust model.

One needs to trust one or more CAs for his secure communications. In that case only the certificates issued by the trusted authorities are acceptable. The framework is illustrated on Figure 4.1.


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4.1.1.1 X.509 certificate structure

An X.509 certificate usually contains information about the certificate holder, the signer, a unique serial number, expiration dates and some other fields [PKIX] as shown in Table 4.2.

FieldDescription
versionThe field that indicates the version of the certificate.
serialNumberThis field holds a unique serial number per certificate.
signatureThe issuing authority’s signature.
issuerHolds the issuer’s distinguished name.
validityThe activation and expiration dates.
subjectThe subject’s distinguished name of the certificate.
extensionsThe extensions are fields only present in version 3 certificates.

Table 4.2: X.509 certificate fields.

The certificate’s subject or issuer name is not just a single string. It is a Distinguished name and in the ASN.1 notation is a sequence of several object identifiers with their corresponding values. Some of available OIDs to be used in an X.509 distinguished name are defined in gnutls/x509.h.

The Version field in a certificate has values either 1 or 3 for version 3 certificates. Version 1 certificates do not support the extensions field so it is not possible to distinguish a CA from a person, thus their usage should be avoided.

The validity dates are there to indicate the date that the specific certificate was activated and the date the certificate’s key would be considered invalid.

Certificate extensions are there to include information about the certificate’s subject that did not fit in the typical certificate fields. Those may be e-mail addresses, flags that indicate whether the belongs to a CA etc. All the supported X.509 version 3 extensions are shown in Table 4.3.

ExtensionOIDDescription
Subject key id2.5.29.14An identifier of the key of the subject.
Authority key id2.5.29.35An identifier of the authority’s key used to sign the certificate.
Subject alternative name2.5.29.17Alternative names to subject’s distinguished name.
Key usage2.5.29.15Constraints the key’s usage of the certificate.
Extended key usage2.5.29.37Constraints the purpose of the certificate.
Basic constraints2.5.29.19Indicates whether this is a CA certificate or not, and specify the maximum path lengths of certificate chains.
CRL distribution points2.5.29.31This extension is set by the CA, in order to inform about the issued CRLs.
Certificate policy2.5.29.32This extension is set to indicate the certificate policy as object identifier and may contain a descriptive string or URL.
Proxy Certification Information1.3.6.1.5.5.7.1.14Proxy Certificates includes this extension that contains the OID of the proxy policy language used, and can specify limits on the maximum lengths of proxy chains. Proxy Certificates are specified in [RFC3820].

Table 4.3: X.509 certificate extensions.

In GnuTLS the X.509 certificate structures are handled using the gnutls_x509_crt_t type and the corresponding private keys with the gnutls_x509_privkey_t type. All the available functions for X.509 certificate handling have their prototypes in gnutls/x509.h. An example program to demonstrate the X.509 parsing capabilities can be found in ex-x509-info.


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4.1.1.2 Importing an X.509 certificate

The certificate structure should be initialized using gnutls_x509_crt_init, and a certificate structure can be imported using gnutls_x509_crt_import.

int gnutls_x509_crt_init (gnutls_x509_crt_t * cert)
int gnutls_x509_crt_import (gnutls_x509_crt_t cert, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format)
void gnutls_x509_crt_deinit (gnutls_x509_crt_t cert)

In several functions an array of certificates is required. To assist in initialization and import the following two functions are provided.

int gnutls_x509_crt_list_import (gnutls_x509_crt_t * certs, unsigned int * cert_max, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, unsigned int flags)
int gnutls_x509_crt_list_import2 (gnutls_x509_crt_t ** certs, unsigned int * size, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, unsigned int flags)

In all cases after use a certificate must be deinitialized using gnutls_x509_crt_deinit. Note that although the functions above apply to gnutls_x509_crt_t structure, similar functions exist for the CRL structure gnutls_x509_crl_t.


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4.1.1.3 X.509 distinguished names

The “subject” of an X.509 certificate is not described by a single name, but rather with a distinguished name. This in X.509 terminology is a list of strings each associated an object identifier. To make things simple GnuTLS provides gnutls_x509_crt_get_dn2 which follows the rules in [RFC4514] and returns a single string. Access to each string by individual object identifiers can be accessed using gnutls_x509_crt_get_dn_by_oid.

Function: int gnutls_x509_crt_get_dn2 (gnutls_x509_crt_t cert, gnutls_datum_t * dn)

cert: should contain a gnutls_x509_crt_t structure

dn: a pointer to a structure to hold the name

This function will allocate buffer and copy the name of the Certificate. The name will be in the form "C=xxxx,O=yyyy,CN=zzzz" as described in RFC4514. The output string will be ASCII or UTF-8 encoded, depending on the certificate data.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value. and a negative error code on error.

Since: 3.1.10

int gnutls_x509_crt_get_dn (gnutls_x509_crt_t cert, char * buf, size_t * buf_size)
int gnutls_x509_crt_get_dn_by_oid (gnutls_x509_crt_t cert, const char * oid, int indx, unsigned int raw_flag, void * buf, size_t * buf_size)
int gnutls_x509_crt_get_dn_oid (gnutls_x509_crt_t cert, int indx, void * oid, size_t * oid_size)

Similar functions exist to access the distinguished name of the issuer of the certificate.

int gnutls_x509_crt_get_issuer_dn (gnutls_x509_crt_t cert, char * buf, size_t * buf_size)
int gnutls_x509_crt_get_issuer_dn2 (gnutls_x509_crt_t cert, gnutls_datum_t * dn)
int gnutls_x509_crt_get_issuer_dn_by_oid (gnutls_x509_crt_t cert, const char * oid, int indx, unsigned int raw_flag, void * buf, size_t * buf_size)
int gnutls_x509_crt_get_issuer_dn_oid (gnutls_x509_crt_t cert, int indx, void * oid, size_t * oid_size)
int gnutls_x509_crt_get_issuer (gnutls_x509_crt_t cert, gnutls_x509_dn_t * dn)

The more powerful gnutls_x509_crt_get_subject and gnutls_x509_dn_get_rdn_ava provide efficient but low-level access to the contents of the distinguished name structure.

int gnutls_x509_crt_get_subject (gnutls_x509_crt_t cert, gnutls_x509_dn_t * dn)
int gnutls_x509_crt_get_issuer (gnutls_x509_crt_t cert, gnutls_x509_dn_t * dn)
Function: int gnutls_x509_dn_get_rdn_ava (gnutls_x509_dn_t dn, int irdn, int iava, gnutls_x509_ava_st * ava)

dn: a pointer to DN

irdn: index of RDN

iava: index of AVA.

ava: Pointer to structure which will hold output information.

Get pointers to data within the DN. The format of the ava structure is shown below.

struct gnutls_x509_ava_st { gnutls_datum_t oid; gnutls_datum_t value; unsigned long value_tag; };

The X.509 distinguished name is a sequence of sequences of strings and this is what the irdn and iava indexes model.

Note that ava will contain pointers into the dn structure which in turns points to the original certificate. Thus you should not modify any data or deallocate any of those.

This is a low-level function that requires the caller to do the value conversions when necessary (e.g. from UCS-2).

Returns: Returns 0 on success, or an error code.


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4.1.1.4 Accessing public and private keys

Each X.509 certificate contains a public key that corresponds to a private key. To get a unique identifier of the public key the gnutls_x509_crt_get_key_id function is provided. To export the public key or its parameters you may need to convert the X.509 structure to a gnutls_pubkey_t. See Abstract public keys for more information.

Function: int gnutls_x509_crt_get_key_id (gnutls_x509_crt_t crt, unsigned int flags, unsigned char * output_data, size_t * output_data_size)

crt: Holds the certificate

flags: should be 0 for now

output_data: will contain the key ID

output_data_size: holds the size of output_data (and will be replaced by the actual size of parameters)

This function will return a unique ID that depends on the public key parameters. This ID can be used in checking whether a certificate corresponds to the given private key.

If the buffer provided is not long enough to hold the output, then *output_data_size is updated and GNUTLS_E_SHORT_MEMORY_BUFFER will be returned. The output will normally be a SHA-1 hash output, which is 20 bytes.

Returns: In case of failure a negative error code will be returned, and 0 on success.

The private key parameters may be directly accessed by using one of the following functions.

int gnutls_x509_privkey_get_pk_algorithm2 (gnutls_x509_privkey_t key, unsigned int * bits)
int gnutls_x509_privkey_export_rsa_raw2 (gnutls_x509_privkey_t key, gnutls_datum_t * m, gnutls_datum_t * e, gnutls_datum_t * d, gnutls_datum_t * p, gnutls_datum_t * q, gnutls_datum_t * u, gnutls_datum_t * e1, gnutls_datum_t * e2)
int gnutls_x509_privkey_export_ecc_raw (gnutls_x509_privkey_t key, gnutls_ecc_curve_t * curve, gnutls_datum_t * x, gnutls_datum_t * y, gnutls_datum_t * k)
int gnutls_x509_privkey_export_dsa_raw (gnutls_x509_privkey_t key, gnutls_datum_t * p, gnutls_datum_t * q, gnutls_datum_t * g, gnutls_datum_t * y, gnutls_datum_t * x)
int gnutls_x509_privkey_get_key_id (gnutls_x509_privkey_t key, unsigned int flags, unsigned char * output_data, size_t * output_data_size)

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4.1.1.5 Verifying X.509 certificate paths

Verifying certificate paths is important in X.509 authentication. For this purpose the following functions are provided.

Function: int gnutls_x509_trust_list_add_cas (gnutls_x509_trust_list_t list, const gnutls_x509_crt_t * clist, int clist_size, unsigned int flags)

list: The structure of the list

clist: A list of CAs

clist_size: The length of the CA list

flags: should be 0.

This function will add the given certificate authorities to the trusted list. The list of CAs must not be deinitialized during this structure’s lifetime.

Returns: The number of added elements is returned.

Since: 3.0

Function: int gnutls_x509_trust_list_add_named_crt (gnutls_x509_trust_list_t list, gnutls_x509_crt_t cert, const void * name, size_t name_size, unsigned int flags)

list: The structure of the list

cert: A certificate

name: An identifier for the certificate

name_size: The size of the identifier

flags: should be 0.

This function will add the given certificate to the trusted list and associate it with a name. The certificate will not be be used for verification with gnutls_x509_trust_list_verify_crt() but only with gnutls_x509_trust_list_verify_named_crt() .

In principle this function can be used to set individual "server" certificates that are trusted by the user for that specific server but for no other purposes.

The certificate must not be deinitialized during the lifetime of the trusted list.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

Function: int gnutls_x509_trust_list_add_crls (gnutls_x509_trust_list_t list, const gnutls_x509_crl_t * crl_list, int crl_size, unsigned int flags, unsigned int verification_flags)

list: The structure of the list

crl_list: A list of CRLs

crl_size: The length of the CRL list

flags: if GNUTLS_TL_VERIFY_CRL is given the CRLs will be verified before being added.

verification_flags: gnutls_certificate_verify_flags if flags specifies GNUTLS_TL_VERIFY_CRL

This function will add the given certificate revocation lists to the trusted list. The list of CRLs must not be deinitialized during this structure’s lifetime.

This function must be called after gnutls_x509_trust_list_add_cas() to allow verifying the CRLs for validity.

Returns: The number of added elements is returned.

Since: 3.0

Function: int gnutls_x509_trust_list_verify_crt (gnutls_x509_trust_list_t list, gnutls_x509_crt_t * cert_list, unsigned int cert_list_size, unsigned int flags, unsigned int * verify, gnutls_verify_output_function func)

list: The structure of the list

cert_list: is the certificate list to be verified

cert_list_size: is the certificate list size

flags: Flags that may be used to change the verification algorithm. Use OR of the gnutls_certificate_verify_flags enumerations.

verify: will hold the certificate verification output.

func: If non-null will be called on each chain element verification with the output.

This function will try to verify the given certificate and return its status. The verify parameter will hold an OR’ed sequence of gnutls_certificate_status_t flags.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

Function: int gnutls_x509_trust_list_verify_named_crt (gnutls_x509_trust_list_t list, gnutls_x509_crt_t cert, const void * name, size_t name_size, unsigned int flags, unsigned int * verify, gnutls_verify_output_function func)

list: The structure of the list

cert: is the certificate to be verified

name: is the certificate’s name

name_size: is the certificate’s name size

flags: Flags that may be used to change the verification algorithm. Use OR of the gnutls_certificate_verify_flags enumerations.

verify: will hold the certificate verification output.

func: If non-null will be called on each chain element verification with the output.

This function will try to find a certificate that is associated with the provided name –see gnutls_x509_trust_list_add_named_crt() . If a match is found the certificate is considered valid. In addition to that this function will also check CRLs. The verify parameter will hold an OR’ed sequence of gnutls_certificate_status_t flags.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

Function: int gnutls_x509_trust_list_add_trust_file (gnutls_x509_trust_list_t list, const char * ca_file, const char * crl_file, gnutls_x509_crt_fmt_t type, unsigned int tl_flags, unsigned int tl_vflags)

list: The structure of the list

ca_file: A file containing a list of CAs (optional)

crl_file: A file containing a list of CRLs (optional)

type: The format of the certificates

tl_flags: GNUTLS_TL_*

tl_vflags: gnutls_certificate_verify_flags if flags specifies GNUTLS_TL_VERIFY_CRL

This function will add the given certificate authorities to the trusted list. pkcs11 URLs are also accepted, instead of files, by this function.

Returns: The number of added elements is returned.

Since: 3.1

Function: int gnutls_x509_trust_list_add_trust_mem (gnutls_x509_trust_list_t list, const gnutls_datum_t * cas, const gnutls_datum_t * crls, gnutls_x509_crt_fmt_t type, unsigned int tl_flags, unsigned int tl_vflags)

list: The structure of the list

cas: A buffer containing a list of CAs (optional)

crls: A buffer containing a list of CRLs (optional)

type: The format of the certificates

tl_flags: GNUTLS_TL_*

tl_vflags: gnutls_certificate_verify_flags if flags specifies GNUTLS_TL_VERIFY_CRL

This function will add the given certificate authorities to the trusted list.

Returns: The number of added elements is returned.

Since: 3.1

Function: int gnutls_x509_trust_list_add_system_trust (gnutls_x509_trust_list_t list, unsigned int tl_flags, unsigned int tl_vflags)

list: The structure of the list

tl_flags: GNUTLS_TL_*

tl_vflags: gnutls_certificate_verify_flags if flags specifies GNUTLS_TL_VERIFY_CRL

This function adds the system’s default trusted certificate authorities to the trusted list. Note that on unsupported system this function returns GNUTLS_E_UNIMPLEMENTED_FEATURE .

Returns: The number of added elements or a negative error code on error.

Since: 3.1

The verification function will verify a given certificate chain against a list of certificate authorities and certificate revocation lists, and output a bit-wise OR of elements of the gnutls_certificate_status_t enumeration shown in Figure 4.2. The GNUTLS_CERT_INVALID flag is always set on a verification error and more detailed flags will also be set when appropriate.

GNUTLS_CERT_INVALID

The certificate is not signed by one of the known authorities or the signature is invalid (deprecated by the flags GNUTLS_CERT_SIGNATURE_FAILURE and GNUTLS_CERT_SIGNER_NOT_FOUND ).

GNUTLS_CERT_REVOKED

Certificate is revoked by its authority. In X.509 this will be set only if CRLs are checked.

GNUTLS_CERT_SIGNER_NOT_FOUND

The certificate’s issuer is not known. This is the case if the issuer is not included in the trusted certificate list.

GNUTLS_CERT_SIGNER_NOT_CA

The certificate’s signer was not a CA. This may happen if this was a version 1 certificate, which is common with some CAs, or a version 3 certificate without the basic constrains extension.

GNUTLS_CERT_INSECURE_ALGORITHM

The certificate was signed using an insecure algorithm such as MD2 or MD5. These algorithms have been broken and should not be trusted.

GNUTLS_CERT_NOT_ACTIVATED

The certificate is not yet activated.

GNUTLS_CERT_EXPIRED

The certificate has expired.

GNUTLS_CERT_SIGNATURE_FAILURE

The signature verification failed.

GNUTLS_CERT_REVOCATION_DATA_SUPERSEDED

The revocation data are old and have been superseded.

GNUTLS_CERT_UNEXPECTED_OWNER

The owner is not the expected one.

GNUTLS_CERT_REVOCATION_DATA_ISSUED_IN_FUTURE

The revocation data have a future issue date.

GNUTLS_CERT_SIGNER_CONSTRAINTS_FAILURE

The certificate’s signer constraints were violated.

GNUTLS_CERT_MISMATCH

The certificate presented isn’t the expected one (TOFU)

Figure 4.2: The gnutls_certificate_status_t enumeration.

An example of certificate verification is shown in ex-verify2. It is also possible to have a set of certificates that are trusted for a particular server but not to authorize other certificates. This purpose is served by the functions gnutls_x509_trust_list_add_named_crt and gnutls_x509_trust_list_verify_named_crt.


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4.1.1.6 Verifying a certificate in the context of TLS session

When operating in the context of a TLS session, the trusted certificate authority list may also be set using:

int gnutls_certificate_set_x509_trust_file (gnutls_certificate_credentials_t cred, const char * cafile, gnutls_x509_crt_fmt_t type)
int gnutls_certificate_set_x509_crl_file (gnutls_certificate_credentials_t res, const char * crlfile, gnutls_x509_crt_fmt_t type)
int gnutls_certificate_set_x509_system_trust (gnutls_certificate_credentials_t cred)

Then it is not required to setup a trusted list as above. The function gnutls_certificate_verify_peers3 may then be used to verify the peer’s certificate chain and identity. The flags are set similarly to the verification functions in the previous section.

There is also the possibility to pass some input to the verification functions in the form of flags. For gnutls_x509_trust_list_verify_crt the flags are passed straightforward, but gnutls_certificate_verify_peers3 depends on the flags set by calling gnutls_certificate_set_verify_flags. All the available flags are part of the enumeration gnutls_certificate_verify_flags shown in Figure 4.3.

GNUTLS_VERIFY_DISABLE_CA_SIGN

If set a signer does not have to be a certificate authority. This flag should normally be disabled, unless you know what this means.

GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT

Allow trusted CA certificates with version 1. This is safer than GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT , and should be used instead. That way only signers in your trusted list will be allowed to have certificates of version 1. This is the default.

GNUTLS_VERIFY_DO_NOT_ALLOW_SAME

If a certificate is not signed by anyone trusted but exists in the trusted CA list do not treat it as trusted.

GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT

Allow CA certificates that have version 1 (both root and intermediate). This might be dangerous since those haven’t the basicConstraints extension. Must be used in combination with GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT .

GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2

Allow certificates to be signed using the broken MD2 algorithm.

GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5

Allow certificates to be signed using the broken MD5 algorithm.

GNUTLS_VERIFY_DISABLE_TIME_CHECKS

Disable checking of activation and expiration validity periods of certificate chains. Don’t set this unless you understand the security implications.

GNUTLS_VERIFY_DISABLE_TRUSTED_TIME_CHECKS

If set a signer in the trusted list is never checked for expiration or activation.

GNUTLS_VERIFY_DO_NOT_ALLOW_X509_V1_CA_CRT

Do not allow trusted CA certificates that have version 1. This option is to be used to deprecate all certificates of version 1.

GNUTLS_VERIFY_DISABLE_CRL_CHECKS

Disable checking for validity using certificate revocation lists or the available OCSP data.

GNUTLS_VERIFY_ALLOW_UNSORTED_CHAIN

A certificate chain is tolerated if unsorted (the case with many TLS servers out there). This is the default since GnuTLS 3.1.4.

GNUTLS_VERIFY_DO_NOT_ALLOW_UNSORTED_CHAIN

Do not tolerate an unsorted certificate chain.

Figure 4.3: The gnutls_certificate_verify_flags enumeration.


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4.1.2 OpenPGP certificates

The OpenPGP key authentication relies on a distributed trust model, called the “web of trust”. The “web of trust” uses a decentralized system of trusted introducers, which are the same as a CA. OpenPGP allows anyone to sign anyone else’s public key. When Alice signs Bob’s key, she is introducing Bob’s key to anyone who trusts Alice. If someone trusts Alice to introduce keys, then Alice is a trusted introducer in the mind of that observer. For example in Figure 4.4, David trusts Alice to be an introducer and Alice signed Bob’s key thus Dave trusts Bob’s key to be the real one.

gnutls-pgp

Figure 4.4: An example of the OpenPGP trust model.

There are some key points that are important in that model. In the example Alice has to sign Bob’s key, only if she is sure that the key belongs to Bob. Otherwise she may also make Dave falsely believe that this is Bob’s key. Dave has also the responsibility to know who to trust. This model is similar to real life relations.

Just see how Charlie behaves in the previous example. Although he has signed Bob’s key - because he knows, somehow, that it belongs to Bob - he does not trust Bob to be an introducer. Charlie decided to trust only Kevin, for some reason. A reason could be that Bob is lazy enough, and signs other people’s keys without being sure that they belong to the actual owner.

FieldDescription
versionThe field that indicates the version of the OpenPGP structure.
user IDAn RFC 2822 string that identifies the owner of the key. There may be multiple user identifiers in a key.
public keyThe main public key of the certificate.
expirationThe expiration time of the main public key.
public subkeyAn additional public key of the certificate. There may be multiple subkeys in a certificate.
public subkey expirationThe expiration time of the subkey.

Table 4.4: OpenPGP certificate fields.

4.1.2.1 OpenPGP certificate structure

In GnuTLS the OpenPGP certificate structures [RFC2440] are handled using the gnutls_openpgp_crt_t type. A typical certificate contains the user ID, which is an RFC 2822 mail and name address, a public key, possibly a number of additional public keys (called subkeys), and a number of signatures. The various fields are shown in Table 4.4.

The additional subkeys may provide key for various different purposes, e.g. one key to encrypt mail, and another to sign a TLS key exchange. Each subkey is identified by a unique key ID. The keys that are to be used in a TLS key exchange that requires signatures are called authentication keys in the OpenPGP jargon. The mapping of TLS key exchange methods to public keys is shown in Table 4.5.

Key exchangePublic key requirements
RSAAn RSA public key that allows encryption.
DHE_RSAAn RSA public key that is marked for authentication.
ECDHE_RSAAn RSA public key that is marked for authentication.
DHE_DSSA DSA public key that is marked for authentication.

Table 4.5: The types of (sub)keys required for the various TLS key exchange methods.

The corresponding private keys are stored in the gnutls_openpgp_privkey_t type. All the prototypes for the key handling functions can be found in gnutls/openpgp.h.

4.1.2.2 Verifying an OpenPGP certificate

The verification functions of OpenPGP keys, included in GnuTLS, are simple ones, and do not use the features of the “web of trust”. For that reason, if the verification needs are complex, the assistance of external tools like GnuPG and GPGME6 is recommended.

In GnuTLS there is a verification function for OpenPGP certificates, the gnutls_openpgp_crt_verify_ring. This checks an OpenPGP key against a given set of public keys (keyring) and returns the key status. The key verification status is the same as in X.509 certificates, although the meaning and interpretation are different. For example an OpenPGP key may be valid, if the self signature is ok, even if no signers were found. The meaning of verification status flags is the same as in the X.509 certificates (see Figure 4.3).

Function: int gnutls_openpgp_crt_verify_ring (gnutls_openpgp_crt_t key, gnutls_openpgp_keyring_t keyring, unsigned int flags, unsigned int * verify)

key: the structure that holds the key.

keyring: holds the keyring to check against

flags: unused (should be 0)

verify: will hold the certificate verification output.

Verify all signatures in the key, using the given set of keys (keyring).

The key verification output will be put in verify and will be one or more of the gnutls_certificate_status_t enumerated elements bitwise or’d.

Note that this function does not verify using any "web of trust". You may use GnuPG for that purpose, or any other external PGP application.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

Function: int gnutls_openpgp_crt_verify_self (gnutls_openpgp_crt_t key, unsigned int flags, unsigned int * verify)

key: the structure that holds the key.

flags: unused (should be 0)

verify: will hold the key verification output.

Verifies the self signature in the key. The key verification output will be put in verify and will be one or more of the gnutls_certificate_status_t enumerated elements bitwise or’d.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

4.1.2.3 Verifying a certificate in the context of a TLS session

Similarly with X.509 certificates, one needs to specify the OpenPGP keyring file in the credentials structure. The certificates in this file will be used by gnutls_certificate_verify_peers3 to verify the signatures in the certificate sent by the peer.

Function: int gnutls_certificate_set_openpgp_keyring_file (gnutls_certificate_credentials_t c, const char * file, gnutls_openpgp_crt_fmt_t format)

c: A certificate credentials structure

file: filename of the keyring.

format: format of keyring.

The function is used to set keyrings that will be used internally by various OpenPGP functions. For example to find a key when it is needed for an operations. The keyring will also be used at the verification functions.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.


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4.1.3 Advanced certificate verification

The verification of X.509 certificates in the HTTPS and other Internet protocols is typically done by loading a trusted list of commercial Certificate Authorities (see gnutls_certificate_set_x509_system_trust), and using them as trusted anchors. However, there are several examples (eg. the Diginotar incident) where one of these authorities was compromised. This risk can be mitigated by using in addition to CA certificate verification, other verification methods. In this section we list the available in GnuTLS methods.


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4.1.3.1 Verifying a certificate using trust on first use authentication

It is possible to use a trust on first use (TOFU) authentication method in GnuTLS. That is the concept used by the SSH programs, where the public key of the peer is not verified, or verified in an out-of-bound way, but subsequent connections to the same peer require the public key to remain the same. Such a system in combination with the typical CA verification of a certificate, and OCSP revocation checks, can help to provide multiple factor verification, where a single point of failure is not enough to compromise the system. For example a server compromise may be detected using OCSP, and a CA compromise can be detected using the trust on first use method. Such a hybrid system with X.509 and trust on first use authentication is shown in Simple client example with SSH-style certificate verification.

See Certificate verification on how to use the available functionality.


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4.1.3.2 Verifying a certificate using DANE (DNSSEC)

The DANE protocol is a protocol that can be used to verify TLS certificates using the DNS (or better DNSSEC) protocols. The DNS security extensions (DNSSEC) provide an alternative public key infrastructure to the commercial CAs that are typically used to sign TLS certificates. The DANE protocol takes advantage of the DNSSEC infrastructure to verify TLS certificates. This can be in addition to the verification by CA infrastructure or may even replace it where DNSSEC is fully deployed. Note however, that DNSSEC deployment is fairly new and it would be better to use it as an additional verification method rather than the only one.

The DANE functionality is provided by the libgnutls-dane library that is shipped with GnuTLS and the function prototypes are in gnutls/dane.h. See Certificate verification for information on how to use the library.

Note however, that the DANE RFC mandates the verification methods one should use in addition to the validation via DNSSEC TLSA entries. GnuTLS doesn’t follow that RFC requirement, and the term DANE verification in this manual refers to the TLSA entry verification. In GnuTLS any other verification methods can be used (e.g., PKIX or TOFU) on top of DANE.


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4.1.4 Digital signatures

In this section we will provide some information about digital signatures, how they work, and give the rationale for disabling some of the algorithms used.

Digital signatures work by using somebody’s secret key to sign some arbitrary data. Then anybody else could use the public key of that person to verify the signature. Since the data may be arbitrary it is not suitable input to a cryptographic digital signature algorithm. For this reason and also for performance cryptographic hash algorithms are used to preprocess the input to the signature algorithm. This works as long as it is difficult enough to generate two different messages with the same hash algorithm output. In that case the same signature could be used as a proof for both messages. Nobody wants to sign an innocent message of donating 1 euro to Greenpeace and find out that he donated 1.000.000 euros to Bad Inc.

For a hash algorithm to be called cryptographic the following three requirements must hold:

  1. Preimage resistance. That means the algorithm must be one way and given the output of the hash function H(x), it is impossible to calculate x.
  2. 2nd preimage resistance. That means that given a pair x,y with y=H(x) it is impossible to calculate an x' such that y=H(x').
  3. Collision resistance. That means that it is impossible to calculate random x and x' such H(x')=H(x).

The last two requirements in the list are the most important in digital signatures. These protect against somebody who would like to generate two messages with the same hash output. When an algorithm is considered broken usually it means that the Collision resistance of the algorithm is less than brute force. Using the birthday paradox the brute force attack takes 2^{((hash size) / 2)} operations. Today colliding certificates using the MD5 hash algorithm have been generated as shown in [WEGER].

There has been cryptographic results for the SHA-1 hash algorithms as well, although they are not yet critical. Before 2004, MD5 had a presumed collision strength of 2^{64}, but it has been showed to have a collision strength well under 2^{50}. As of November 2005, it is believed that SHA-1’s collision strength is around 2^{63}. We consider this sufficiently hard so that we still support SHA-1. We anticipate that SHA-256/386/512 will be used in publicly-distributed certificates in the future. When 2^{63} can be considered too weak compared to the computer power available sometime in the future, SHA-1 will be disabled as well. The collision attacks on SHA-1 may also get better, given the new interest in tools for creating them.

4.1.4.1 Trading security for interoperability

If you connect to a server and use GnuTLS’ functions to verify the certificate chain, and get a GNUTLS_CERT_INSECURE_ALGORITHM validation error (see Verifying X.509 certificate paths), it means that somewhere in the certificate chain there is a certificate signed using RSA-MD2 or RSA-MD5. These two digital signature algorithms are considered broken, so GnuTLS fails verifying the certificate. In some situations, it may be useful to be able to verify the certificate chain anyway, assuming an attacker did not utilize the fact that these signatures algorithms are broken. This section will give help on how to achieve that.

It is important to know that you do not have to enable any of the flags discussed here to be able to use trusted root CA certificates self-signed using RSA-MD2 or RSA-MD5. The certificates in the trusted list are considered trusted irrespective of the signature.

If you are using gnutls_certificate_verify_peers3 to verify the certificate chain, you can call gnutls_certificate_set_verify_flags with the flags:

as in the following example:

  gnutls_certificate_set_verify_flags (x509cred,
                                       GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5);

This will signal the verifier algorithm to enable RSA-MD5 when verifying the certificates.

If you are using gnutls_x509_crt_verify or gnutls_x509_crt_list_verify, you can pass the GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5 parameter directly in the flags parameter.

If you are using these flags, it may also be a good idea to warn the user when verification failure occur for this reason. The simplest is to not use the flags by default, and only fall back to using them after warning the user. If you wish to inspect the certificate chain yourself, you can use gnutls_certificate_get_peers to extract the raw server’s certificate chain, gnutls_x509_crt_list_import to parse each of the certificates, and then gnutls_x509_crt_get_signature_algorithm to find out the signing algorithm used for each certificate. If any of the intermediary certificates are using GNUTLS_SIGN_RSA_MD2 or GNUTLS_SIGN_RSA_MD5, you could present a warning.


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4.2 More on certificate authentication

Certificates are not the only structures involved in a public key infrastructure. Several other structures that are used for certificate requests, encrypted private keys, revocation lists, GnuTLS abstract key structures, etc., are discussed in this chapter.


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4.2.1 PKCS #10 certificate requests

A certificate request is a structure, which contain information about an applicant of a certificate service. It usually contains a private key, a distinguished name and secondary data such as a challenge password. GnuTLS supports the requests defined in PKCS #10 [RFC2986]. Other formats of certificate requests are not currently supported.

A certificate request can be generated by associating it with a private key, setting the subject’s information and finally self signing it. The last step ensures that the requester is in possession of the private key.

int gnutls_x509_crq_set_version (gnutls_x509_crq_t crq, unsigned int version)
int gnutls_x509_crq_set_dn (gnutls_x509_crq_t crq, const char * dn, const char ** err)
int gnutls_x509_crq_set_dn_by_oid (gnutls_x509_crq_t crq, const char * oid, unsigned int raw_flag, const void * data, unsigned int sizeof_data)
int gnutls_x509_crq_set_key_usage (gnutls_x509_crq_t crq, unsigned int usage)
int gnutls_x509_crq_set_key_purpose_oid (gnutls_x509_crq_t crq, const void * oid, unsigned int critical)
int gnutls_x509_crq_set_basic_constraints (gnutls_x509_crq_t crq, unsigned int ca, int pathLenConstraint)

The gnutls_x509_crq_set_key and gnutls_x509_crq_sign2 functions associate the request with a private key and sign it. If a request is to be signed with a key residing in a PKCS #11 token it is recommended to use the signing functions shown in Abstract key types.

Function: int gnutls_x509_crq_set_key (gnutls_x509_crq_t crq, gnutls_x509_privkey_t key)

crq: should contain a gnutls_x509_crq_t structure

key: holds a private key

This function will set the public parameters from the given private key to the request.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Function: int gnutls_x509_crq_sign2 (gnutls_x509_crq_t crq, gnutls_x509_privkey_t key, gnutls_digest_algorithm_t dig, unsigned int flags)

crq: should contain a gnutls_x509_crq_t structure

key: holds a private key

dig: The message digest to use, i.e., GNUTLS_DIG_SHA1

flags: must be 0

This function will sign the certificate request with a private key. This must be the same key as the one used in gnutls_x509_crt_set_key() since a certificate request is self signed.

This must be the last step in a certificate request generation since all the previously set parameters are now signed.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code. GNUTLS_E_ASN1_VALUE_NOT_FOUND is returned if you didn’t set all information in the certificate request (e.g., the version using gnutls_x509_crq_set_version() ).

The following example is about generating a certificate request, and a private key. A certificate request can be later be processed by a CA which should return a signed certificate.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <gnutls/abstract.h>
#include <time.h>

/* This example will generate a private key and a certificate
 * request.
 */

int main(void)
{
        gnutls_x509_crq_t crq;
        gnutls_x509_privkey_t key;
        unsigned char buffer[10 * 1024];
        size_t buffer_size = sizeof(buffer);
        unsigned int bits;

        gnutls_global_init();

        /* Initialize an empty certificate request, and
         * an empty private key.
         */
        gnutls_x509_crq_init(&crq);

        gnutls_x509_privkey_init(&key);

        /* Generate an RSA key of moderate security.
         */
        bits =
            gnutls_sec_param_to_pk_bits(GNUTLS_PK_RSA,
                                        GNUTLS_SEC_PARAM_NORMAL);
        gnutls_x509_privkey_generate(key, GNUTLS_PK_RSA, bits, 0);

        /* Add stuff to the distinguished name
         */
        gnutls_x509_crq_set_dn_by_oid(crq, GNUTLS_OID_X520_COUNTRY_NAME,
                                      0, "GR", 2);

        gnutls_x509_crq_set_dn_by_oid(crq, GNUTLS_OID_X520_COMMON_NAME,
                                      0, "Nikos", strlen("Nikos"));

        /* Set the request version.
         */
        gnutls_x509_crq_set_version(crq, 1);

        /* Set a challenge password.
         */
        gnutls_x509_crq_set_challenge_password(crq,
                                               "something to remember here");

        /* Associate the request with the private key
         */
        gnutls_x509_crq_set_key(crq, key);

        /* Self sign the certificate request.
         */
        gnutls_x509_crq_sign2(crq, key, GNUTLS_DIG_SHA1, 0);

        /* Export the PEM encoded certificate request, and
         * display it.
         */
        gnutls_x509_crq_export(crq, GNUTLS_X509_FMT_PEM, buffer,
                               &buffer_size);

        printf("Certificate Request: \n%s", buffer);


        /* Export the PEM encoded private key, and
         * display it.
         */
        buffer_size = sizeof(buffer);
        gnutls_x509_privkey_export(key, GNUTLS_X509_FMT_PEM, buffer,
                                   &buffer_size);

        printf("\n\nPrivate key: \n%s", buffer);

        gnutls_x509_crq_deinit(crq);
        gnutls_x509_privkey_deinit(key);

        return 0;

}

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4.2.2 PKIX certificate revocation lists

A certificate revocation list (CRL) is a structure issued by an authority periodically containing a list of revoked certificates serial numbers. The CRL structure is signed with the issuing authorities’ keys. A typical CRL contains the fields as shown in Table 4.6. Certificate revocation lists are used to complement the expiration date of a certificate, in order to account for other reasons of revocation, such as compromised keys, etc.

Each CRL is valid for limited amount of time and is required to provide, except for the current issuing time, also the issuing time of the next update.

FieldDescription
versionThe field that indicates the version of the CRL structure.
signatureA signature by the issuing authority.
issuerHolds the issuer’s distinguished name.
thisUpdateThe issuing time of the revocation list.
nextUpdateThe issuing time of the revocation list that will update that one.
revokedCertificatesList of revoked certificates serial numbers.
extensionsOptional CRL structure extensions.

Table 4.6: Certificate revocation list fields.

The basic CRL structure functions follow.

int gnutls_x509_crl_init (gnutls_x509_crl_t * crl)
int gnutls_x509_crl_import (gnutls_x509_crl_t crl, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format)
int gnutls_x509_crl_export (gnutls_x509_crl_t crl, gnutls_x509_crt_fmt_t format, void * output_data, size_t * output_data_size)
int gnutls_x509_crl_export (gnutls_x509_crl_t crl, gnutls_x509_crt_fmt_t format, void * output_data, size_t * output_data_size)

Reading a CRL

The most important function that extracts the certificate revocation information from a CRL is gnutls_x509_crl_get_crt_serial. Other functions that return other fields of the CRL structure are also provided.

Function: int gnutls_x509_crl_get_crt_serial (gnutls_x509_crl_t crl, int indx, unsigned char * serial, size_t * serial_size, time_t * t)

crl: should contain a gnutls_x509_crl_t structure

indx: the index of the certificate to extract (starting from 0)

serial: where the serial number will be copied

serial_size: initially holds the size of serial

t: if non null, will hold the time this certificate was revoked

This function will retrieve the serial number of the specified, by the index, revoked certificate.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value. and a negative error code on error.

int gnutls_x509_crl_get_version (gnutls_x509_crl_t crl)
int gnutls_x509_crl_get_issuer_dn (const gnutls_x509_crl_t crl, char * buf, size_t * sizeof_buf)
int gnutls_x509_crl_get_issuer_dn2 (gnutls_x509_crl_t crl, gnutls_datum_t * dn)
time_t gnutls_x509_crl_get_this_update (gnutls_x509_crl_t crl)
time_t gnutls_x509_crl_get_next_update (gnutls_x509_crl_t crl)
int gnutls_x509_crl_get_crt_count (gnutls_x509_crl_t crl)

Generation of a CRL

The following functions can be used to generate a CRL.

int gnutls_x509_crl_set_version (gnutls_x509_crl_t crl, unsigned int version)
int gnutls_x509_crl_set_crt_serial (gnutls_x509_crl_t crl, const void * serial, size_t serial_size, time_t revocation_time)
int gnutls_x509_crl_set_crt (gnutls_x509_crl_t crl, gnutls_x509_crt_t crt, time_t revocation_time)
int gnutls_x509_crl_set_next_update (gnutls_x509_crl_t crl, time_t exp_time)
int gnutls_x509_crl_set_this_update (gnutls_x509_crl_t crl, time_t act_time)

The gnutls_x509_crl_sign2 and gnutls_x509_crl_privkey_sign functions sign the revocation list with a private key. The latter function can be used to sign with a key residing in a PKCS #11 token.

Function: int gnutls_x509_crl_sign2 (gnutls_x509_crl_t crl, gnutls_x509_crt_t issuer, gnutls_x509_privkey_t issuer_key, gnutls_digest_algorithm_t dig, unsigned int flags)

crl: should contain a gnutls_x509_crl_t structure

issuer: is the certificate of the certificate issuer

issuer_key: holds the issuer’s private key

dig: The message digest to use. GNUTLS_DIG_SHA1 is the safe choice unless you know what you’re doing.

flags: must be 0

This function will sign the CRL with the issuer’s private key, and will copy the issuer’s information into the CRL.

This must be the last step in a certificate CRL since all the previously set parameters are now signed.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Function: int gnutls_x509_crl_privkey_sign (gnutls_x509_crl_t crl, gnutls_x509_crt_t issuer, gnutls_privkey_t issuer_key, gnutls_digest_algorithm_t dig, unsigned int flags)

crl: should contain a gnutls_x509_crl_t structure

issuer: is the certificate of the certificate issuer

issuer_key: holds the issuer’s private key

dig: The message digest to use. GNUTLS_DIG_SHA1 is the safe choice unless you know what you’re doing.

flags: must be 0

This function will sign the CRL with the issuer’s private key, and will copy the issuer’s information into the CRL.

This must be the last step in a certificate CRL since all the previously set parameters are now signed.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since 2.12.0

Few extensions on the CRL structure are supported, including the CRL number extension and the authority key identifier.

int gnutls_x509_crl_set_number (gnutls_x509_crl_t crl, const void * nr, size_t nr_size)
int gnutls_x509_crl_set_authority_key_id (gnutls_x509_crl_t crl, const void * id, size_t id_size)

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4.2.3 OCSP certificate status checking

Certificates may be revoked before their expiration time has been reached. There are several reasons for revoking certificates, but a typical situation is when the private key associated with a certificate has been compromised. Traditionally, Certificate Revocation Lists (CRLs) have been used by application to implement revocation checking, however, several problems with CRLs have been identified [RIVESTCRL].

The Online Certificate Status Protocol, or OCSP [RFC2560], is a widely implemented protocol to perform certificate revocation status checking. An application that wish to verify the identity of a peer will verify the certificate against a set of trusted certificates and then check whether the certificate is listed in a CRL and/or perform an OCSP check for the certificate.

Note that in the context of a TLS session the server may provide an OCSP response that will used during the TLS certificate verification (see gnutls_certificate_verify_peers3). You may obtain this response using gnutls_ocsp_status_request_get.

Before performing the OCSP query, the application will need to figure out the address of the OCSP server. The OCSP server address can be provided by the local user in manual configuration or may be stored in the certificate that is being checked. When stored in a certificate the OCSP server is in the extension field called the Authority Information Access (AIA). The following function extracts this information from a certificate.

int gnutls_x509_crt_get_authority_info_access (gnutls_x509_crt_t crt, unsigned int seq, int what, gnutls_datum_t * data, unsigned int * critical)

There are several functions in GnuTLS for creating and manipulating OCSP requests and responses. The general idea is that a client application create an OCSP request object, store some information about the certificate to check in the request, and then export the request in DER format. The request will then need to be sent to the OCSP responder, which needs to be done by the application (GnuTLS does not send and receive OCSP packets). Normally an OCSP response is received that the application will need to import into an OCSP response object. The digital signature in the OCSP response needs to be verified against a set of trust anchors before the information in the response can be trusted.

The ASN.1 structure of OCSP requests are briefly as follows. It is useful to review the structures to get an understanding of which fields are modified by GnuTLS functions.

OCSPRequest     ::=     SEQUENCE {
    tbsRequest                  TBSRequest,
    optionalSignature   [0]     EXPLICIT Signature OPTIONAL }

TBSRequest      ::=     SEQUENCE {
    version             [0]     EXPLICIT Version DEFAULT v1,
    requestorName       [1]     EXPLICIT GeneralName OPTIONAL,
    requestList                 SEQUENCE OF Request,
    requestExtensions   [2]     EXPLICIT Extensions OPTIONAL }

Request         ::=     SEQUENCE {
    reqCert                     CertID,
    singleRequestExtensions     [0] EXPLICIT Extensions OPTIONAL }

CertID          ::=     SEQUENCE {
    hashAlgorithm       AlgorithmIdentifier,
    issuerNameHash      OCTET STRING, -- Hash of Issuer's DN
    issuerKeyHash       OCTET STRING, -- Hash of Issuers public key
    serialNumber        CertificateSerialNumber }

The basic functions to initialize, import, export and deallocate OCSP requests are the following.

int gnutls_ocsp_req_init (gnutls_ocsp_req_t * req)
void gnutls_ocsp_req_deinit (gnutls_ocsp_req_t req)
int gnutls_ocsp_req_import (gnutls_ocsp_req_t req, const gnutls_datum_t * data)
int gnutls_ocsp_req_export (gnutls_ocsp_req_t req, gnutls_datum_t * data)
int gnutls_ocsp_req_print (gnutls_ocsp_req_t req, gnutls_ocsp_print_formats_t format, gnutls_datum_t * out)

To generate an OCSP request the issuer name hash, issuer key hash, and the checked certificate’s serial number are required. There are two interfaces available for setting those in an OCSP request. The is a low-level function when you have the issuer name hash, issuer key hash, and certificate serial number in binary form. The second is more useful if you have the certificate (and its issuer) in a gnutls_x509_crt_t type. There is also a function to extract this information from existing an OCSP request.

int gnutls_ocsp_req_add_cert_id (gnutls_ocsp_req_t req, gnutls_digest_algorithm_t digest, const gnutls_datum_t * issuer_name_hash, const gnutls_datum_t * issuer_key_hash, const gnutls_datum_t * serial_number)
int gnutls_ocsp_req_add_cert (gnutls_ocsp_req_t req, gnutls_digest_algorithm_t digest, gnutls_x509_crt_t issuer, gnutls_x509_crt_t cert)
int gnutls_ocsp_req_get_cert_id (gnutls_ocsp_req_t req, unsigned indx, gnutls_digest_algorithm_t * digest, gnutls_datum_t * issuer_name_hash, gnutls_datum_t * issuer_key_hash, gnutls_datum_t * serial_number)

Each OCSP request may contain a number of extensions. Extensions are identified by an Object Identifier (OID) and an opaque data buffer whose syntax and semantics is implied by the OID. You can extract or set those extensions using the following functions.

int gnutls_ocsp_req_get_extension (gnutls_ocsp_req_t req, unsigned indx, gnutls_datum_t * oid, unsigned int * critical, gnutls_datum_t * data)
int gnutls_ocsp_req_set_extension (gnutls_ocsp_req_t req, const char * oid, unsigned int critical, const gnutls_datum_t * data)

A common OCSP Request extension is the nonce extension (OID 1.3.6.1.5.5.7.48.1.2), which is used to avoid replay attacks of earlier recorded OCSP responses. The nonce extension carries a value that is intended to be sufficiently random and unique so that an attacker will not be able to give a stale response for the same nonce.

int gnutls_ocsp_req_get_nonce (gnutls_ocsp_req_t req, unsigned int * critical, gnutls_datum_t * nonce)
int gnutls_ocsp_req_set_nonce (gnutls_ocsp_req_t req, unsigned int critical, const gnutls_datum_t * nonce)
int gnutls_ocsp_req_randomize_nonce (gnutls_ocsp_req_t req)

The OCSP response structures is a complex structure. A simplified overview of it is in Table 4.7. Note that a response may contain information on multiple certificates.

FieldDescription
versionThe OCSP response version number (typically 1).
responder IDAn identifier of the responder (DN name or a hash of its key).
issue timeThe time the response was generated.
thisUpdateThe issuing time of the revocation information.
nextUpdateThe issuing time of the revocation information that will update that one.
Revoked certificates
certificate statusThe status of the certificate.
certificate serialThe certificate’s serial number.
revocationTimeThe time the certificate was revoked.
revocationReasonThe reason the certificate was revoked.

Table 4.7: The most important OCSP response fields.

We provide basic functions for initialization, importing, exporting and deallocating OCSP responses.

int gnutls_ocsp_resp_init (gnutls_ocsp_resp_t * resp)
void gnutls_ocsp_resp_deinit (gnutls_ocsp_resp_t resp)
int gnutls_ocsp_resp_import (gnutls_ocsp_resp_t resp, const gnutls_datum_t * data)
int gnutls_ocsp_resp_export (gnutls_ocsp_resp_t resp, gnutls_datum_t * data)
int gnutls_ocsp_resp_print (gnutls_ocsp_resp_t resp, gnutls_ocsp_print_formats_t format, gnutls_datum_t * out)

The utility function that extracts the revocation as well as other information from a response is shown below.

Function: int gnutls_ocsp_resp_get_single (gnutls_ocsp_resp_t resp, unsigned indx, gnutls_digest_algorithm_t * digest, gnutls_datum_t * issuer_name_hash, gnutls_datum_t * issuer_key_hash, gnutls_datum_t * serial_number, unsigned int * cert_status, time_t * this_update, time_t * next_update, time_t * revocation_time, unsigned int * revocation_reason)

resp: should contain a gnutls_ocsp_resp_t structure

indx: Specifies response number to get. Use (0) to get the first one.

digest: output variable with gnutls_digest_algorithm_t hash algorithm

issuer_name_hash: output buffer with hash of issuer’s DN

issuer_key_hash: output buffer with hash of issuer’s public key

serial_number: output buffer with serial number of certificate to check

cert_status: a certificate status, a gnutls_ocsp_cert_status_t enum.

this_update: time at which the status is known to be correct.

next_update: when newer information will be available, or (time_t)-1 if unspecified

revocation_time: when cert_status is GNUTLS_OCSP_CERT_REVOKED , holds time of revocation.

revocation_reason: revocation reason, a gnutls_x509_crl_reason_t enum.

This function will return the certificate information of the indx ’ed response in the Basic OCSP Response resp . The information returned corresponds to the OCSP SingleResponse structure except the final singleExtensions.

Each of the pointers to output variables may be NULL to indicate that the caller is not interested in that value.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned. If you have reached the last CertID available GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE will be returned.

The possible revocation reasons available in an OCSP response are shown below.

GNUTLS_X509_CRLREASON_UNSPECIFIED

Unspecified reason.

GNUTLS_X509_CRLREASON_KEYCOMPROMISE

Private key compromised.

GNUTLS_X509_CRLREASON_CACOMPROMISE

CA compromised.

GNUTLS_X509_CRLREASON_AFFILIATIONCHANGED

Affiliation has changed.

GNUTLS_X509_CRLREASON_SUPERSEDED

Certificate superseded.

GNUTLS_X509_CRLREASON_CESSATIONOFOPERATION

Operation has ceased.

GNUTLS_X509_CRLREASON_CERTIFICATEHOLD

Certificate is on hold.

GNUTLS_X509_CRLREASON_REMOVEFROMCRL

Will be removed from delta CRL.

GNUTLS_X509_CRLREASON_PRIVILEGEWITHDRAWN

Privilege withdrawn.

GNUTLS_X509_CRLREASON_AACOMPROMISE

AA compromised.

Figure 4.5: The revocation reasons

Note, that the OCSP response needs to be verified against some set of trust anchors before it can be relied upon. It is also important to check whether the received OCSP response corresponds to the certificate being checked.

int gnutls_ocsp_resp_verify (gnutls_ocsp_resp_t resp, gnutls_x509_trust_list_t trustlist, unsigned int * verify, unsigned int flags)
int gnutls_ocsp_resp_verify_direct (gnutls_ocsp_resp_t resp, gnutls_x509_crt_t issuer, unsigned int * verify, unsigned int flags)
int gnutls_ocsp_resp_check_crt (gnutls_ocsp_resp_t resp, unsigned int indx, gnutls_x509_crt_t crt)

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4.2.4 Managing encrypted keys

Transferring or storing private keys in plain may not be a good idea, since any compromise is irreparable. Storing the keys in hardware security modules (see Smart cards and HSMs) could solve the storage problem but it is not always practical or efficient enough. This section describes ways to store and transfer encrypted private keys.

There are methods for key encryption, namely the PKCS #8, PKCS #12 and OpenSSL’s custom encrypted private key formats. The PKCS #8 and the OpenSSL’s method allow encryption of the private key, while the PKCS #12 method allows, in addition, the bundling of accompanying data into the structure. That is typically the corresponding certificate, as well as a trusted CA certificate.

High level functionality

Generic and higher level private key import functions are available, that import plain or encrypted keys and will auto-detect the encrypted key format.

Function: int gnutls_privkey_import_x509_raw (gnutls_privkey_t pkey, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags)

pkey: The private key

data: The private key data to be imported

format: The format of the private key

password: A password (optional)

flags: an ORed sequence of gnutls_pkcs_encrypt_flags_t

This function will import the given private key to the abstract gnutls_privkey_t structure.

The supported formats are basic unencrypted key, PKCS8, PKCS12, and the openssl format.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0

Function: int gnutls_x509_privkey_import2 (gnutls_x509_privkey_t key, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags)

key: The structure to store the parsed key

data: The DER or PEM encoded key.

format: One of DER or PEM

password: A password (optional)

flags: an ORed sequence of gnutls_pkcs_encrypt_flags_t

This function will import the given DER or PEM encoded key, to the native gnutls_x509_privkey_t format, irrespective of the input format. The input format is auto-detected.

The supported formats are basic unencrypted key, PKCS8, PKCS12, and the openssl format.

If the provided key is encrypted but no password was given, then GNUTLS_E_DECRYPTION_FAILED is returned.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Any keys imported using those functions can be imported to a certificate credentials structure using gnutls_certificate_set_key, or alternatively they can be directly imported using gnutls_certificate_set_x509_key_file2.

PKCS #8 structures

PKCS #8 keys can be imported and exported as normal private keys using the functions below. An addition to the normal import functions, are a password and a flags argument. The flags can be any element of the gnutls_pkcs_encrypt_flags_t enumeration. Note however, that GnuTLS only supports the PKCS #5 PBES2 encryption scheme. Keys encrypted with the obsolete PBES1 scheme cannot be decrypted.

int gnutls_x509_privkey_import_pkcs8 (gnutls_x509_privkey_t key, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags)
int gnutls_x509_privkey_export_pkcs8 (gnutls_x509_privkey_t key, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags, void * output_data, size_t * output_data_size)
int gnutls_x509_privkey_export2_pkcs8 (gnutls_x509_privkey_t key, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags, gnutls_datum_t * out)
GNUTLS_PKCS_PLAIN

Unencrypted private key.

GNUTLS_PKCS_USE_PKCS12_3DES

PKCS-12 3DES.

GNUTLS_PKCS_USE_PKCS12_ARCFOUR

PKCS-12 ARCFOUR.

GNUTLS_PKCS_USE_PKCS12_RC2_40

PKCS-12 RC2-40.

GNUTLS_PKCS_USE_PBES2_3DES

PBES2 3DES.

GNUTLS_PKCS_USE_PBES2_AES_128

PBES2 AES-128.

GNUTLS_PKCS_USE_PBES2_AES_192

PBES2 AES-192.

GNUTLS_PKCS_USE_PBES2_AES_256

PBES2 AES-256.

GNUTLS_PKCS_NULL_PASSWORD

Some schemas distinguish between an empty and a NULL password.

Figure 4.6: Encryption flags

PKCS #12 structures

A PKCS #12 structure [PKCS12] usually contains a user’s private keys and certificates. It is commonly used in browsers to export and import the user’s identities. A file containing such a key can be directly imported to a certificate credentials structure by using gnutls_certificate_set_x509_simple_pkcs12_file.

In GnuTLS the PKCS #12 structures are handled using the gnutls_pkcs12_t type. This is an abstract type that may hold several gnutls_pkcs12_bag_t types. The bag types are the holders of the actual data, which may be certificates, private keys or encrypted data. A bag of type encrypted should be decrypted in order for its data to be accessed.

To reduce the complexity in parsing the structures the simple helper function gnutls_pkcs12_simple_parse is provided. For more advanced uses, manual parsing of the structure is required using the functions below.

int gnutls_pkcs12_get_bag (gnutls_pkcs12_t pkcs12, int indx, gnutls_pkcs12_bag_t bag)
int gnutls_pkcs12_verify_mac (gnutls_pkcs12_t pkcs12, const char * pass)
int gnutls_pkcs12_bag_decrypt (gnutls_pkcs12_bag_t bag, const char * pass)
int gnutls_pkcs12_bag_get_count (gnutls_pkcs12_bag_t bag)
Function: int gnutls_pkcs12_simple_parse (gnutls_pkcs12_t p12, const char * password, gnutls_x509_privkey_t * key, gnutls_x509_crt_t ** chain, unsigned int * chain_len, gnutls_x509_crt_t ** extra_certs, unsigned int * extra_certs_len, gnutls_x509_crl_t * crl, unsigned int flags)

p12: the PKCS12 blob.

password: optional password used to decrypt PKCS12 blob, bags and keys.

key: a structure to store the parsed private key.

chain: the corresponding to key certificate chain (may be NULL )

chain_len: will be updated with the number of additional (may be NULL )

extra_certs: optional pointer to receive an array of additional certificates found in the PKCS12 blob (may be NULL ).

extra_certs_len: will be updated with the number of additional certs (may be NULL ).

crl: an optional structure to store the parsed CRL (may be NULL ).

flags: should be zero or one of GNUTLS_PKCS12_SP_*

This function parses a PKCS12 blob in p12blob and extracts the private key, the corresponding certificate chain, and any additional certificates and a CRL.

The extra_certs_ret and extra_certs_len parameters are optional and both may be set to NULL . If either is non-NULL , then both must be set.

Encrypted PKCS12 bags and PKCS8 private keys are supported. However, only password based security, and the same password for all operations, are supported.

A PKCS12 file may contain many keys and/or certificates, and there is no way to identify which key/certificate pair you want. You should make sure the PKCS12 file only contain one key/certificate pair and/or one CRL.

It is believed that the limitations of this function are acceptable for common usage, and that any more flexibility would introduce complexity that would make it harder to use this functionality at all.

If the provided structure has encrypted fields but no password is provided then this function returns GNUTLS_E_DECRYPTION_FAILED .

Note that normally the chain constructed does not include self signed certificates, to comply with TLS’ requirements. If, however, the flag GNUTLS_PKCS12_SP_INCLUDE_SELF_SIGNED is specified then self signed certificates will be included in the chain.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1

int gnutls_pkcs12_bag_get_data (gnutls_pkcs12_bag_t bag, int indx, gnutls_datum_t * data)
int gnutls_pkcs12_bag_get_key_id (gnutls_pkcs12_bag_t bag, int indx, gnutls_datum_t * id)
int gnutls_pkcs12_bag_get_friendly_name (gnutls_pkcs12_bag_t bag, int indx, char ** name)

The functions below are used to generate a PKCS #12 structure. An example of their usage is shown at PKCS12 structure generation example.

int gnutls_pkcs12_set_bag (gnutls_pkcs12_t pkcs12, gnutls_pkcs12_bag_t bag)
int gnutls_pkcs12_bag_encrypt (gnutls_pkcs12_bag_t bag, const char * pass, unsigned int flags)
int gnutls_pkcs12_generate_mac (gnutls_pkcs12_t pkcs12, const char * pass)
int gnutls_pkcs12_bag_set_data (gnutls_pkcs12_bag_t bag, gnutls_pkcs12_bag_type_t type, const gnutls_datum_t * data)
int gnutls_pkcs12_bag_set_crl (gnutls_pkcs12_bag_t bag, gnutls_x509_crl_t crl)
int gnutls_pkcs12_bag_set_crt (gnutls_pkcs12_bag_t bag, gnutls_x509_crt_t crt)
int gnutls_pkcs12_bag_set_key_id (gnutls_pkcs12_bag_t bag, int indx, const gnutls_datum_t * id)
int gnutls_pkcs12_bag_set_friendly_name (gnutls_pkcs12_bag_t bag, int indx, const char * name)

OpenSSL encrypted keys

Unfortunately the structures discussed in the previous sections are not the only structures that may hold an encrypted private key. For example the OpenSSL library offers a custom key encryption method. Those structures are also supported in GnuTLS with gnutls_x509_privkey_import_openssl.

Function: int gnutls_x509_privkey_import_openssl (gnutls_x509_privkey_t key, const gnutls_datum_t * data, const char * password)

key: The structure to store the parsed key

data: The DER or PEM encoded key.

password: the password to decrypt the key (if it is encrypted).

This function will convert the given PEM encrypted to the native gnutls_x509_privkey_t format. The output will be stored in key .

The password should be in ASCII. If the password is not provided or wrong then GNUTLS_E_DECRYPTION_FAILED will be returned.

If the Certificate is PEM encoded it should have a header of "PRIVATE KEY" and the "DEK-Info" header.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.


Next: , Previous: , Up: More on certificate authentication   [Contents][Index]

4.2.5 Invoking certtool

Tool to parse and generate X.509 certificates, requests and private keys. It can be used interactively or non interactively by specifying the template command line option.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the certtool program. This software is released under the GNU General Public License, version 3 or later.

certtool help/usage (--help)

This is the automatically generated usage text for certtool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

certtool - GnuTLS certificate tool
Usage:  certtool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -V, --verbose              More verbose output
                                - may appear multiple times
       --infile=file          Input file
                                - file must pre-exist
       --outfile=str          Output file
   -s, --generate-self-signed  Generate a self-signed certificate
   -c, --generate-certificate  Generate a signed certificate
       --generate-proxy       Generates a proxy certificate
       --generate-crl         Generate a CRL
   -u, --update-certificate   Update a signed certificate
   -p, --generate-privkey     Generate a private key
   -q, --generate-request     Generate a PKCS #10 certificate request
                                - prohibits the option 'infile'
   -e, --verify-chain         Verify a PEM encoded certificate chain
       --verify               Verify a PEM encoded certificate chain using a trusted list
       --verify-crl           Verify a CRL using a trusted list
                                - requires the option 'load-ca-certificate'
       --generate-dh-params   Generate PKCS #3 encoded Diffie-Hellman parameters
       --get-dh-params        Get the included PKCS #3 encoded Diffie-Hellman parameters
       --dh-info              Print information PKCS #3 encoded Diffie-Hellman parameters
       --load-privkey=str     Loads a private key file
       --load-pubkey=str      Loads a public key file
       --load-request=file    Loads a certificate request file
                                - file must pre-exist
       --load-certificate=str Loads a certificate file
       --load-ca-privkey=str  Loads the certificate authority's private key file
       --load-ca-certificate=str Loads the certificate authority's certificate file
       --password=str         Password to use
       --hex-numbers          Print big number in an easier format to parse
       --cprint               In certain operations it prints the information in C-friendly format
       --null-password        Enforce a NULL password
       --empty-password       Enforce an empty password
   -i, --certificate-info     Print information on the given certificate
       --certificate-pubkey   Print certificate's public key
       --pgp-certificate-info  Print information on the given OpenPGP certificate
       --pgp-ring-info        Print information on the given OpenPGP keyring structure
   -l, --crl-info             Print information on the given CRL structure
       --crq-info             Print information on the given certificate request
       --no-crq-extensions    Do not use extensions in certificate requests
       --p12-info             Print information on a PKCS #12 structure
       --p7-info              Print information on a PKCS #7 structure
       --smime-to-p7          Convert S/MIME to PKCS #7 structure
   -k, --key-info             Print information on a private key
       --pgp-key-info         Print information on an OpenPGP private key
       --pubkey-info          Print information on a public key
       --v1                   Generate an X.509 version 1 certificate (with no extensions)
       --to-p12               Generate a PKCS #12 structure
                                - requires the option 'load-certificate'
   -!, --to-p8                Generate a PKCS #8 structure
   -8, --pkcs8                Use PKCS #8 format for private keys
   -", --rsa                  Generate RSA key
   -#, --dsa                  Generate DSA key
   -$, --ecc                  Generate ECC (ECDSA) key
   -%, --ecdsa                an alias for the 'ecc' option
   -&, --hash=str             Hash algorithm to use for signing
   -', --inder                Use DER format for input certificates, private keys, and DH parameters
                                - disabled as '--no-inder'
   -(, --inraw                an alias for the 'inder' option
   -), --outder               Use DER format for output certificates, private keys, and DH parameters
                                - disabled as '--no-outder'
   -*, --outraw               an alias for the 'outder' option
   -+, --bits=num             Specify the number of bits for key generate
   -,, --sec-param=str        Specify the security level [low, legacy, normal, high, ultra]
   --, --disable-quick-random  No effect
   -., --template=file        Template file to use for non-interactive operation
                                - file must pre-exist
   -/, --ask-pass             Enable interaction for entering password when in batch mode.
   -0, --pkcs-cipher=str      Cipher to use for PKCS #8 and #12 operations
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Tool to parse and generate X.509 certificates, requests and private keys.
It can be used interactively or non interactively by specifying the
template command line option.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

generate-request option (-q)

This is the “generate a pkcs #10 certificate request” option.

This option has some usage constraints. It:

Will generate a PKCS #10 certificate request. To specify a private key use –load-privkey.

verify-chain option (-e)

This is the “verify a pem encoded certificate chain” option. The last certificate in the chain must be a self signed one.

verify option

This is the “verify a pem encoded certificate chain using a trusted list” option. The trusted certificate list can be loaded with –load-ca-certificate. If no certificate list is provided, then the system’s certificate list is used.

verify-crl option

This is the “verify a crl using a trusted list” option.

This option has some usage constraints. It:

The trusted certificate list must be loaded with –load-ca-certificate.

get-dh-params option

This is the “get the included pkcs #3 encoded diffie-hellman parameters” option. Returns stored DH parameters in GnuTLS. Those parameters are used in the SRP protocol. The parameters returned by fresh generation are more efficient since GnuTLS 3.0.9.

load-privkey option

This is the “loads a private key file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

load-pubkey option

This is the “loads a public key file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

load-certificate option

This is the “loads a certificate file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

load-ca-privkey option

This is the “loads the certificate authority’s private key file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

load-ca-certificate option

This is the “loads the certificate authority’s certificate file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

password option

This is the “password to use” option. This option takes a string argument. You can use this option to specify the password in the command line instead of reading it from the tty. Note, that the command line arguments are available for view in others in the system. Specifying password as ” is the same as specifying no password.

cprint option

This is the “in certain operations it prints the information in c-friendly format” option. In certain operations it prints the information in C-friendly format, suitable for including into C programs.

null-password option

This is the “enforce a null password” option. This option enforces a NULL password. This is different than the empty or no password in schemas like PKCS #8.

empty-password option

This is the “enforce an empty password” option. This option enforces an empty password. This is different than the NULL or no password in schemas like PKCS #8.

pubkey-info option

This is the “print information on a public key” option. The option combined with –load-request, –load-pubkey, –load-privkey and –load-certificate will extract the public key of the object in question.

to-p12 option

This is the “generate a pkcs #12 structure” option.

This option has some usage constraints. It:

It requires a certificate, a private key and possibly a CA certificate to be specified.

rsa option

This is the “generate rsa key” option. When combined with –generate-privkey generates an RSA private key.

dsa option

This is the “generate dsa key” option. When combined with –generate-privkey generates a DSA private key.

ecc option

This is the “generate ecc (ecdsa) key” option. When combined with –generate-privkey generates an elliptic curve private key to be used with ECDSA.

ecdsa option

This is an alias for the ecc option, see the ecc option documentation.

hash option

This is the “hash algorithm to use for signing” option. This option takes a string argument. Available hash functions are SHA1, RMD160, SHA256, SHA384, SHA512.

inder option

This is the “use der format for input certificates, private keys, and dh parameters ” option.

This option has some usage constraints. It:

The input files will be assumed to be in DER or RAW format. Unlike options that in PEM input would allow multiple input data (e.g. multiple certificates), when reading in DER format a single data structure is read.

inraw option

This is an alias for the inder option, see the inder option documentation.

outder option

This is the “use der format for output certificates, private keys, and dh parameters” option.

This option has some usage constraints. It:

The output will be in DER or RAW format.

outraw option

This is an alias for the outder option, see the outder option documentation.

sec-param option

This is the “specify the security level [low, legacy, normal, high, ultra]” option. This option takes a string argument Security parameter. This is alternative to the bits option.

ask-pass option

This is the “enable interaction for entering password when in batch mode.” option. This option will enable interaction to enter password when in batch mode. That is useful when the template option has been specified.

pkcs-cipher option

This is the “cipher to use for pkcs #8 and #12 operations” option. This option takes a string argument Cipher. Cipher may be one of 3des, 3des-pkcs12, aes-128, aes-192, aes-256, rc2-40, arcfour.

certtool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

certtool See Also

p11tool (1)

certtool Examples

Generating private keys

To create an RSA private key, run:

$ certtool --generate-privkey --outfile key.pem --rsa

To create a DSA or elliptic curves (ECDSA) private key use the above command combined with ’dsa’ or ’ecc’ options.

Generating certificate requests

To create a certificate request (needed when the certificate is issued by another party), run:

certtool --generate-request --load-privkey key.pem \
   --outfile request.pem

If the private key is stored in a smart card you can generate a request by specifying the private key object URL.

$ ./certtool --generate-request --load-privkey "pkcs11:..." \
  --load-pubkey "pkcs11:..." --outfile request.pem

Generating a self-signed certificate

To create a self signed certificate, use the command:

$ certtool --generate-privkey --outfile ca-key.pem
$ certtool --generate-self-signed --load-privkey ca-key.pem \
   --outfile ca-cert.pem

Note that a self-signed certificate usually belongs to a certificate authority, that signs other certificates.

Generating a certificate

To generate a certificate using the previous request, use the command:

$ certtool --generate-certificate --load-request request.pem \
   --outfile cert.pem --load-ca-certificate ca-cert.pem \
   --load-ca-privkey ca-key.pem

To generate a certificate using the private key only, use the command:

$ certtool --generate-certificate --load-privkey key.pem \
   --outfile cert.pem --load-ca-certificate ca-cert.pem \
   --load-ca-privkey ca-key.pem

Certificate information

To view the certificate information, use:

$ certtool --certificate-info --infile cert.pem

PKCS #12 structure generation

To generate a PKCS #12 structure using the previous key and certificate, use the command:

$ certtool --load-certificate cert.pem --load-privkey key.pem \
   --to-p12 --outder --outfile key.p12

Some tools (reportedly web browsers) have problems with that file because it does not contain the CA certificate for the certificate. To work around that problem in the tool, you can use the –load-ca-certificate parameter as follows:

$ certtool --load-ca-certificate ca.pem \
  --load-certificate cert.pem --load-privkey key.pem \
  --to-p12 --outder --outfile key.p12

Diffie-Hellman parameter generation

To generate parameters for Diffie-Hellman key exchange, use the command:

$ certtool --generate-dh-params --outfile dh.pem --sec-param normal

Proxy certificate generation

Proxy certificate can be used to delegate your credential to a temporary, typically short-lived, certificate. To create one from the previously created certificate, first create a temporary key and then generate a proxy certificate for it, using the commands:

$ certtool --generate-privkey > proxy-key.pem
$ certtool --generate-proxy --load-ca-privkey key.pem \
  --load-privkey proxy-key.pem --load-certificate cert.pem \
  --outfile proxy-cert.pem

Certificate revocation list generation

To create an empty Certificate Revocation List (CRL) do:

$ certtool --generate-crl --load-ca-privkey x509-ca-key.pem \
           --load-ca-certificate x509-ca.pem

To create a CRL that contains some revoked certificates, place the certificates in a file and use --load-certificate as follows:

$ certtool --generate-crl --load-ca-privkey x509-ca-key.pem \
  --load-ca-certificate x509-ca.pem --load-certificate revoked-certs.pem

To verify a Certificate Revocation List (CRL) do:

$ certtool --verify-crl --load-ca-certificate x509-ca.pem < crl.pem

certtool Files

Certtool’s template file format

A template file can be used to avoid the interactive questions of certtool. Initially create a file named ’cert.cfg’ that contains the information about the certificate. The template can be used as below:

$ certtool --generate-certificate --load-privkey key.pem  \
   --template cert.cfg --outfile cert.pem \
   --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem

An example certtool template file that can be used to generate a certificate request or a self signed certificate follows.

# X.509 Certificate options
#
# DN options

# The organization of the subject.
organization = "Koko inc."

# The organizational unit of the subject.
unit = "sleeping dept."

# The locality of the subject.
# locality =

# The state of the certificate owner.
state = "Attiki"

# The country of the subject. Two letter code.
country = GR

# The common name of the certificate owner.
cn = "Cindy Lauper"

# A user id of the certificate owner.
#uid = "clauper"

# Set domain components
#dc = "name"
#dc = "domain"

# If the supported DN OIDs are not adequate you can set
# any OID here.
# For example set the X.520 Title and the X.520 Pseudonym
# by using OID and string pairs.
#dn_oid = 2.5.4.12 Dr. 
#dn_oid = 2.5.4.65 jackal

# This is deprecated and should not be used in new
# certificates.
# pkcs9_email = "none@none.org"

# An alternative way to set the certificate's distinguished name directly
# is with the "dn" option. The attribute names allowed are:
# C (country), street, O (organization), OU (unit), title, CN (common name),
# L (locality), ST (state), placeOfBirth, gender, countryOfCitizenship, 
# countryOfResidence, serialNumber, telephoneNumber, surName, initials, 
# generationQualifier, givenName, pseudonym, dnQualifier, postalCode, name, 
# businessCategory, DC, UID, jurisdictionOfIncorporationLocalityName, 
# jurisdictionOfIncorporationStateOrProvinceName,
# jurisdictionOfIncorporationCountryName, XmppAddr, and numeric OIDs.

#dn = "cn=Nik,st=Attiki,C=GR,surName=Mavrogiannopoulos,2.5.4.9=Arkadias"

# The serial number of the certificate
# Comment the field for a time-based serial number.
serial = 007

# In how many days, counting from today, this certificate will expire.
# Use -1 if there is no expiration date.
expiration_days = 700

# Alternatively you may set concrete dates and time. The GNU date string 
# formats are accepted. See:
# http://www.gnu.org/software/tar/manual/html_node/Date-input-formats.html

#activation_date = "2004-02-29 16:21:42"
#expiration_date = "2025-02-29 16:24:41"

# X.509 v3 extensions

# A dnsname in case of a WWW server.
#dns_name = "www.none.org"
#dns_name = "www.morethanone.org"

# A subject alternative name URI
#uri = "http://www.example.com"

# An IP address in case of a server.
#ip_address = "192.168.1.1"

# An email in case of a person
email = "none@none.org"

# Challenge password used in certificate requests
challenge_password = 123456

# Password when encrypting a private key
#password = secret

# An URL that has CRLs (certificate revocation lists)
# available. Needed in CA certificates.
#crl_dist_points = "http://www.getcrl.crl/getcrl/"

# Whether this is a CA certificate or not
#ca

# for microsoft smart card logon
# key_purpose_oid = 1.3.6.1.4.1.311.20.2.2

### Other predefined key purpose OIDs

# Whether this certificate will be used for a TLS client
#tls_www_client

# Whether this certificate will be used for a TLS server
#tls_www_server

# Whether this certificate will be used to sign data (needed
# in TLS DHE ciphersuites).
signing_key

# Whether this certificate will be used to encrypt data (needed
# in TLS RSA ciphersuites). Note that it is preferred to use different
# keys for encryption and signing.
encryption_key

# Whether this key will be used to sign other certificates.
#cert_signing_key

# Whether this key will be used to sign CRLs.
#crl_signing_key

# Whether this key will be used to sign code.
#code_signing_key

# Whether this key will be used to sign OCSP data.
#ocsp_signing_key

# Whether this key will be used for time stamping.
#time_stamping_key

# Whether this key will be used for IPsec IKE operations.
#ipsec_ike_key

### end of key purpose OIDs

# When generating a certificate from a certificate
# request, then honor the extensions stored in the request
# and store them in the real certificate.
#honor_crq_extensions

# Path length contraint. Sets the maximum number of
# certificates that can be used to certify this certificate.
# (i.e. the certificate chain length)
#path_len = -1
#path_len = 2

# OCSP URI
# ocsp_uri = http://my.ocsp.server/ocsp

# CA issuers URI
# ca_issuers_uri = http://my.ca.issuer

# Certificate policies
# policy1 = 1.3.6.1.4.1.5484.1.10.99.1.0
# policy1_txt = "This is a long policy to summarize"
# policy1_url = http://www.example.com/a-policy-to-read

# policy2 = 1.3.6.1.4.1.5484.1.10.99.1.1
# policy2_txt = "This is a short policy"
# policy2_url = http://www.example.com/another-policy-to-read


# Options for proxy certificates
# proxy_policy_language = 1.3.6.1.5.5.7.21.1


# Options for generating a CRL

# next CRL update will be in 43 days (wow)
#crl_next_update = 43

# this is the 5th CRL by this CA
#crl_number = 5


Next: , Previous: , Up: More on certificate authentication   [Contents][Index]

4.2.6 Invoking ocsptool

Ocsptool is a program that can parse and print information about OCSP requests/responses, generate requests and verify responses.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the ocsptool program. This software is released under the GNU General Public License, version 3 or later.

ocsptool help/usage (--help)

This is the automatically generated usage text for ocsptool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

ocsptool - GnuTLS OCSP tool
Usage:  ocsptool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -V, --verbose              More verbose output
                                - may appear multiple times
       --infile=file          Input file
                                - file must pre-exist
       --outfile=str          Output file
       --ask[=arg]            Ask an OCSP/HTTP server on a certificate validity
                                - requires these options:
                                load-cert
                                load-issuer
   -e, --verify-response      Verify response
   -i, --request-info         Print information on a OCSP request
   -j, --response-info        Print information on a OCSP response
   -q, --generate-request     Generate an OCSP request
       --nonce                Use (or not) a nonce to OCSP request
                                - disabled as '--no-nonce'
       --load-issuer=file     Read issuer certificate from file
                                - file must pre-exist
       --load-cert=file       Read certificate to check from file
                                - file must pre-exist
       --load-trust=file      Read OCSP trust anchors from file
                                - prohibits the option 'load-signer'
                                - file must pre-exist
       --load-signer=file     Read OCSP response signer from file
                                - prohibits the option 'load-trust'
                                - file must pre-exist
       --inder                Use DER format for input certificates and private keys
                                - disabled as '--no-inder'
   -Q, --load-request=file    Read DER encoded OCSP request from file
                                - file must pre-exist
   -S, --load-response=file   Read DER encoded OCSP response from file
                                - file must pre-exist
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Ocsptool is a program that can parse and print information about OCSP
requests/responses, generate requests and verify responses.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

ask option

This is the “ask an ocsp/http server on a certificate validity” option. This option takes an optional string argument server name|url.

This option has some usage constraints. It:

Connects to the specified HTTP OCSP server and queries on the validity of the loaded certificate.

ocsptool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

ocsptool See Also

certtool (1)

ocsptool Examples

Print information about an OCSP request

To parse an OCSP request and print information about the content, the -i or --request-info parameter may be used as follows. The -Q parameter specify the name of the file containing the OCSP request, and it should contain the OCSP request in binary DER format.

$ ocsptool -i -Q ocsp-request.der

The input file may also be sent to standard input like this:

$ cat ocsp-request.der | ocsptool --request-info

Print information about an OCSP response

Similar to parsing OCSP requests, OCSP responses can be parsed using the -j or --response-info as follows.

$ ocsptool -j -Q ocsp-response.der
$ cat ocsp-response.der | ocsptool --response-info

Generate an OCSP request

The -q or --generate-request parameters are used to generate an OCSP request. By default the OCSP request is written to standard output in binary DER format, but can be stored in a file using --outfile. To generate an OCSP request the issuer of the certificate to check needs to be specified with --load-issuer and the certificate to check with --load-cert. By default PEM format is used for these files, although --inder can be used to specify that the input files are in DER format.

$ ocsptool -q --load-issuer issuer.pem --load-cert client.pem \
           --outfile ocsp-request.der

When generating OCSP requests, the tool will add an OCSP extension containing a nonce. This behaviour can be disabled by specifying --no-nonce.

Verify signature in OCSP response

To verify the signature in an OCSP response the -e or --verify-response parameter is used. The tool will read an OCSP response in DER format from standard input, or from the file specified by --load-response. The OCSP response is verified against a set of trust anchors, which are specified using --load-trust. The trust anchors are concatenated certificates in PEM format. The certificate that signed the OCSP response needs to be in the set of trust anchors, or the issuer of the signer certificate needs to be in the set of trust anchors and the OCSP Extended Key Usage bit has to be asserted in the signer certificate.

$ ocsptool -e --load-trust issuer.pem \
           --load-response ocsp-response.der

The tool will print status of verification.

Verify signature in OCSP response against given certificate

It is possible to override the normal trust logic if you know that a certain certificate is supposed to have signed the OCSP response, and you want to use it to check the signature. This is achieved using --load-signer instead of --load-trust. This will load one certificate and it will be used to verify the signature in the OCSP response. It will not check the Extended Key Usage bit.

$ ocsptool -e --load-signer ocsp-signer.pem \
           --load-response ocsp-response.der

This approach is normally only relevant in two situations. The first is when the OCSP response does not contain a copy of the signer certificate, so the --load-trust code would fail. The second is if you want to avoid the indirect mode where the OCSP response signer certificate is signed by a trust anchor.

Real-world example

Here is an example of how to generate an OCSP request for a certificate and to verify the response. For illustration we’ll use the blog.josefsson.org host, which (as of writing) uses a certificate from CACert. First we’ll use gnutls-cli to get a copy of the server certificate chain. The server is not required to send this information, but this particular one is configured to do so.

$ echo | gnutls-cli -p 443 blog.josefsson.org --print-cert > chain.pem

Use a text editor on chain.pem to create three files for each separate certificates, called cert.pem for the first certificate for the domain itself, secondly issuer.pem for the intermediate certificate and root.pem for the final root certificate.

The domain certificate normally contains a pointer to where the OCSP responder is located, in the Authority Information Access Information extension. For example, from certtool -i < cert.pem there is this information:

Authority Information Access Information (not critical):
Access Method: 1.3.6.1.5.5.7.48.1 (id-ad-ocsp)
Access Location URI: http://ocsp.CAcert.org/

This means the CA support OCSP queries over HTTP. We are now ready to create a OCSP request for the certificate.

$ ocsptool --ask ocsp.CAcert.org --load-issuer issuer.pem \
           --load-cert cert.pem --outfile ocsp-response.der

The request is sent via HTTP to the OCSP server address specified. If the address is ommited ocsptool will use the address stored in the certificate.


Previous: , Up: More on certificate authentication   [Contents][Index]

4.2.7 Invoking danetool

Tool to generate and check DNS resource records for the DANE protocol.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the danetool program. This software is released under the GNU General Public License, version 3 or later.

danetool help/usage (--help)

This is the automatically generated usage text for danetool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

danetool - GnuTLS DANE tool
Usage:  danetool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -V, --verbose              More verbose output
                                - may appear multiple times
       --infile=file          Input file
                                - file must pre-exist
       --outfile=str          Output file
       --load-pubkey=str      Loads a public key file
       --load-certificate=str Loads a certificate file
       --dlv=str              Sets a DLV file
       --hash=str             Hash algorithm to use for signing
       --check=str            Check a host's DANE TLSA entry
       --check-ee             Check only the end-entity's certificate
       --check-ca             Check only the CA's certificate
       --insecure             Do not verify any DNSSEC signature
       --local-dns            Use the local DNS server for DNSSEC resolving
                                - disabled as '--no-local-dns'
       --inder                Use DER format for input certificates and private keys
                                - disabled as '--no-inder'
       --inraw                an alias for the 'inder' option
       --tlsa-rr              Print the DANE RR data on a certificate or public key
                                - requires the option 'host'
       --host=str             Specify the hostname to be used in the DANE RR
       --proto=str            The protocol set for DANE data (tcp, udp etc.)
       --port=num             Specify the port number for the DANE data
       --ca                   Whether the provided certificate or public key is a Certificate
Authority
       --x509                 Use the hash of the X.509 certificate, rather than the public key
       --local                an alias for the 'domain' option
                                - enabled by default
       --domain               The provided certificate or public key is issued by the local domain
                                - disabled as '--no-domain'
                                - enabled by default
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Tool to generate and check DNS resource records for the DANE protocol.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

load-pubkey option

This is the “loads a public key file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

load-certificate option

This is the “loads a certificate file” option. This option takes a string argument. This can be either a file or a PKCS #11 URL

dlv option

This is the “sets a dlv file” option. This option takes a string argument. This sets a DLV file to be used for DNSSEC verification.

hash option

This is the “hash algorithm to use for signing” option. This option takes a string argument. Available hash functions are SHA1, RMD160, SHA256, SHA384, SHA512.

check option

This is the “check a host’s dane tlsa entry” option. This option takes a string argument. Obtains the DANE TLSA entry from the given hostname and prints information. Note that the actual certificate of the host has to be provided using –load-certificate.

check-ee option

This is the “check only the end-entity’s certificate” option. Checks the end-entity’s certificate only. Trust anchors or CAs are not considered.

check-ca option

This is the “check only the ca’s certificate” option. Checks the trust anchor’s and CA’s certificate only. End-entities are not considered.

insecure option

This is the “do not verify any dnssec signature” option. Ignores any DNSSEC signature verification results.

local-dns option

This is the “use the local dns server for dnssec resolving” option.

This option has some usage constraints. It:

This option will use the local DNS server for DNSSEC. This is disabled by default due to many servers not allowing DNSSEC.

inder option

This is the “use der format for input certificates and private keys” option.

This option has some usage constraints. It:

The input files will be assumed to be in DER or RAW format. Unlike options that in PEM input would allow multiple input data (e.g. multiple certificates), when reading in DER format a single data structure is read.

inraw option

This is an alias for the inder option, see the inder option documentation.

tlsa-rr option

This is the “print the dane rr data on a certificate or public key” option.

This option has some usage constraints. It:

This command prints the DANE RR data needed to enable DANE on a DNS server.

host option

This is the “specify the hostname to be used in the dane rr” option. This option takes a string argument Hostname. This command sets the hostname for the DANE RR.

proto option

This is the “the protocol set for dane data (tcp, udp etc.)” option. This option takes a string argument Protocol. This command specifies the protocol for the service set in the DANE data.

ca option

This is the “whether the provided certificate or public key is a certificate authority” option. Marks the DANE RR as a CA certificate if specified.

x509 option

This is the “use the hash of the x.509 certificate, rather than the public key” option. This option forces the generated record to contain the hash of the full X.509 certificate. By default only the hash of the public key is used.

local option

This is an alias for the domain option, see the domain option documentation.

domain option

This is the “the provided certificate or public key is issued by the local domain” option.

This option has some usage constraints. It:

DANE distinguishes certificates and public keys offered via the DNSSEC to trusted and local entities. This flag indicates that this is a domain-issued certificate, meaning that there could be no CA involved.

danetool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

danetool See Also

certtool (1)

danetool Examples

DANE TLSA RR generation

To create a DANE TLSA resource record for a certificate (or public key) that was issued localy and may or may not be signed by a CA use the following command.

$ danetool --tlsa-rr --host www.example.com --load-certificate cert.pem

To create a DANE TLSA resource record for a CA signed certificate, which will be marked as such use the following command.

$ danetool --tlsa-rr --host www.example.com --load-certificate cert.pem \
  --no-domain

The former is useful to add in your DNS entry even if your certificate is signed by a CA. That way even users who do not trust your CA will be able to verify your certificate using DANE.

In order to create a record for the CA signer of your certificate use the following.

$ danetool --tlsa-rr --host www.example.com --load-certificate cert.pem \
  --ca --no-domain

To read a server’s DANE TLSA entry, use:

$ danetool --check www.example.com --proto tcp --port 443

To verify a server’s DANE TLSA entry, use:

$ danetool --check www.example.com --proto tcp --port 443 --load-certificate chain.pem

Next: , Previous: , Up: Authentication methods   [Contents][Index]

4.3 Shared-key and anonymous authentication

In addition to certificate authentication, the TLS protocol may be used with password, shared-key and anonymous authentication methods. The rest of this chapter discusses details of these methods.


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4.3.1 SRP authentication


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4.3.1.1 Authentication using SRP

GnuTLS supports authentication via the Secure Remote Password or SRP protocol (see [RFC2945,TOMSRP] for a description). The SRP key exchange is an extension to the TLS protocol, and it provides an authenticated with a password key exchange. The peers can be identified using a single password, or there can be combinations where the client is authenticated using SRP and the server using a certificate.

The advantage of SRP authentication, over other proposed secure password authentication schemes, is that SRP is not susceptible to off-line dictionary attacks. Moreover, SRP does not require the server to hold the user’s password. This kind of protection is similar to the one used traditionally in the UNIX /etc/passwd file, where the contents of this file did not cause harm to the system security if they were revealed. The SRP needs instead of the plain password something called a verifier, which is calculated using the user’s password, and if stolen cannot be used to impersonate the user.

Typical conventions in SRP are a password file, called tpasswd that holds the SRP verifiers (encoded passwords) and another file, tpasswd.conf, which holds the allowed SRP parameters. The included in GnuTLS helper follow those conventions. The srptool program, discussed in the next section is a tool to manipulate the SRP parameters.

The implementation in GnuTLS is based on [TLSSRP]. The supported key exchange methods are shown below.

SRP:

Authentication using the SRP protocol.

SRP_DSS:

Client authentication using the SRP protocol. Server is authenticated using a certificate with DSA parameters.

SRP_RSA:

Client authentication using the SRP protocol. Server is authenticated using a certificate with RSA parameters.

Function: int gnutls_srp_verifier (const char * username, const char * password, const gnutls_datum_t * salt, const gnutls_datum_t * generator, const gnutls_datum_t * prime, gnutls_datum_t * res)

username: is the user’s name

password: is the user’s password

salt: should be some randomly generated bytes

generator: is the generator of the group

prime: is the group’s prime

res: where the verifier will be stored.

This function will create an SRP verifier, as specified in RFC2945. The prime and generator should be one of the static parameters defined in gnutls/gnutls.h or may be generated.

The verifier will be allocated with gnutls_malloc () and will be stored in res using binary format.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, or an error code.

int gnutls_srp_base64_encode_alloc (const gnutls_datum_t * data, gnutls_datum_t * result)
int gnutls_srp_base64_decode_alloc (const gnutls_datum_t * b64_data, gnutls_datum_t * result)

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4.3.1.2 Invoking srptool

Simple program that emulates the programs in the Stanford SRP (Secure Remote Password) libraries using GnuTLS. It is intended for use in places where you don’t expect SRP authentication to be the used for system users.

In brief, to use SRP you need to create two files. These are the password file that holds the users and the verifiers associated with them and the configuration file to hold the group parameters (called tpasswd.conf).

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the srptool program. This software is released under the GNU General Public License, version 3 or later.

srptool help/usage (--help)

This is the automatically generated usage text for srptool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

srptool - GnuTLS SRP tool
Usage:  srptool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -i, --index=num            specify the index of the group parameters in tpasswd.conf to use
   -u, --username=str         specify a username
   -p, --passwd=str           specify a password file
   -s, --salt=num             specify salt size
       --verify               just verify the password.
   -v, --passwd-conf=str      specify a password conf file.
       --create-conf=str      Generate a password configuration file.
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Simple program that emulates the programs in the Stanford SRP (Secure
Remote Password) libraries using GnuTLS.  It is intended for use in places
where you don't expect SRP authentication to be the used for system users.

In brief, to use SRP you need to create two files.  These are the password
file that holds the users and the verifiers associated with them and the
configuration file to hold the group parameters (called tpasswd.conf).

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

verify option

This is the “just verify the password.” option. Verifies the password provided against the password file.

passwd-conf option (-v)

This is the “specify a password conf file.” option. This option takes a string argument. Specify a filename or a PKCS #11 URL to read the CAs from.

create-conf option

This is the “generate a password configuration file.” option. This option takes a string argument. This generates a password configuration file (tpasswd.conf) containing the required for TLS parameters.

srptool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

srptool See Also

gnutls-cli-debug (1), gnutls-serv (1), srptool (1), psktool (1), certtool (1)

srptool Examples

To create tpasswd.conf which holds the g and n values for SRP protocol (generator and a large prime), run:

$ srptool --create-conf /etc/tpasswd.conf

This command will create /etc/tpasswd and will add user ’test’ (you will also be prompted for a password). Verifiers are stored by default in the way libsrp expects.

$ srptool --passwd /etc/tpasswd --passwd-conf /etc/tpasswd.conf -u test

This command will check against a password. If the password matches the one in /etc/tpasswd you will get an ok.

$ srptool --passwd /etc/tpasswd --passwd\-conf /etc/tpasswd.conf --verify -u test

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4.3.2 PSK authentication


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4.3.2.1 Authentication using PSK

Authentication using Pre-shared keys is a method to authenticate using usernames and binary keys. This protocol avoids making use of public key infrastructure and expensive calculations, thus it is suitable for constraint clients.

The implementation in GnuTLS is based on [TLSPSK]. The supported PSK key exchange methods are:

PSK:

Authentication using the PSK protocol.

DHE-PSK:

Authentication using the PSK protocol and Diffie-Hellman key exchange. This method offers perfect forward secrecy.

ECDHE-PSK:

Authentication using the PSK protocol and Elliptic curve Diffie-Hellman key exchange. This method offers perfect forward secrecy.

RSA-PSK:

Authentication using the PSK protocol for the client and an RSA certificate for the server.

Helper functions to generate and maintain PSK keys are also included in GnuTLS.

int gnutls_key_generate (gnutls_datum_t * key, unsigned int key_size)
int gnutls_hex_encode (const gnutls_datum_t * data, char * result, size_t * result_size)
int gnutls_hex_decode (const gnutls_datum_t * hex_data, void * result, size_t * result_size)

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4.3.2.2 Invoking psktool

Program that generates random keys for use with TLS-PSK. The keys are stored in hexadecimal format in a key file.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the psktool program. This software is released under the GNU General Public License, version 3 or later.

psktool help/usage (--help)

This is the automatically generated usage text for psktool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

psktool - GnuTLS PSK tool
Usage:  psktool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -s, --keysize=num          specify the key size in bytes
                                - it must be in the range:
                                  0 to 512
   -u, --username=str         specify a username
   -p, --passwd=str           specify a password file
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Program that generates random keys for use with TLS-PSK.  The keys are
stored in hexadecimal format in a key file.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

psktool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

psktool See Also

gnutls-cli-debug (1), gnutls-serv (1), srptool (1), certtool (1)

psktool Examples

To add a user ’psk_identity’ in passwd.psk for use with GnuTLS run:

$ ./psktool -u psk_identity -p passwd.psk
Generating a random key for user 'psk_identity'
Key stored to passwd.psk
$ cat psks.txt
psk_identity:88f3824b3e5659f52d00e959bacab954b6540344
$

This command will create passwd.psk if it does not exist and will add user ’psk_identity’ (you will also be prompted for a password).


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4.3.3 Anonymous authentication

The anonymous key exchange offers encryption without any indication of the peer’s identity. This kind of authentication is vulnerable to a man in the middle attack, but can be used even if there is no prior communication or shared trusted parties with the peer. It is useful to establish a session over which certificate authentication will occur in order to hide the indentities of the participants from passive eavesdroppers.

Unless in the above case, it is not recommended to use anonymous authentication. In the cases where there is no prior communication with the peers, an alternative with better properties, such as key continuity, is trust on first use (see Verifying a certificate using trust on first use authentication).

The available key exchange algorithms for anonymous authentication are shown below, but note that few public servers support them, and they have to be explicitly enabled.

ANON_DH:

This algorithm exchanges Diffie-Hellman parameters.

ANON_ECDH:

This algorithm exchanges elliptic curve Diffie-Hellman parameters. It is more efficient than ANON_DH on equivalent security levels.


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4.4 Selecting an appropriate authentication method

This section provides some guidance on how to use the available authentication methods in GnuTLS in various scenarios.

4.4.1 Two peers with an out-of-band channel

Let’s consider two peers need to communicate over an untrusted channel (the Internet), but have an out-of-band channel available. The latter channel is considered safe from eavesdropping and message modification and thus can be used for an initial bootstrapping of the protocol. The options available are:

Provided that the out-of-band channel is trusted all of the above provide a similar level of protection. An out-of-band channel may be the initial bootstrapping of a user’s PC in a corporate environment, in-person communication, communication over an alternative network (e.g. the phone network), etc.

4.4.2 Two peers without an out-of-band channel

When an out-of-band channel is not available a peer cannot be reliably authenticated. What can be done, however, is to allow some form of registration of users connecting for the first time and ensure that their keys remain the same after that initial connection. This is termed key continuity or trust on first use (TOFU).

The available option is to use public key authentication (see Certificate authentication). The client and the server store each other’s public keys (or fingerprints of them) and associate them with their identity. On future sessions over the untrusted channel they verify the keys being the same (see Verifying a certificate using trust on first use authentication).

To mitigate the uncertainty of the information exchanged in the first connection other channels over the Internet may be used, e.g., DNSSEC (see Verifying a certificate using DANE).

4.4.3 Two peers and a trusted third party

When a trusted third party is available (or a certificate authority) the most suitable option is to use certificate authentication (see Certificate authentication). The client and the server obtain certificates that associate their identity and public keys using a digital signature by the trusted party and use them to on the subsequent communications with each other. Each party verifies the peer’s certificate using the trusted third party’s signature. The parameters of the third party’s signature are present in its certificate which must be available to all communicating parties.

While the above is the typical authentication method for servers in the Internet by using the commercial CAs, the users that act as clients in the protocol rarely possess such certificates. In that case a hybrid method can be used where the server is authenticated by the client using the commercial CAs and the client is authenticated based on some information the client provided over the initial server-authenticated channel. The available options are:


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5 Hardware security modules and abstract key types

In several cases storing the long term cryptographic keys in a hard disk or even in memory poses a significant risk. Once the system they are stored is compromised the keys must be replaced as the secrecy of future sessions is no longer guarranteed. Moreover, past sessions that were not protected by a perfect forward secrecy offering ciphersuite are also to be assumed compromised.

If such threats need to be addressed, then it may be wise storing the keys in a security module such as a smart card, an HSM or the TPM chip. Those modules ensure the protection of the cryptographic keys by only allowing operations on them and preventing their extraction.


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5.1 Abstract key types

Since there are many forms of a public or private keys supported by GnuTLS such as X.509, OpenPGP, PKCS #11 or TPM it is desirable to allow common operations on them. For these reasons the abstract gnutls_privkey_t and gnutls_pubkey_t were introduced in gnutls/abstract.h header. Those types are initialized using a specific type of key and then can be used to perform operations in an abstract way. For example in order to sign an X.509 certificate with a key that resides in a token the following steps must be used.

#inlude <gnutls/abstract.h>

void sign_cert( gnutls_x509_crt_t to_be_signed)
{
gnutls_x509_crt_t ca_cert;
gnutls_privkey_t abs_key;

  /* initialize the abstract key */
  gnutls_privkey_init(&abs_key);

  /* keys stored in tokens are identified by URLs */
  gnutls_privkey_import_url(abs_key, key_url);

  gnutls_x509_crt_init(&ca_cert);
  gnutls_x509_crt_import_pkcs11_url(&ca_cert, cert_url);

  /* sign the certificate to be signed */
  gnutls_x509_crt_privkey_sign(to_be_signed, ca_cert, abs_key, 
                               GNUTLS_DIG_SHA256, 0);
}

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5.1.1 Public keys

An abstract gnutls_pubkey_t can be initialized using the functions below. It can be imported through an existing structure like gnutls_x509_crt_t, or through an ASN.1 encoding of the X.509 SubjectPublicKeyInfo sequence.

int gnutls_pubkey_import_x509 (gnutls_pubkey_t key, gnutls_x509_crt_t crt, unsigned int flags)
int gnutls_pubkey_import_openpgp (gnutls_pubkey_t key, gnutls_openpgp_crt_t crt, unsigned int flags)
int gnutls_pubkey_import_pkcs11 (gnutls_pubkey_t key, gnutls_pkcs11_obj_t obj, unsigned int flags)
int gnutls_pubkey_import_url (gnutls_pubkey_t key, const char * url, unsigned int flags)
int gnutls_pubkey_import_privkey (gnutls_pubkey_t key, gnutls_privkey_t pkey, unsigned int usage, unsigned int flags)
int gnutls_pubkey_import (gnutls_pubkey_t key, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format)
int gnutls_pubkey_export (gnutls_pubkey_t key, gnutls_x509_crt_fmt_t format, void * output_data, size_t * output_data_size)
Function: int gnutls_pubkey_export2 (gnutls_pubkey_t key, gnutls_x509_crt_fmt_t format, gnutls_datum_t * out)

key: Holds the certificate

format: the format of output params. One of PEM or DER.

out: will contain a certificate PEM or DER encoded

This function will export the public key to DER or PEM format. The contents of the exported data is the SubjectPublicKeyInfo X.509 structure.

The output buffer will be allocated using gnutls_malloc() .

If the structure is PEM encoded, it will have a header of "BEGIN CERTIFICATE".

Returns: In case of failure a negative error code will be returned, and 0 on success.

Since: 3.1.3

Other helper functions that allow directly importing from raw X.509 or OpenPGP structures are shown below.

int gnutls_pubkey_import_x509_raw (gnutls_pubkey_t pkey, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, unsigned int flags)
int gnutls_pubkey_import_openpgp_raw (gnutls_pubkey_t pkey, const gnutls_datum_t * data, gnutls_openpgp_crt_fmt_t format, const gnutls_openpgp_keyid_t keyid, unsigned int flags)

An important function is gnutls_pubkey_import_url which will import public keys from URLs that identify objects stored in tokens (see Smart cards and HSMs and Trusted Platform Module). A function to check for a supported by GnuTLS URL is gnutls_url_is_supported.

Function: int gnutls_url_is_supported (const char * url)

url: A PKCS 11 url

Check whether url is supported. Depending on the system libraries GnuTLS may support pkcs11 or tpmkey URLs.

Returns: return non-zero if the given URL is supported, and zero if it is not known.

Since: 3.1.0

Additional functions are available that will return information over a public key, such as a unique key ID, as well as a function that given a public key fingerprint would provide a memorable sketch.

Note that gnutls_pubkey_get_key_id calculates a SHA1 digest of the public key as a DER-formatted, subjectPublicKeyInfo object. Other implementations use different approaches, e.g., some use the “common method” described in section 4.2.1.2 of [RFC5280] which calculates a digest on a part of the subjectPublicKeyInfo object.

int gnutls_pubkey_get_pk_algorithm (gnutls_pubkey_t key, unsigned int * bits)
int gnutls_pubkey_get_preferred_hash_algorithm (gnutls_pubkey_t key, gnutls_digest_algorithm_t * hash, unsigned int * mand)
int gnutls_pubkey_get_key_id (gnutls_pubkey_t key, unsigned int flags, unsigned char * output_data, size_t * output_data_size)
int gnutls_random_art (gnutls_random_art_t type, const char * key_type, unsigned int key_size, void * fpr, size_t fpr_size, gnutls_datum_t * art)

To export the key-specific parameters, or obtain a unique key ID the following functions are provided.

int gnutls_pubkey_get_pk_rsa_raw (gnutls_pubkey_t key, gnutls_datum_t * m, gnutls_datum_t * e)
int gnutls_pubkey_get_pk_dsa_raw (gnutls_pubkey_t key, gnutls_datum_t * p, gnutls_datum_t * q, gnutls_datum_t * g, gnutls_datum_t * y)
int gnutls_pubkey_get_pk_ecc_raw (gnutls_pubkey_t key, gnutls_ecc_curve_t * curve, gnutls_datum_t * x, gnutls_datum_t * y)
int gnutls_pubkey_get_pk_ecc_x962 (gnutls_pubkey_t key, gnutls_datum_t * parameters, gnutls_datum_t * ecpoint)

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5.1.2 Private keys

An abstract gnutls_privkey_t can be initialized using the functions below. It can be imported through an existing structure like gnutls_x509_privkey_t, but unlike public keys it cannot be exported. That is to allow abstraction over keys stored in hardware that makes available only operations.

int gnutls_privkey_import_x509 (gnutls_privkey_t pkey, gnutls_x509_privkey_t key, unsigned int flags)
int gnutls_privkey_import_openpgp (gnutls_privkey_t pkey, gnutls_openpgp_privkey_t key, unsigned int flags)
int gnutls_privkey_import_pkcs11 (gnutls_privkey_t pkey, gnutls_pkcs11_privkey_t key, unsigned int flags)

Other helper functions that allow directly importing from raw X.509 or OpenPGP structures are shown below. Again, as with public keys, private keys can be imported from a hardware module using URLs.

int gnutls_privkey_import_x509_raw (gnutls_privkey_t pkey, const gnutls_datum_t * data, gnutls_x509_crt_fmt_t format, const char * password, unsigned int flags)
int gnutls_privkey_import_openpgp_raw (gnutls_privkey_t pkey, const gnutls_datum_t * data, gnutls_openpgp_crt_fmt_t format, const gnutls_openpgp_keyid_t keyid, const char * password)
Function: int gnutls_privkey_import_url (gnutls_privkey_t key, const char * url, unsigned int flags)

key: A key of type gnutls_privkey_t

url: A PKCS 11 url

flags: should be zero

This function will import a PKCS11 or TPM URL as a private key. The supported URL types can be checked using gnutls_url_is_supported() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0

int gnutls_privkey_get_pk_algorithm (gnutls_privkey_t key, unsigned int * bits)
gnutls_privkey_type_t gnutls_privkey_get_type (gnutls_privkey_t key)
int gnutls_privkey_status (gnutls_privkey_t key)

In order to support cryptographic operations using an external API, the following function is provided. This allows for a simple extensibility API without resorting to PKCS #11.

Function: int gnutls_privkey_import_ext2 (gnutls_privkey_t pkey, gnutls_pk_algorithm_t pk, void * userdata, gnutls_privkey_sign_func sign_func, gnutls_privkey_decrypt_func decrypt_func, gnutls_privkey_deinit_func deinit_func, unsigned int flags)

pkey: The private key

pk: The public key algorithm

userdata: private data to be provided to the callbacks

sign_func: callback for signature operations

decrypt_func: callback for decryption operations

deinit_func: a deinitialization function

flags: Flags for the import

This function will associate the given callbacks with the gnutls_privkey_t structure. At least one of the two callbacks must be non-null. If a deinitialization function is provided then flags is assumed to contain GNUTLS_PRIVKEY_IMPORT_AUTO_RELEASE .

Note that the signing function is supposed to "raw" sign data, i.e., without any hashing or preprocessing. In case of RSA the DigestInfo will be provided, and the signing function is expected to do the PKCS 1 1.5 padding and the exponentiation.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1


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5.1.3 Operations

The abstract key types can be used to access signing and signature verification operations with the underlying keys.

Function: int gnutls_pubkey_verify_data2 (gnutls_pubkey_t pubkey, gnutls_sign_algorithm_t algo, unsigned int flags, const gnutls_datum_t * data, const gnutls_datum_t * signature)

pubkey: Holds the public key

algo: The signature algorithm used

flags: Zero or one of gnutls_pubkey_flags_t

data: holds the signed data

signature: contains the signature

This function will verify the given signed data, using the parameters from the certificate.

Returns: In case of a verification failure GNUTLS_E_PK_SIG_VERIFY_FAILED is returned, and zero or positive code on success.

Since: 3.0

Function: int gnutls_pubkey_verify_hash2 (gnutls_pubkey_t key, gnutls_sign_algorithm_t algo, unsigned int flags, const gnutls_datum_t * hash, const gnutls_datum_t * signature)

key: Holds the public key

algo: The signature algorithm used

flags: Zero or one of gnutls_pubkey_flags_t

hash: holds the hash digest to be verified

signature: contains the signature

This function will verify the given signed digest, using the parameters from the public key.

Returns: In case of a verification failure GNUTLS_E_PK_SIG_VERIFY_FAILED is returned, and zero or positive code on success.

Since: 3.0

Function: int gnutls_pubkey_encrypt_data (gnutls_pubkey_t key, unsigned int flags, const gnutls_datum_t * plaintext, gnutls_datum_t * ciphertext)

key: Holds the public key

flags: should be 0 for now

plaintext: The data to be encrypted

ciphertext: contains the encrypted data

This function will encrypt the given data, using the public key.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

Function: int gnutls_privkey_sign_data (gnutls_privkey_t signer, gnutls_digest_algorithm_t hash, unsigned int flags, const gnutls_datum_t * data, gnutls_datum_t * signature)

signer: Holds the key

hash: should be a digest algorithm

flags: Zero or one of gnutls_privkey_flags_t

data: holds the data to be signed

signature: will contain the signature allocate with gnutls_malloc()

This function will sign the given data using a signature algorithm supported by the private key. Signature algorithms are always used together with a hash functions. Different hash functions may be used for the RSA algorithm, but only the SHA family for the DSA keys.

You may use gnutls_pubkey_get_preferred_hash_algorithm() to determine the hash algorithm.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Function: int gnutls_privkey_sign_hash (gnutls_privkey_t signer, gnutls_digest_algorithm_t hash_algo, unsigned int flags, const gnutls_datum_t * hash_data, gnutls_datum_t * signature)

signer: Holds the signer’s key

hash_algo: The hash algorithm used

flags: Zero or one of gnutls_privkey_flags_t

hash_data: holds the data to be signed

signature: will contain newly allocated signature

This function will sign the given hashed data using a signature algorithm supported by the private key. Signature algorithms are always used together with a hash functions. Different hash functions may be used for the RSA algorithm, but only SHA-XXX for the DSA keys.

You may use gnutls_pubkey_get_preferred_hash_algorithm() to determine the hash algorithm.

Note that if GNUTLS_PRIVKEY_SIGN_FLAG_TLS1_RSA flag is specified this function will ignore hash_algo and perform a raw PKCS1 signature.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Function: int gnutls_privkey_decrypt_data (gnutls_privkey_t key, unsigned int flags, const gnutls_datum_t * ciphertext, gnutls_datum_t * plaintext)

key: Holds the key

flags: zero for now

ciphertext: holds the data to be decrypted

plaintext: will contain the decrypted data, allocated with gnutls_malloc()

This function will decrypt the given data using the algorithm supported by the private key.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Signing existing structures, such as certificates, CRLs, or certificate requests, as well as associating public keys with structures is also possible using the key abstractions.

Function: int gnutls_x509_crq_set_pubkey (gnutls_x509_crq_t crq, gnutls_pubkey_t key)

crq: should contain a gnutls_x509_crq_t structure

key: holds a public key

This function will set the public parameters from the given public key to the request.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Function: int gnutls_x509_crt_set_pubkey (gnutls_x509_crt_t crt, gnutls_pubkey_t key)

crt: should contain a gnutls_x509_crt_t structure

key: holds a public key

This function will set the public parameters from the given public key to the request.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

int gnutls_x509_crt_privkey_sign (gnutls_x509_crt_t crt, gnutls_x509_crt_t issuer, gnutls_privkey_t issuer_key, gnutls_digest_algorithm_t dig, unsigned int flags)
int gnutls_x509_crl_privkey_sign (gnutls_x509_crl_t crl, gnutls_x509_crt_t issuer, gnutls_privkey_t issuer_key, gnutls_digest_algorithm_t dig, unsigned int flags)
int gnutls_x509_crq_privkey_sign (gnutls_x509_crq_t crq, gnutls_privkey_t key, gnutls_digest_algorithm_t dig, unsigned int flags)

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5.2 Smart cards and HSMs

In this section we present the smart-card and hardware security module (HSM) support in GnuTLS using PKCS #11 [PKCS11]. Hardware security modules and smart cards provide a way to store private keys and perform operations on them without exposing them. This decouples cryptographic keys from the applications that use them and provide an additional security layer against cryptographic key extraction. Since this can also be achieved in software components such as in Gnome keyring, we will use the term security module to describe any cryptographic key separation subsystem.

PKCS #11 is plugin API allowing applications to access cryptographic operations on a security module, as well as to objects residing on it. PKCS #11 modules exist for hardware tokens such as smart cards7, cryptographic tokens, as well as for software modules like Gnome Keyring. The objects residing on a security module may be certificates, public keys, private keys or secret keys. Of those certificates and public/private key pairs can be used with GnuTLS. PKCS #11’s main advantage is that it allows operations on private key objects such as decryption and signing without exposing the key. In GnuTLS the PKCS #11 functionality is available in gnutls/pkcs11.h.

Moreover PKCS #11 can be (ab)used to allow all applications in the same operating system to access shared cryptographic keys and certificates in a uniform way, as in Figure 5.1. That way applications could load their trusted certificate list, as well as user certificates from a common PKCS #11 module. Such a provider exists in the Gnome system, being the Gnome Keyring.

pkcs11-vision

Figure 5.1: PKCS #11 module usage.


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5.2.1 Initialization

To allow all GnuTLS applications to transparently access smard cards and tokens, PKCS #11 is automatically initialized during the global initialization (see gnutls_global_init). The initialization function, to select which modules to load reads certain module configuration files. Those are stored in /etc/pkcs11/modules/ and are the configuration files of p11-kit8. For example a file that will load the OpenSC module, could be named /etc/pkcs11/modules/opensc.module and contain the following:

module: /usr/lib/opensc-pkcs11.so

If you use these configuration files, then there is no need for other initialization in GnuTLS, except for the PIN and token functions (see next section). In several cases, however, it is desirable to limit badly behaving modules (e.g., modules that add an unacceptable delay on initialization) to single applications. That can be done using the “enable-in:” option followed by the base name of applications that this module should be used.

In all cases, you can also manually initialize the PKCS #11 subsystem if the default settings are not desirable. To completely disable PKCS #11 support you need to call gnutls_pkcs11_init with the flag GNUTLS_PKCS11_FLAG_MANUAL prior to gnutls_global_init.

Function: int gnutls_pkcs11_init (unsigned int flags, const char * deprecated_config_file)

flags: GNUTLS_PKCS11_FLAG_MANUAL or GNUTLS_PKCS11_FLAG_AUTO

deprecated_config_file: either NULL or the location of a deprecated configuration file

This function will initialize the PKCS 11 subsystem in gnutls. It will read configuration files if GNUTLS_PKCS11_FLAG_AUTO is used or allow you to independently load PKCS 11 modules using gnutls_pkcs11_add_provider() if GNUTLS_PKCS11_FLAG_MANUAL is specified.

Normally you don’t need to call this function since it is being called by gnutls_global_init() using the GNUTLS_PKCS11_FLAG_AUTO . If other option is required then it must be called before it.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Note that PKCS #11 modules must be reinitialized on the child processes after a fork. GnuTLS provides gnutls_pkcs11_reinit to be called for this purpose.

Function: int gnutls_pkcs11_reinit ( void)

This function will reinitialize the PKCS 11 subsystem in gnutls. This is required by PKCS 11 when an application uses fork() . The reinitialization function must be called on the child.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0


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5.2.2 Accessing objects that require a PIN

Objects stored in token such as a private keys are typically protected from access by a PIN or password. This PIN may be required to either read the object (if allowed) or to perform operations with it. To allow obtaining the PIN when accessing a protected object, as well as probe the user to insert the token the following functions allow to set a callback.

void gnutls_pkcs11_set_token_function (gnutls_pkcs11_token_callback_t fn, void * userdata)
void gnutls_pkcs11_set_pin_function (gnutls_pin_callback_t fn, void * userdata)
int gnutls_pkcs11_add_provider (const char * name, const char * params)
gnutls_pin_callback_t gnutls_pkcs11_get_pin_function (void ** userdata)

The callback is of type gnutls_pin_callback_t and will have as input the provided userdata, the PIN attempt number, a URL describing the token, a label describing the object and flags. The PIN must be at most of pin_max size and must be copied to pin variable. The function must return 0 on success or a negative error code otherwise.

typedef int (*gnutls_pin_callback_t) (void *userdata, int attempt,
                                      const char *token_url,
                                      const char *token_label,
                                      unsigned int flags,
                                      char *pin, size_t pin_max);

The flags are of gnutls_pin_flag_t type and are explained below.

GNUTLS_PIN_USER

The PIN for the user.

GNUTLS_PIN_SO

The PIN for the security officer (admin).

GNUTLS_PIN_FINAL_TRY

This is the final try before blocking.

GNUTLS_PIN_COUNT_LOW

Few tries remain before token blocks.

GNUTLS_PIN_CONTEXT_SPECIFIC

The PIN is for a specific action and key like signing.

GNUTLS_PIN_WRONG

Last given PIN was not correct.

Figure 5.2: The gnutls_pin_flag_t enumeration.

Note that due to limitations of PKCS #11 there are issues when multiple libraries are sharing a module. To avoid this problem GnuTLS uses p11-kit that provides a middleware to control access to resources over the multiple users.

To avoid conflicts with multiple registered callbacks for PIN functions, gnutls_pkcs11_get_pin_function may be used to check for any previously set functions. In addition context specific PIN functions are allowed, e.g., by using functions below.

void gnutls_certificate_set_pin_function (gnutls_certificate_credentials_t cred, gnutls_pin_callback_t fn, void * userdata)
void gnutls_pubkey_set_pin_function (gnutls_pubkey_t key, gnutls_pin_callback_t fn, void * userdata)
void gnutls_privkey_set_pin_function (gnutls_privkey_t key, gnutls_pin_callback_t fn, void * userdata)
void gnutls_pkcs11_obj_set_pin_function (gnutls_pkcs11_obj_t obj, gnutls_pin_callback_t fn, void * userdata)
void gnutls_x509_crt_set_pin_function (gnutls_x509_crt_t crt, gnutls_pin_callback_t fn, void * userdata)

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5.2.3 Reading objects

All PKCS #11 objects are referenced by GnuTLS functions by URLs as described in [PKCS11URI]. This allows for a consistent naming of objects across systems and applications in the same system. For example a public key on a smart card may be referenced as:

pkcs11:token=Nikos;serial=307521161601031;model=PKCS%2315; \
manufacturer=EnterSafe;object=test1;objecttype=public;\
id=32f153f3e37990b08624141077ca5dec2d15faed

while the smart card itself can be referenced as:

pkcs11:token=Nikos;serial=307521161601031;model=PKCS%2315;manufacturer=EnterSafe

Objects stored in a PKCS #11 token can be extracted if they are not marked as sensitive. Usually only private keys are marked as sensitive and cannot be extracted, while certificates and other data can be retrieved. The functions that can be used to access objects are shown below.

int gnutls_pkcs11_obj_import_url (gnutls_pkcs11_obj_t obj, const char * url, unsigned int flags)
int gnutls_pkcs11_obj_export_url (gnutls_pkcs11_obj_t obj, gnutls_pkcs11_url_type_t detailed, char ** url)
Function: int gnutls_pkcs11_obj_get_info (gnutls_pkcs11_obj_t crt, gnutls_pkcs11_obj_info_t itype, void * output, size_t * output_size)

crt: should contain a gnutls_pkcs11_obj_t structure

itype: Denotes the type of information requested

output: where output will be stored

output_size: contains the maximum size of the output and will be overwritten with actual

This function will return information about the PKCS11 certificate such as the label, id as well as token information where the key is stored. When output is text it returns null terminated string although output_size contains the size of the actual data only.

Returns: GNUTLS_E_SUCCESS (0) on success or a negative error code on error.

Since: 2.12.0

int gnutls_x509_crt_import_pkcs11 (gnutls_x509_crt_t crt, gnutls_pkcs11_obj_t pkcs11_crt)
int gnutls_x509_crt_import_pkcs11_url (gnutls_x509_crt_t crt, const char * url, unsigned int flags)
int gnutls_x509_crt_list_import_pkcs11 (gnutls_x509_crt_t * certs, unsigned int cert_max, gnutls_pkcs11_obj_t * const objs, unsigned int flags)

Properties of the physical token can also be accessed and altered with GnuTLS. For example data in a token can be erased (initialized), PIN can be altered, etc.

int gnutls_pkcs11_token_init (const char * token_url, const char * so_pin, const char * label)
int gnutls_pkcs11_token_get_url (unsigned int seq, gnutls_pkcs11_url_type_t detailed, char ** url)
int gnutls_pkcs11_token_get_info (const char * url, gnutls_pkcs11_token_info_t ttype, void * output, size_t * output_size)
int gnutls_pkcs11_token_get_flags (const char * url, unsigned int * flags)
int gnutls_pkcs11_token_set_pin (const char * token_url, const char * oldpin, const char * newpin, unsigned int flags)

The following examples demonstrate the usage of the API. The first example will list all available PKCS #11 tokens in a system and the latter will list all certificates in a token that have a corresponding private key.

int i;
char* url;

gnutls_global_init();

for (i=0;;i++) 
  {
    ret = gnutls_pkcs11_token_get_url(i, &url);
    if (ret == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE)
      break;

    if (ret < 0)
      exit(1);
		
    fprintf(stdout, "Token[%d]: URL: %s\n", i, url);
    gnutls_free(url);
  }
gnutls_global_deinit();
/* This example code is placed in the public domain. */

#include <config.h>
#include <gnutls/gnutls.h>
#include <gnutls/pkcs11.h>
#include <stdio.h>
#include <stdlib.h>

#define URL "pkcs11:URL"

int main(int argc, char **argv)
{
        gnutls_pkcs11_obj_t *obj_list;
        gnutls_x509_crt_t xcrt;
        unsigned int obj_list_size = 0;
        gnutls_datum_t cinfo;
        int ret;
        unsigned int i;

        obj_list_size = 0;
        ret = gnutls_pkcs11_obj_list_import_url(NULL, &obj_list_size, URL,
                                                GNUTLS_PKCS11_OBJ_ATTR_CRT_WITH_PRIVKEY,
                                                0);
        if (ret < 0 && ret != GNUTLS_E_SHORT_MEMORY_BUFFER)
                return -1;

/* no error checking from now on */
        obj_list = malloc(sizeof(*obj_list) * obj_list_size);

        gnutls_pkcs11_obj_list_import_url(obj_list, &obj_list_size, URL,
                                          GNUTLS_PKCS11_OBJ_ATTR_CRT_WITH_PRIVKEY,
                                          0);

/* now all certificates are in obj_list */
        for (i = 0; i < obj_list_size; i++) {

                gnutls_x509_crt_init(&xcrt);

                gnutls_x509_crt_import_pkcs11(xcrt, obj_list[i]);

                gnutls_x509_crt_print(xcrt, GNUTLS_CRT_PRINT_FULL, &cinfo);

                fprintf(stdout, "cert[%d]:\n %s\n\n", i, cinfo.data);

                gnutls_free(cinfo.data);
                gnutls_x509_crt_deinit(xcrt);
        }

        return 0;
}

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5.2.4 Writing objects

With GnuTLS you can copy existing private keys and certificates to a token. Note that when copying private keys it is recommended to mark them as sensitive using the GNUTLS_PKCS11_OBJ_FLAG_MARK_SENSITIVE to prevent its extraction. An object can be marked as private using the flag GNUTLS_PKCS11_OBJ_FLAG_MARK_PRIVATE, to require PIN to be entered before accessing the object (for operations or otherwise).

Function: int gnutls_pkcs11_copy_x509_privkey (const char * token_url, gnutls_x509_privkey_t key, const char * label, unsigned int key_usage, unsigned int flags)

token_url: A PKCS 11 URL specifying a token

key: A private key

label: A name to be used for the stored data

key_usage: One of GNUTLS_KEY_*

flags: One of GNUTLS_PKCS11_OBJ_* flags

This function will copy a private key into a PKCS 11 token specified by a URL. It is highly recommended flags to contain GNUTLS_PKCS11_OBJ_FLAG_MARK_SENSITIVE unless there is a strong reason not to.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Function: int gnutls_pkcs11_copy_x509_crt (const char * token_url, gnutls_x509_crt_t crt, const char * label, unsigned int flags)

token_url: A PKCS 11 URL specifying a token

crt: A certificate

label: A name to be used for the stored data

flags: One of GNUTLS_PKCS11_OBJ_FLAG_*

This function will copy a certificate into a PKCS 11 token specified by a URL. The certificate can be marked as trusted or not.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 2.12.0

Function: int gnutls_pkcs11_delete_url (const char * object_url, unsigned int flags)

object_url: The URL of the object to delete.

flags: One of GNUTLS_PKCS11_OBJ_* flags

This function will delete objects matching the given URL. Note that not all tokens support the delete operation.

Returns: On success, the number of objects deleted is returned, otherwise a negative error value.

Since: 2.12.0


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5.2.5 Using a PKCS #11 token with TLS

It is possible to use a PKCS #11 token to a TLS session, as shown in ex-pkcs11-client. In addition the following functions can be used to load PKCS #11 key and certificates by specifying a PKCS #11 URL instead of a filename.

int gnutls_certificate_set_x509_trust_file (gnutls_certificate_credentials_t cred, const char * cafile, gnutls_x509_crt_fmt_t type)
int gnutls_certificate_set_x509_key_file2 (gnutls_certificate_credentials_t res, const char * certfile, const char * keyfile, gnutls_x509_crt_fmt_t type, const char * pass, unsigned int flags)
Function: int gnutls_certificate_set_x509_system_trust (gnutls_certificate_credentials_t cred)

cred: is a gnutls_certificate_credentials_t structure.

This function adds the system’s default trusted CAs in order to verify client or server certificates.

In the case the system is currently unsupported GNUTLS_E_UNIMPLEMENTED_FEATURE is returned.

Returns: the number of certificates processed or a negative error code on error.

Since: 3.0


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5.2.6 Invoking p11tool

Program that allows handling data from PKCS #11 smart cards and security modules.

To use PKCS #11 tokens with gnutls the configuration file /etc/gnutls/pkcs11.conf has to exist and contain a number of lines of the form ’load=/usr/lib/opensc-pkcs11.so’. Alternatively the p11-kit configuration files have to be setup.

To provide the PIN for all the operations below use the environment variable GNUTLS_PIN.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the p11tool program. This software is released under the GNU General Public License, version 3 or later.

p11tool help/usage (--help)

This is the automatically generated usage text for p11tool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

p11tool - GnuTLS PKCS #11 tool
Usage:  p11tool [ -<flag> [<val>] | --<name>[{=| }<val>] ]... [url]

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
       --outfile=str          Output file
       --list-tokens          List all available tokens
       --export               Export the object specified by the URL
       --export-chain         Export the certificate specified by the URL and its chain of trust
       --list-mechanisms      List all available mechanisms in a token
       --list-all             List all available objects in a token
       --list-all-certs       List all available certificates in a token
       --list-certs           List all certificates that have an associated private key
       --list-all-privkeys    List all available private keys in a token
       --list-privkeys        an alias for the 'list-all-privkeys' option
       --list-keys            an alias for the 'list-all-privkeys' option
       --list-all-trusted     List all available certificates marked as trusted
       --initialize           Initializes a PKCS #11 token
       --write                Writes the loaded objects to a PKCS #11 token
       --delete               Deletes the objects matching the PKCS #11 URL
       --generate-random=num  Generate random data
       --generate-rsa         Generate an RSA private-public key pair
       --generate-dsa         Generate an RSA private-public key pair
       --generate-ecc         Generate an RSA private-public key pair
       --label=str            Sets a label for the write operation
       --trusted              Marks the object to be written as trusted
                                - disabled as '--no-trusted'
       --private              Marks the object to be written as private
                                - disabled as '--no-private'
                                - enabled by default
       --login                Force (user) login to token
                                - disabled as '--no-login'
       --so-login             Force security officer login to token
                                - disabled as '--no-so-login'
       --admin-login          an alias for the 'so-login' option
       --detailed-url         Print detailed URLs
                                - disabled as '--no-detailed-url'
       --secret-key=str       Provide a hex encoded secret key
       --load-privkey=file    Private key file to use
                                - file must pre-exist
       --load-pubkey=file     Public key file to use
                                - file must pre-exist
       --load-certificate=file Certificate file to use
                                - file must pre-exist
   -8, --pkcs8                Use PKCS #8 format for private keys
       --bits=num             Specify the number of bits for key generate
       --sec-param=str        Specify the security level
   -!, --inder                Use DER/RAW format for input
                                - disabled as '--no-inder'
   -", --inraw                an alias for the 'inder' option
   -#, --outder               Use DER format for output certificates, private keys, and DH parameters
                                - disabled as '--no-outder'
   -$, --outraw               an alias for the 'outder' option
   -%, --provider=file        Specify the PKCS #11 provider library
                                - file must pre-exist
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.
Operands and options may be intermixed.  They will be reordered.

Program that allows handling data from PKCS #11 smart cards and security
modules.

To use PKCS #11 tokens with gnutls the configuration file
/etc/gnutls/pkcs11.conf has to exist and contain a number of lines of the
form 'load=/usr/lib/opensc-pkcs11.so'.  Alternatively the p11-kit
configuration files have to be setup.

To provide the PIN for all the operations below use the environment
variable GNUTLS_PIN.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

export-chain option

This is the “export the certificate specified by the url and its chain of trust” option. Exports the certificate specified by the URL and generates its chain of trust based on the stored certificates in the module.

list-all-privkeys option

This is the “list all available private keys in a token” option. Lists all the private keys in a token that match the specified URL.

list-privkeys option

This is an alias for the list-all-privkeys option, see the list-all-privkeys option documentation.

list-keys option

This is an alias for the list-all-privkeys option, see the list-all-privkeys option documentation.

write option

This is the “writes the loaded objects to a pkcs #11 token” option. It can be used to write private keys, certificates or secret keys to a token.

generate-random option

This is the “generate random data” option. This option takes a number argument. Asks the token to generate a number of bytes of random bytes.

generate-rsa option

This is the “generate an rsa private-public key pair” option. Generates an RSA private-public key pair on the specified token.

generate-dsa option

This is the “generate an rsa private-public key pair” option. Generates an RSA private-public key pair on the specified token.

generate-ecc option

This is the “generate an rsa private-public key pair” option. Generates an RSA private-public key pair on the specified token.

private option

This is the “marks the object to be written as private” option.

This option has some usage constraints. It:

The written object will require a PIN to be used.

so-login option

This is the “force security officer login to token” option.

This option has some usage constraints. It:

Forces login to the token as security officer (admin).

admin-login option

This is an alias for the so-login option, see the so-login option documentation.

sec-param option

This is the “specify the security level” option. This option takes a string argument Security parameter. This is alternative to the bits option. Available options are [low, legacy, normal, high, ultra].

inder option

This is the “use der/raw format for input” option.

This option has some usage constraints. It:

Use DER/RAW format for input certificates and private keys.

inraw option

This is an alias for the inder option, see the inder option documentation.

outder option

This is the “use der format for output certificates, private keys, and dh parameters” option.

This option has some usage constraints. It:

The output will be in DER or RAW format.

outraw option

This is an alias for the outder option, see the outder option documentation.

provider option

This is the “specify the pkcs #11 provider library” option. This option takes a file argument. This will override the default options in /etc/gnutls/pkcs11.conf

p11tool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

p11tool See Also

certtool (1)

p11tool Examples

To view all tokens in your system use:

$ p11tool --list-tokens

To view all objects in a token use:

$ p11tool --login --list-all "pkcs11:TOKEN-URL"

To store a private key and a certificate in a token run:

$ p11tool --login --write "pkcs11:URL" --load-privkey key.pem \
          --label "Mykey"
$ p11tool --login --write "pkcs11:URL" --load-certificate cert.pem \
          --label "Mykey"

Note that some tokens require the same label to be used for the certificate and its corresponding private key.

To generate an RSA private key inside the token use:

$ p11tool --login --generate-rsa --bits 1024 --label "MyNewKey" \
          --outfile MyNewKey.pub "pkcs11:TOKEN-URL"

The bits parameter in the above example is explicitly set because some tokens only support a limited number of bits. The output file is the corresponding public key. This key can be used to general a certificate request with certtool.

certtool --generate-request --load-privkey "pkcs11:KEY-URL" \
   --load-pubkey MyNewKey.pub --outfile request.pem

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5.3 Trusted Platform Module (TPM)

In this section we present the Trusted Platform Module (TPM) support in GnuTLS.

There was a big hype when the TPM chip was introduced into computers. Briefly it is a co-processor in your PC that allows it to perform calculations independently of the main processor. This has good and bad side-effects. In this section we focus on the good ones; these are the fact that you can use the TPM chip to perform cryptographic operations on keys stored in it, without accessing them. That is very similar to the operation of a PKCS #11 smart card. The chip allows for storage and usage of RSA keys, but has quite some operational differences from PKCS #11 module, and thus require different handling. The basic TPM operations supported and used by GnuTLS, are key generation and signing.

The next sections assume that the TPM chip in the system is already initialized and in a operational state.

In GnuTLS the TPM functionality is available in gnutls/tpm.h.


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5.3.1 Keys in TPM

The RSA keys in the TPM module may either be stored in a flash memory within TPM or stored in a file in disk. In the former case the key can provide operations as with PKCS #11 and is identified by a URL. The URL is described in [TPMURI] and is of the following form.

tpmkey:uuid=42309df8-d101-11e1-a89a-97bb33c23ad1;storage=user

It consists from a unique identifier of the key as well as the part of the flash memory the key is stored at. The two options for the storage field are ‘user’ and ‘system’. The user keys are typically only available to the generating user and the system keys to all users. The stored in TPM keys are called registered keys.

The keys that are stored in the disk are exported from the TPM but in an encrypted form. To access them two passwords are required. The first is the TPM Storage Root Key (SRK), and the other is a key-specific password. Also those keys are identified by a URL of the form:

tpmkey:file=/path/to/file

When objects require a PIN to be accessed the same callbacks as with PKCS #11 objects are expected (see Accessing objects that require a PIN). Note that the PIN function may be called multiple times to unlock the SRK and the specific key in use. The label in the key function will then be set to ‘SRK’ when unlocking the SRK key, or to ‘TPM’ when unlocking any other key.


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5.3.2 Key generation

All keys used by the TPM must be generated by the TPM. This can be done using gnutls_tpm_privkey_generate.

Function: int gnutls_tpm_privkey_generate (gnutls_pk_algorithm_t pk, unsigned int bits, const char * srk_password, const char * key_password, gnutls_tpmkey_fmt_t format, gnutls_x509_crt_fmt_t pub_format, gnutls_datum_t * privkey, gnutls_datum_t * pubkey, unsigned int flags)

pk: the public key algorithm

bits: the security bits

srk_password: a password to protect the exported key (optional)

key_password: the password for the TPM (optional)

format: the format of the private key

pub_format: the format of the public key

privkey: the generated key

pubkey: the corresponding public key (may be null)

flags: should be a list of GNUTLS_TPM_* flags

This function will generate a private key in the TPM chip. The private key will be generated within the chip and will be exported in a wrapped with TPM’s master key form. Furthermore the wrapped key can be protected with the provided password .

Note that bits in TPM is quantized value. If the input value is not one of the allowed values, then it will be quantized to one of 512, 1024, 2048, 4096, 8192 and 16384.

Allowed flags are:

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0

int gnutls_tpm_get_registered (gnutls_tpm_key_list_t * list)
void gnutls_tpm_key_list_deinit (gnutls_tpm_key_list_t list)
int gnutls_tpm_key_list_get_url (gnutls_tpm_key_list_t list, unsigned int idx, char ** url, unsigned int flags)
Function: int gnutls_tpm_privkey_delete (const char * url, const char * srk_password)

url: the URL describing the key

srk_password: a password for the SRK key

This function will unregister the private key from the TPM chip.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0


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5.3.3 Using keys

Importing keys

The TPM keys can be used directly by the abstract key types and do not require any special structures. Moreover functions like gnutls_certificate_set_x509_key_file2 can access TPM URLs.

int gnutls_privkey_import_tpm_raw (gnutls_privkey_t pkey, const gnutls_datum_t * fdata, gnutls_tpmkey_fmt_t format, const char * srk_password, const char * key_password, unsigned int flags)
int gnutls_pubkey_import_tpm_raw (gnutls_pubkey_t pkey, const gnutls_datum_t * fdata, gnutls_tpmkey_fmt_t format, const char * srk_password, unsigned int flags)
Function: int gnutls_privkey_import_tpm_url (gnutls_privkey_t pkey, const char * url, const char * srk_password, const char * key_password, unsigned int flags)

pkey: The private key

url: The URL of the TPM key to be imported

srk_password: The password for the SRK key (optional)

key_password: A password for the key (optional)

flags: One of the GNUTLS_PRIVKEY_* flags

This function will import the given private key to the abstract gnutls_privkey_t structure.

Note that unless GNUTLS_PRIVKEY_DISABLE_CALLBACKS is specified, if incorrect (or NULL) passwords are given the PKCS11 callback functions will be used to obtain the correct passwords. Otherwise if the SRK password is wrong GNUTLS_E_TPM_SRK_PASSWORD_ERROR is returned and if the key password is wrong or not provided then GNUTLS_E_TPM_KEY_PASSWORD_ERROR is returned.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0

Function: int gnutls_pubkey_import_tpm_url (gnutls_pubkey_t pkey, const char * url, const char * srk_password, unsigned int flags)

pkey: The public key

url: The URL of the TPM key to be imported

srk_password: The password for the SRK key (optional)

flags: should be zero

This function will import the given private key to the abstract gnutls_privkey_t structure.

Note that unless GNUTLS_PUBKEY_DISABLE_CALLBACKS is specified, if incorrect (or NULL) passwords are given the PKCS11 callback functions will be used to obtain the correct passwords. Otherwise if the SRK password is wrong GNUTLS_E_TPM_SRK_PASSWORD_ERROR is returned.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0

Listing and deleting keys

The registered keys (that are stored in the TPM) can be listed using one of the following functions. Those keys are unfortunately only identified by their UUID and have no label or other human friendly identifier. Keys can be deleted from permament storage using gnutls_tpm_privkey_delete.

int gnutls_tpm_get_registered (gnutls_tpm_key_list_t * list)
void gnutls_tpm_key_list_deinit (gnutls_tpm_key_list_t list)
int gnutls_tpm_key_list_get_url (gnutls_tpm_key_list_t list, unsigned int idx, char ** url, unsigned int flags)
Function: int gnutls_tpm_privkey_delete (const char * url, const char * srk_password)

url: the URL describing the key

srk_password: a password for the SRK key

This function will unregister the private key from the TPM chip.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.0


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5.3.4 Invoking tpmtool

Program that allows handling cryptographic data from the TPM chip.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the tpmtool program. This software is released under the GNU General Public License, version 3 or later.

tpmtool help/usage (--help)

This is the automatically generated usage text for tpmtool.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

tpmtool - GnuTLS TPM tool
Usage:  tpmtool [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
       --infile=file          Input file
                                - file must pre-exist
       --outfile=str          Output file
       --generate-rsa         Generate an RSA private-public key pair
       --register             Any generated key will be registered in the TPM
                                - requires the option 'generate-rsa'
       --signing              Any generated key will be a signing key
                                - requires the option 'generate-rsa'
                                -- and prohibits the option 'legacy'
       --legacy               Any generated key will be a legacy key
                                - requires the option 'generate-rsa'
                                -- and prohibits the option 'signing'
       --user                 Any registered key will be a user key
                                - requires the option 'register'
                                -- and prohibits the option 'system'
       --system               Any registred key will be a system key
                                - requires the option 'register'
                                -- and prohibits the option 'user'
       --pubkey=str           Prints the public key of the provided key
       --list                 Lists all stored keys in the TPM
       --delete=str           Delete the key identified by the given URL (UUID).
       --sec-param=str        Specify the security level [low, legacy, normal, high, ultra].
       --bits=num             Specify the number of bits for key generate
       --inder                Use the DER format for keys.
                                - disabled as '--no-inder'
       --outder               Use DER format for output keys
                                - disabled as '--no-outder'
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Program that allows handling cryptographic data from the TPM chip.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

generate-rsa option

This is the “generate an rsa private-public key pair” option. Generates an RSA private-public key pair in the TPM chip. The key may be stored in filesystem and protected by a PIN, or stored (registered) in the TPM chip flash.

user option

This is the “any registered key will be a user key” option.

This option has some usage constraints. It:

The generated key will be stored in a user specific persistent storage.

system option

This is the “any registred key will be a system key” option.

This option has some usage constraints. It:

The generated key will be stored in system persistent storage.

sec-param option

This is the “specify the security level [low, legacy, normal, high, ultra].” option. This option takes a string argument Security parameter. This is alternative to the bits option. Note however that the values allowed by the TPM chip are quantized and given values may be rounded up.

inder option

This is the “use the der format for keys.” option.

This option has some usage constraints. It:

The input files will be assumed to be in the portable DER format of TPM. The default format is a custom format used by various TPM tools

outder option

This is the “use der format for output keys” option.

This option has some usage constraints. It:

The output will be in the TPM portable DER format.

tpmtool exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

tpmtool See Also

p11tool (1), certtool (1)

tpmtool Examples

To generate a key that is to be stored in filesystem use:

$ tpmtool --generate-rsa --bits 2048 --outfile tpmkey.pem

To generate a key that is to be stored in TPM’s flash use:

$ tpmtool --generate-rsa --bits 2048 --register --user

To get the public key of a TPM key use:

$ tpmtool --pubkey tpmkey:uuid=58ad734b-bde6-45c7-89d8-756a55ad1891;storage=user \
          --outfile pubkey.pem

or if the key is stored in the filesystem:

$ tpmtool --pubkey tpmkey:file=tmpkey.pem --outfile pubkey.pem

To list all keys stored in TPM use:

$ tpmtool --list

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6 How to use GnuTLS in applications


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6.1 Introduction

This chapter tries to explain the basic functionality of the current GnuTLS library. Note that there may be additional functionality not discussed here but included in the library. Checking the header files in /usr/include/gnutls/ and the manpages is recommended.


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6.1.1 General idea

A brief description of how GnuTLS sessions operate is shown at Figure 6.1. This section will become more clear when it is completely read. As shown in the figure, there is a read-only global state that is initialized once by the global initialization function. This global structure, among others, contains the memory allocation functions used, structures needed for the ASN.1 parser and depending on the system’s CPU, pointers to hardware accelerated encryption functions. This structure is never modified by any GnuTLS function, except for the deinitialization function which frees all allocated memory and must be called after the program has permanently finished using GnuTLS.

gnutls-internals

Figure 6.1: High level design of GnuTLS.

The credentials structures are used by the authentication methods, such as certificate authentication. They store certificates, privates keys, and other information that is needed to prove the identity to the peer, and/or verify the indentity of the peer. The information stored in the credentials structures is initialized once and then can be shared by many TLS sessions.

A GnuTLS session contains all the required information to handle one secure connection. The session communicates with the peers using the provided functions of the transport layer. Every session has a unique session ID shared with the peer.

Since TLS sessions can be resumed, servers need a database back-end to hold the session’s parameters. Every GnuTLS session after a successful handshake calls the appropriate back-end function (see resume) to store the newly negotiated session. The session database is examined by the server just after having received the client hello9, and if the session ID sent by the client, matches a stored session, the stored session will be retrieved, and the new session will be a resumed one, and will share the same session ID with the previous one.


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6.1.2 Error handling

In GnuTLS most functions return an integer type as a result. In almost all cases a zero or a positive number means success, and a negative number indicates failure, or a situation that some action has to be taken. Thus negative error codes may be fatal or not.

Fatal errors terminate the connection immediately and further sends and receives will be disallowed. Such an example is GNUTLS_E_DECRYPTION_FAILED. Non-fatal errors may warn about something, i.e., a warning alert was received, or indicate the some action has to be taken. This is the case with the error code GNUTLS_E_REHANDSHAKE returned by gnutls_record_recv. This error code indicates that the server requests a re-handshake. The client may ignore this request, or may reply with an alert. You can test if an error code is a fatal one by using the gnutls_error_is_fatal. All errors can be converted to a descriptive string using gnutls_strerror.

If any non fatal errors, that require an action, are to be returned by a function, these error codes will be documented in the function’s reference. For example the error codes GNUTLS_E_WARNING_ALERT_RECEIVED and GNUTLS_E_FATAL_ALERT_RECEIVED that may returned when receiving data, should be handled by notifying the user of the alert (as explained in Handling alerts). See Error codes, for a description of the available error codes.


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6.1.3 Common types

All strings that are to provided as input to GnuTLS functions should be in UTF-8 unless otherwise specified. Output strings are also in UTF-8 format unless otherwise specified.

When data of a fixed size are provided to GnuTLS functions then the helper structure gnutls_datum_t is often used. Its definition is shown below.

  typedef struct
  {
    unsigned char *data;
    unsigned int size;
  } gnutls_datum_t;

Other functions that require data for scattered read use a structure similar to struct iovec typically used by readv. It is shown below.

  typedef struct
  {
    void *iov_base;             /* Starting address */
    size_t iov_len;             /* Number of bytes to transfer */
  } giovec_t;

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6.1.4 Debugging and auditing

In many cases things may not go as expected and further information, to assist debugging, from GnuTLS is desired. Those are the cases where the gnutls_global_set_log_level and gnutls_global_set_log_function are to be used. Those will print verbose information on the GnuTLS functions internal flow.

void gnutls_global_set_log_level (int level)
void gnutls_global_set_log_function (gnutls_log_func log_func)

Alternatively the environment variable GNUTLS_DEBUG_LEVEL can be set to a logging level and GnuTLS will output debugging output to standard error.

When debugging is not required, important issues, such as detected attacks on the protocol still need to be logged. This is provided by the logging function set by gnutls_global_set_audit_log_function. The provided function will receive an message and the corresponding TLS session. The session information might be used to derive IP addresses or other information about the peer involved.

Function: void gnutls_global_set_audit_log_function (gnutls_audit_log_func log_func)

log_func: it is the audit log function

This is the function to set the audit logging function. This is a function to report important issues, such as possible attacks in the protocol. This is different from gnutls_global_set_log_function() because it will report also session-specific events. The session parameter will be null if there is no corresponding TLS session.

gnutls_audit_log_func is of the form, void (*gnutls_audit_log_func)( gnutls_session_t, const char*);

Since: 3.0


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6.1.5 Thread safety

The GnuTLS library is thread safe by design, meaning that objects of the library such as TLS sessions, can be safely divided across threads as long as a single thread accesses a single object. This is sufficient to support a server which handles several sessions per thread. If, however, an object needs to be shared across threads then access must be protected with a mutex. Read-only access to objects, for example the credentials holding structures, is also thread-safe.

The random generator of the cryptographic back-end, is not thread safe and requires mutex locks which are setup by GnuTLS. Applications can either call gnutls_global_init which will initialize the default operating system provided locks (i.e. pthreads on GNU/Linux and CriticalSection on Windows), or manually specify the locking system using the function gnutls_global_set_mutex before calling gnutls_global_init. Setting mutexes manually is recommended only for applications that have full control of the underlying libraries. If this is not the case, the use of the operating system defaults is recommended. An example of non-native thread usage is shown below.

#include <gnutls/gnutls.h>

int main()
{
   /* When the system mutexes are not to be used 
    * gnutls_global_set_mutex() must be called explicitly
    */
   gnutls_global_set_mutex (mutex_init, mutex_deinit, 
                            mutex_lock, mutex_unlock);
   gnutls_global_init();
}

Note that gnutls_global_init is itself not thread safe. It is also not recommended to initialize it on every available thread, but if need to, it should be protected using mutex locks.

Function: void gnutls_global_set_mutex (mutex_init_func init, mutex_deinit_func deinit, mutex_lock_func lock, mutex_unlock_func unlock)

init: mutex initialization function

deinit: mutex deinitialization function

lock: mutex locking function

unlock: mutex unlocking function

With this function you are allowed to override the default mutex locks used in some parts of gnutls and dependent libraries. This function should be used if you have complete control of your program and libraries. Do not call this function from a library. Instead only initialize gnutls and the default OS mutex locks will be used.

This function must be called before gnutls_global_init() .

Since: 2.12.0


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6.1.6 Callback functions

There are several cases where GnuTLS may need out of band input from your program. This is now implemented using some callback functions, which your program is expected to register.

An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer.

void gnutls_transport_set_push_function (gnutls_session_t session, gnutls_push_func push_func)
void gnutls_transport_set_pull_function (gnutls_session_t session, gnutls_pull_func pull_func)

Other callback functions may require more complicated input and data to be allocated. Such an example is gnutls_srp_set_server_credentials_function. All callbacks should allocate and free memory using gnutls_malloc and gnutls_free.


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6.2 Preparation

To use GnuTLS, you have to perform some changes to your sources and your build system. The necessary changes are explained in the following subsections.


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6.2.1 Headers

All the data types and functions of the GnuTLS library are defined in the header file gnutls/gnutls.h. This must be included in all programs that make use of the GnuTLS library.


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6.2.2 Initialization

GnuTLS must be initialized before it can be used. The library is initialized by calling gnutls_global_init. That call typically enables CPU-specific acceleration, performs any required precalculations needed, and initializes subsystems that could be used later (e.g., PKCS #11 – see PKCS11 Initialization).

The resources allocated by the initialization process can be released if the application no longer has a need to call GnuTLS functions, this is done by calling gnutls_global_deinit.

In order to take advantage of the internationalization features in GnuTLS, such as translated error messages, the application must set the current locale using setlocale before initializing GnuTLS.


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6.2.3 Version check

It is often desirable to check that the version of ‘gnutls’ used is indeed one which fits all requirements. Even with binary compatibility new features may have been introduced but due to problem with the dynamic linker an old version is actually used. So you may want to check that the version is okay right after program start-up. See the function gnutls_check_version.

On the other hand, it is often desirable to support more than one versions of the library. In that case you could utilize compile-time feature checks using the the GNUTLS_VERSION_NUMBER macro. For example, to conditionally add code for GnuTLS 3.2.1 or later, you may use:

#if GNUTLS_VERSION_NUMBER >= 0x030201
 ...
#endif

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6.2.4 Building the source

If you want to compile a source file including the gnutls/gnutls.h header file, you must make sure that the compiler can find it in the directory hierarchy. This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).

However, the path to the include file is determined at the time the source is configured. To solve this problem, the library uses the external package pkg-config that knows the path to the include file and other configuration options. The options that need to be added to the compiler invocation at compile time are output by the --cflags option to pkg-config gnutls. The following example shows how it can be used at the command line:

gcc -c foo.c `pkg-config gnutls --cflags`

Adding the output of ‘pkg-config gnutls --cflags’ to the compilers command line will ensure that the compiler can find the gnutls/gnutls.h header file.

A similar problem occurs when linking the program with the library. Again, the compiler has to find the library files. For this to work, the path to the library files has to be added to the library search path (via the -L option). For this, the option --libs to pkg-config gnutls can be used. For convenience, this option also outputs all other options that are required to link the program with the library (for instance, the ‘-ltasn1’ option). The example shows how to link foo.o with the library to a program foo.

gcc -o foo foo.o `pkg-config gnutls --libs`

Of course you can also combine both examples to a single command by specifying both options to pkg-config:

gcc -o foo foo.c `pkg-config gnutls --cflags --libs`

When a program uses the GNU autoconf system, then the following line or similar can be used to detect the presence of GnuTLS.

PKG_CHECK_MODULES([LIBGNUTLS], [gnutls >= 3.0.0])

AC_SUBST([LIBGNUTLS_CFLAGS])
AC_SUBST([LIBGNUTLS_LIBS])

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6.3 Session initialization

In the previous sections we have discussed the global initialization required for GnuTLS as well as the initialization required for each authentication method’s credentials (see Authentication). In this section we elaborate on the TLS or DTLS session initiation. Each session is initialized using gnutls_init which among others is used to specify the type of the connection (server or client), and the underlying protocol type, i.e., datagram (UDP) or reliable (TCP).

Function: int gnutls_init (gnutls_session_t * session, unsigned int flags)

session: is a pointer to a gnutls_session_t structure.

flags: indicate if this session is to be used for server or client.

This function initializes the current session to null. Every session must be initialized before use, so internal structures can be allocated. This function allocates structures which can only be free’d by calling gnutls_deinit() . Returns GNUTLS_E_SUCCESS (0) on success.

flags can be one of GNUTLS_CLIENT and GNUTLS_SERVER . For a DTLS entity, the flags GNUTLS_DATAGRAM and GNUTLS_NONBLOCK are also available. The latter flag will enable a non-blocking operation of the DTLS timers.

The flag GNUTLS_NO_REPLAY_PROTECTION will disable any replay protection in DTLS mode. That must only used when replay protection is achieved using other means.

Note that since version 3.1.2 this function enables some common TLS extensions such as session tickets and OCSP certificate status request in client side by default. To prevent that use the GNUTLS_NO_EXTENSIONS flag.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

After the session initialization details on the allowed ciphersuites and protocol versions should be set using the priority functions such as gnutls_priority_set_direct. We elaborate on them in Priority Strings. The credentials used for the key exchange method, such as certificates or usernames and passwords should also be associated with the session current session using gnutls_credentials_set.

Function: int gnutls_credentials_set (gnutls_session_t session, gnutls_credentials_type_t type, void * cred)

session: is a gnutls_session_t structure.

type: is the type of the credentials

cred: is a pointer to a structure.

Sets the needed credentials for the specified type. Eg username, password - or public and private keys etc. The cred parameter is a structure that depends on the specified type and on the current session (client or server).

In order to minimize memory usage, and share credentials between several threads gnutls keeps a pointer to cred, and not the whole cred structure. Thus you will have to keep the structure allocated until you call gnutls_deinit() .

For GNUTLS_CRD_ANON , cred should be gnutls_anon_client_credentials_t in case of a client. In case of a server it should be gnutls_anon_server_credentials_t .

For GNUTLS_CRD_SRP , cred should be gnutls_srp_client_credentials_t in case of a client, and gnutls_srp_server_credentials_t , in case of a server.

For GNUTLS_CRD_CERTIFICATE , cred should be gnutls_certificate_credentials_t .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.


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6.4 Associating the credentials

Each authentication method is associated with a key exchange method, and a credentials type. The contents of the credentials is method-dependent, e.g. certificates for certificate authentication and should be initialized and associated with a session (see gnutls_credentials_set). A mapping of the key exchange methods with the credential types is shown in Table 6.1.

Authentication methodKey exchangeClient credentialsServer credentials
CertificateKX_RSA, KX_DHE_RSA, KX_DHE_DSS, KX_ECDHE_RSA, KX_ECDHE_ECDSA, KX_RSA_EXPORTCRD_CERTIFICATECRD_CERTIFICATE
Password and certificateKX_SRP_RSA, KX_SRP_DSSCRD_SRPCRD_CERTIFICATE, CRD_SRP
PasswordKX_SRPCRD_SRPCRD_SRP
AnonymousKX_ANON_DH, KX_ANON_ECDHCRD_ANONCRD_ANON
Pre-shared keyKX_PSK, KX_DHE_PSK, KX_ECDHE_PSKCRD_PSKCRD_PSK

Table 6.1: Key exchange algorithms and the corresponding credential types.


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6.4.1 Certificates

Server certificate authentication

When using certificates the server is required to have at least one certificate and private key pair. Clients may not hold such a pair, but a server could require it. In this section we discuss general issues applying to both client and server certificates. The next section will elaborate on issues arising from client authentication only.

int gnutls_certificate_allocate_credentials (gnutls_certificate_credentials_t * res)
void gnutls_certificate_free_credentials (gnutls_certificate_credentials_t sc)

After the credentials structures are initialized, the certificate and key pair must be loaded. This occurs before any TLS session is initialized, and the same structures are reused for multiple sessions. Depending on the certificate type different loading functions are available, as shown below. For X.509 certificates, the functions will accept and use a certificate chain that leads to a trusted authority. The certificate chain must be ordered in such way that every certificate certifies the one before it. The trusted authority’s certificate need not to be included since the peer should possess it already.

int gnutls_certificate_set_x509_key_mem2 (gnutls_certificate_credentials_t res, const gnutls_datum_t * cert, const gnutls_datum_t * key, gnutls_x509_crt_fmt_t type, const char * pass, unsigned int flags)
int gnutls_certificate_set_x509_key (gnutls_certificate_credentials_t res, gnutls_x509_crt_t * cert_list, int cert_list_size, gnutls_x509_privkey_t key)
int gnutls_certificate_set_x509_key_file2 (gnutls_certificate_credentials_t res, const char * certfile, const char * keyfile, gnutls_x509_crt_fmt_t type, const char * pass, unsigned int flags)
int gnutls_certificate_set_openpgp_key_mem (gnutls_certificate_credentials_t res, const gnutls_datum_t * cert, const gnutls_datum_t * key, gnutls_openpgp_crt_fmt_t format)
int gnutls_certificate_set_openpgp_key (gnutls_certificate_credentials_t res, gnutls_openpgp_crt_t crt, gnutls_openpgp_privkey_t pkey)
int gnutls_certificate_set_openpgp_key_file (gnutls_certificate_credentials_t res, const char * certfile, const char * keyfile, gnutls_openpgp_crt_fmt_t format)

Note however, that since functions like gnutls_certificate_set_x509_key_file2 may accept URLs that specify objects stored in token, another important function is gnutls_certificate_set_pin_function. That allows setting a callback function to retrieve a PIN if the input keys are protected by PIN by the token.

Function: void gnutls_certificate_set_pin_function (gnutls_certificate_credentials_t cred, gnutls_pin_callback_t fn, void * userdata)

cred: is a gnutls_certificate_credentials_t structure.

fn: A PIN callback

userdata: Data to be passed in the callback

This function will set a callback function to be used when required to access a protected object. This function overrides any other global PIN functions.

Note that this function must be called right after initialization to have effect.

Since: 3.1.0

If the imported keys and certificates need to be accessed before any TLS session is established, it is convenient to use gnutls_certificate_set_key in combination with gnutls_pcert_import_x509_raw and gnutls_privkey_import_x509_raw.

Function: int gnutls_certificate_set_key (gnutls_certificate_credentials_t res, const char ** names, int names_size, gnutls_pcert_st * pcert_list, int pcert_list_size, gnutls_privkey_t key)

res: is a gnutls_certificate_credentials_t structure.

names: is an array of DNS name of the certificate (NULL if none)

names_size: holds the size of the names list

pcert_list: contains a certificate list (path) for the specified private key

pcert_list_size: holds the size of the certificate list

key: is a gnutls_privkey_t key

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server. For clients that wants to send more than its own end entity certificate (e.g., also an intermediate CA cert) then put the certificate chain in pcert_list .

Note that the pcert_list and key will become part of the credentials structure and must not be deallocated. They will be automatically deallocated when the res structure is deinitialized.

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

Since: 3.0

If multiple certificates are used with the functions above each client’s request will be served with the certificate that matches the requested name (see Server name indication).

As an alternative to loading from files or buffers, a callback may be used for the server or the client to specify the certificate and the key at the handshake time. In that case a certificate should be selected according the peer’s signature algorithm preferences. To get those preferences use gnutls_sign_algorithm_get_requested. Both functions are shown below.

void gnutls_certificate_set_retrieve_function (gnutls_certificate_credentials_t cred, gnutls_certificate_retrieve_function * func)
void gnutls_certificate_set_retrieve_function2 (gnutls_certificate_credentials_t cred, gnutls_certificate_retrieve_function2 * func)
int gnutls_sign_algorithm_get_requested (gnutls_session_t session, size_t indx, gnutls_sign_algorithm_t * algo)

The functions above do not handle the requested server name automatically. A server would need to check the name requested by the client using gnutls_server_name_get, and serve the appropriate certificate. Note that some of these functions require the gnutls_pcert_st structure to be filled in. Helper functions to make the required structures are listed below.

typedef struct gnutls_pcert_st
{
  gnutls_pubkey_t pubkey;
  gnutls_datum_t cert;
  gnutls_certificate_type_t type;
} gnutls_pcert_st;
int gnutls_pcert_import_x509 (gnutls_pcert_st * pcert, gnutls_x509_crt_t crt, unsigned int flags)
int gnutls_pcert_import_openpgp (gnutls_pcert_st * pcert, gnutls_openpgp_crt_t crt, unsigned int flags)
int gnutls_pcert_import_x509_raw (gnutls_pcert_st * pcert, const gnutls_datum_t * cert, gnutls_x509_crt_fmt_t format, unsigned int flags)
int gnutls_pcert_import_openpgp_raw (gnutls_pcert_st * pcert, const gnutls_datum_t * cert, gnutls_openpgp_crt_fmt_t format, gnutls_openpgp_keyid_t keyid, unsigned int flags)
void gnutls_pcert_deinit (gnutls_pcert_st * pcert)

In a handshake, the negotiated cipher suite depends on the certificate’s parameters, so some key exchange methods might not be available with all certificates. GnuTLS will disable ciphersuites that are not compatible with the key, or the enabled authentication methods. For example keys marked as sign-only, will not be able to access the plain RSA ciphersuites, that require decryption. It is not recommended to use RSA keys for both signing and encryption. If possible use a different key for the DHE-RSA which uses signing and RSA that requires decryption. All the key exchange methods shown in Table 4.1 are available in certificate authentication.

Client certificate authentication

If a certificate is to be requested from the client during the handshake, the server will send a certificate request message. This behavior is controlled gnutls_certificate_server_set_request. The request contains a list of the acceptable by the server certificate signers. This list is constructed using the trusted certificate authorities of the server. In cases where the server supports a large number of certificate authorities it makes sense not to advertise all of the names to save bandwidth. That can be controlled using the function gnutls_certificate_send_x509_rdn_sequence. This however will have the side-effect of not restricting the client to certificates signed by server’s acceptable signers.

Function: void gnutls_certificate_server_set_request (gnutls_session_t session, gnutls_certificate_request_t req)

session: is a gnutls_session_t structure.

req: is one of GNUTLS_CERT_REQUEST, GNUTLS_CERT_REQUIRE

This function specifies if we (in case of a server) are going to send a certificate request message to the client. If req is GNUTLS_CERT_REQUIRE then the server will return an error if the peer does not provide a certificate. If you do not call this function then the client will not be asked to send a certificate.

Function: void gnutls_certificate_send_x509_rdn_sequence (gnutls_session_t session, int status)

session: is a pointer to a gnutls_session_t structure.

status: is 0 or 1

If status is non zero, this function will order gnutls not to send the rdnSequence in the certificate request message. That is the server will not advertise its trusted CAs to the peer. If status is zero then the default behaviour will take effect, which is to advertise the server’s trusted CAs.

This function has no effect in clients, and in authentication methods other than certificate with X.509 certificates.

Client or server certificate verification

Certificate verification is possible by loading the trusted authorities into the credentials structure by using the following functions, applicable to X.509 and OpenPGP certificates.

int gnutls_certificate_set_x509_system_trust (gnutls_certificate_credentials_t cred)
int gnutls_certificate_set_x509_trust_file (gnutls_certificate_credentials_t cred, const char * cafile, gnutls_x509_crt_fmt_t type)
int gnutls_certificate_set_openpgp_keyring_file (gnutls_certificate_credentials_t c, const char * file, gnutls_openpgp_crt_fmt_t format)

The peer’s certificate is not automatically verified and one must call gnutls_certificate_verify_peers3 after a successful handshake to verify the certificate’s signature and the owner of the certificate. The verification status returned can be printed using gnutls_certificate_verification_status_print.

Alternatively the verification can occur during the handshake by using gnutls_certificate_set_verify_function.

The functions above provide a brief verification output. If a detailed output is required one should call gnutls_certificate_get_peers to obtain the raw certificate of the peer and verify it using the functions discussed in X.509 certificates.

Function: int gnutls_certificate_verify_peers3 (gnutls_session_t session, const char * hostname, unsigned int * status)

session: is a gnutls session

hostname: is the expected name of the peer; may be NULL

status: is the output of the verification

This function will verify the peer’s certificate and store the status in the status variable as a bitwise or’d gnutls_certificate_status_t values or zero if the certificate is trusted. Note that value in status is set only when the return value of this function is success (i.e, failure to trust a certificate does not imply a negative return value).

If the hostname provided is non-NULL then this function will compare the hostname in the certificate against the given. If they do not match the GNUTLS_CERT_UNEXPECTED_OWNER status flag will be set.

If available the OCSP Certificate Status extension will be utilized by this function.

To avoid denial of service attacks some default upper limits regarding the certificate key size and chain size are set. To override them use gnutls_certificate_set_verify_limits() .

Returns: a negative error code on error and GNUTLS_E_SUCCESS (0) on success.

Since: 3.1.4

Function: void gnutls_certificate_set_verify_function (gnutls_certificate_credentials_t cred, gnutls_certificate_verify_function * func)

cred: is a gnutls_certificate_credentials_t structure.

func: is the callback function

This function sets a callback to be called when peer’s certificate has been received in order to verify it on receipt rather than doing after the handshake is completed.

The callback’s function prototype is: int (*callback)(gnutls_session_t);

If the callback function is provided then gnutls will call it, in the handshake, just after the certificate message has been received. To verify or obtain the certificate the gnutls_certificate_verify_peers2() , gnutls_certificate_type_get() , gnutls_certificate_get_peers() functions can be used.

The callback function should return 0 for the handshake to continue or non-zero to terminate.

Since: 2.10.0


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6.4.2 SRP

The initialization functions in SRP credentials differ between client and server. Clients supporting SRP should set the username and password prior to connection, to the credentials structure. Alternatively gnutls_srp_set_client_credentials_function may be used instead, to specify a callback function that should return the SRP username and password. The callback is called once during the TLS handshake.

int gnutls_srp_allocate_server_credentials (gnutls_srp_server_credentials_t * sc)
int gnutls_srp_allocate_client_credentials (gnutls_srp_client_credentials_t * sc)
void gnutls_srp_free_server_credentials (gnutls_srp_server_credentials_t sc)
void gnutls_srp_free_client_credentials (gnutls_srp_client_credentials_t sc)
int gnutls_srp_set_client_credentials (gnutls_srp_client_credentials_t res, const char * username, const char * password)
Function: void gnutls_srp_set_client_credentials_function (gnutls_srp_client_credentials_t cred, gnutls_srp_client_credentials_function * func)

cred: is a gnutls_srp_server_credentials_t structure.

func: is the callback function

This function can be used to set a callback to retrieve the username and password for client SRP authentication. The callback’s function form is:

int (*callback)(gnutls_session_t, char** username, char**password);

The username and password must be allocated using gnutls_malloc() . username and password should be ASCII strings or UTF-8 strings prepared using the "SASLprep" profile of "stringprep".

The callback function will be called once per handshake before the initial hello message is sent.

The callback should not return a negative error code the second time called, since the handshake procedure will be aborted.

The callback function should return 0 on success. -1 indicates an error.

In server side the default behavior of GnuTLS is to read the usernames and SRP verifiers from password files. These password file format is compatible the with the Stanford srp libraries format. If a different password file format is to be used, then gnutls_srp_set_server_credentials_function should be called, to set an appropriate callback.

Function: int gnutls_srp_set_server_credentials_file (gnutls_srp_server_credentials_t res, const char * password_file, const char * password_conf_file)

res: is a gnutls_srp_server_credentials_t structure.

password_file: is the SRP password file (tpasswd)

password_conf_file: is the SRP password conf file (tpasswd.conf)

This function sets the password files, in a gnutls_srp_server_credentials_t structure. Those password files hold usernames and verifiers and will be used for SRP authentication.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, or an error code.

Function: void gnutls_srp_set_server_credentials_function (gnutls_srp_server_credentials_t cred, gnutls_srp_server_credentials_function * func)

cred: is a gnutls_srp_server_credentials_t structure.

func: is the callback function

This function can be used to set a callback to retrieve the user’s SRP credentials. The callback’s function form is:

int (*callback)(gnutls_session_t, const char* username, gnutls_datum_t* salt, gnutls_datum_t *verifier, gnutls_datum_t* generator, gnutls_datum_t* prime);

username contains the actual username. The salt , verifier , generator and prime must be filled in using the gnutls_malloc() . For convenience prime and generator may also be one of the static parameters defined in gnutls.h.

In case the callback returned a negative number then gnutls will assume that the username does not exist.

In order to prevent attackers from guessing valid usernames, if a user does not exist, g and n values should be filled in using a random user’s parameters. In that case the callback must return the special value (1).

The callback function will only be called once per handshake. The callback function should return 0 on success, while -1 indicates an error.


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6.4.3 PSK

The initialization functions in PSK credentials differ between client and server.

int gnutls_psk_allocate_server_credentials (gnutls_psk_server_credentials_t * sc)
int gnutls_psk_allocate_client_credentials (gnutls_psk_client_credentials_t * sc)
void gnutls_psk_free_server_credentials (gnutls_psk_server_credentials_t sc)
void gnutls_psk_free_client_credentials (gnutls_psk_client_credentials_t sc)

Clients supporting PSK should supply the username and key before a TLS session is established. Alternatively gnutls_psk_set_client_credentials_function can be used to specify a callback function. This has the advantage that the callback will be called only if PSK has been negotiated.

int gnutls_psk_set_client_credentials (gnutls_psk_client_credentials_t res, const char * username, const gnutls_datum_t * key, gnutls_psk_key_flags flags)
Function: void gnutls_psk_set_client_credentials_function (gnutls_psk_client_credentials_t cred, gnutls_psk_client_credentials_function * func)

cred: is a gnutls_psk_server_credentials_t structure.

func: is the callback function

This function can be used to set a callback to retrieve the username and password for client PSK authentication. The callback’s function form is: int (*callback)(gnutls_session_t, char** username, gnutls_datum_t* key);

The username and key ->data must be allocated using gnutls_malloc() . username should be ASCII strings or UTF-8 strings prepared using the "SASLprep" profile of "stringprep".

The callback function will be called once per handshake.

The callback function should return 0 on success. -1 indicates an error.

In server side the default behavior of GnuTLS is to read the usernames and PSK keys from a password file. The password file should contain usernames and keys in hexadecimal format. The name of the password file can be stored to the credentials structure by calling gnutls_psk_set_server_credentials_file. If a different password file format is to be used, then a callback should be set instead by gnutls_psk_set_server_credentials_function.

The server can help the client chose a suitable username and password, by sending a hint. Note that there is no common profile for the PSK hint and applications are discouraged to use it. A server, may specify the hint by calling gnutls_psk_set_server_credentials_hint. The client can retrieve the hint, for example in the callback function, using gnutls_psk_client_get_hint.

Function: int gnutls_psk_set_server_credentials_file (gnutls_psk_server_credentials_t res, const char * password_file)

res: is a gnutls_psk_server_credentials_t structure.

password_file: is the PSK password file (passwd.psk)

This function sets the password file, in a gnutls_psk_server_credentials_t structure. This password file holds usernames and keys and will be used for PSK authentication.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

void gnutls_psk_set_server_credentials_function (gnutls_psk_server_credentials_t cred, gnutls_psk_server_credentials_function * func)
int gnutls_psk_set_server_credentials_hint (gnutls_psk_server_credentials_t res, const char * hint)
const char * gnutls_psk_client_get_hint (gnutls_session_t session)

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6.4.4 Anonymous

The key exchange methods for anonymous authentication might require Diffie-Hellman parameters to be generated by the server and associated with an anonymous credentials structure. Check Parameter generation for more information. The initialization functions for the credentials are shown below.

int gnutls_anon_allocate_server_credentials (gnutls_anon_server_credentials_t * sc)
int gnutls_anon_allocate_client_credentials (gnutls_anon_client_credentials_t * sc)
void gnutls_anon_free_server_credentials (gnutls_anon_server_credentials_t sc)
void gnutls_anon_free_client_credentials (gnutls_anon_client_credentials_t sc)

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6.5 Setting up the transport layer

The next step is to setup the underlying transport layer details. The Berkeley sockets are implicitly used by GnuTLS, thus a call to gnutls_transport_set_int would be sufficient to specify the socket descriptor.

void gnutls_transport_set_int (gnutls_session_t session, int i)
void gnutls_transport_set_int2 (gnutls_session_t session, int recv_int, int send_int)

If however another transport layer than TCP is selected, then a pointer should be used instead to express the parameter to be passed to custom functions. In that case the following functions should be used instead.

void gnutls_transport_set_ptr (gnutls_session_t session, gnutls_transport_ptr_t ptr)
void gnutls_transport_set_ptr2 (gnutls_session_t session, gnutls_transport_ptr_t recv_ptr, gnutls_transport_ptr_t send_ptr)

Moreover all of the following push and pull callbacks should be set.

Function: void gnutls_transport_set_push_function (gnutls_session_t session, gnutls_push_func push_func)

session: is a gnutls_session_t structure.

push_func: a callback function similar to write()

This is the function where you set a push function for gnutls to use in order to send data. If you are going to use berkeley style sockets, you do not need to use this function since the default send(2) will probably be ok. Otherwise you should specify this function for gnutls to be able to send data. The callback should return a positive number indicating the bytes sent, and -1 on error.

push_func is of the form, ssize_t (*gnutls_push_func)(gnutls_transport_ptr_t, const void*, size_t);

Function: void gnutls_transport_set_vec_push_function (gnutls_session_t session, gnutls_vec_push_func vec_func)

session: is a gnutls_session_t structure.

vec_func: a callback function similar to writev()

Using this function you can override the default writev(2) function for gnutls to send data. Setting this callback instead of gnutls_transport_set_push_function() is recommended since it introduces less overhead in the TLS handshake process.

vec_func is of the form, ssize_t (*gnutls_vec_push_func) (gnutls_transport_ptr_t, const giovec_t * iov, int iovcnt);

Since: 2.12.0

Function: void gnutls_transport_set_pull_function (gnutls_session_t session, gnutls_pull_func pull_func)

session: is a gnutls_session_t structure.

pull_func: a callback function similar to read()

This is the function where you set a function for gnutls to receive data. Normally, if you use berkeley style sockets, do not need to use this function since the default recv(2) will probably be ok. The callback should return 0 on connection termination, a positive number indicating the number of bytes received, and -1 on error.

gnutls_pull_func is of the form, ssize_t (*gnutls_pull_func)(gnutls_transport_ptr_t, void*, size_t);

Function: void gnutls_transport_set_pull_timeout_function (gnutls_session_t session, gnutls_pull_timeout_func func)

session: is a gnutls_session_t structure.

func: a callback function

This is the function where you set a function for gnutls to know whether data are ready to be received. It should wait for data a given time frame in milliseconds. The callback should return 0 on timeout, a positive number if data can be received, and -1 on error. You’ll need to override this function if select() is not suitable for the provided transport calls.

As with select() , if the timeout value is zero the callback should return zero if no data are immediately available.

gnutls_pull_timeout_func is of the form, int (*gnutls_pull_timeout_func)(gnutls_transport_ptr_t, unsigned int ms);

Since: 3.0

The functions above accept a callback function which should return the number of bytes written, or -1 on error and should set errno appropriately. In some environments, setting errno is unreliable. For example Windows have several errno variables in different CRTs, or in other systems it may be a non thread-local variable. If this is a concern to you, call gnutls_transport_set_errno with the intended errno value instead of setting errno directly.

Function: void gnutls_transport_set_errno (gnutls_session_t session, int err)

session: is a gnutls_session_t structure.

err: error value to store in session-specific errno variable.

Store err in the session-specific errno variable. Useful values for err is EAGAIN and EINTR, other values are treated will be treated as real errors in the push/pull function.

This function is useful in replacement push and pull functions set by gnutls_transport_set_push_function() and gnutls_transport_set_pull_function() under Windows, where the replacements may not have access to the same errno variable that is used by GnuTLS (e.g., the application is linked to msvcr71.dll and gnutls is linked to msvcrt.dll).

GnuTLS currently only interprets the EINTR, EAGAIN and EMSGSIZE errno values and returns the corresponding GnuTLS error codes:

The EINTR and EAGAIN values are returned by interrupted system calls, or when non blocking IO is used. All GnuTLS functions can be resumed (called again), if any of the above error codes is returned. The EMSGSIZE value is returned when attempting to send a large datagram.

In the case of DTLS it is also desirable to override the generic transport functions with functions that emulate the operation of recvfrom and sendto. In addition DTLS requires timers during the receive of a handshake message, set using the gnutls_transport_set_pull_timeout_function function. To check the retransmission timers the function gnutls_dtls_get_timeout is provided, which returns the time remaining until the next retransmission, or better the time until gnutls_handshake should be called again.

Function: void gnutls_transport_set_pull_timeout_function (gnutls_session_t session, gnutls_pull_timeout_func func)

session: is a gnutls_session_t structure.

func: a callback function

This is the function where you set a function for gnutls to know whether data are ready to be received. It should wait for data a given time frame in milliseconds. The callback should return 0 on timeout, a positive number if data can be received, and -1 on error. You’ll need to override this function if select() is not suitable for the provided transport calls.

As with select() , if the timeout value is zero the callback should return zero if no data are immediately available.

gnutls_pull_timeout_func is of the form, int (*gnutls_pull_timeout_func)(gnutls_transport_ptr_t, unsigned int ms);

Since: 3.0

Function: unsigned int gnutls_dtls_get_timeout (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function will return the milliseconds remaining for a retransmission of the previously sent handshake message. This function is useful when DTLS is used in non-blocking mode, to estimate when to call gnutls_handshake() if no packets have been received.

Returns: the remaining time in milliseconds.

Since: 3.0


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6.5.1 Asynchronous operation

GnuTLS can be used with asynchronous socket or event-driven programming. The approach is similar to using Berkeley sockets under such an environment. The blocking, due to network interaction, calls such as gnutls_handshake, gnutls_record_recv, can be set to non-blocking by setting the underlying sockets to non-blocking. If other push and pull functions are setup, then they should behave the same way as recv and send when used in a non-blocking way, i.e., set errno to EAGAIN. Since, during a TLS protocol session GnuTLS does not block except for network interaction, the non blocking EAGAIN errno will be propagated and GnuTLS functions will return the GNUTLS_E_AGAIN error code. Such calls can be resumed the same way as a system call would. The only exception is gnutls_record_send, which if interrupted subsequent calls need not to include the data to be sent (can be called with NULL argument).

The select system call can also be used in combination with the GnuTLS functions. select allows monitoring of sockets and notifies on them being ready for reading or writing data. Note however that this system call cannot notify on data present in GnuTLS read buffers, it is only applicable to the kernel sockets API. Thus if you are using it for reading from a GnuTLS session, make sure that any cached data are read completely. That can be achieved by checking there are no data waiting to be read (using gnutls_record_check_pending), either before the select system call, or after a call to gnutls_record_recv. GnuTLS does not keep a write buffer, thus when writing no additional actions are required.

Although in the TLS protocol implementation each call to receive or send function implies to restoring the same function that was interrupted, in the DTLS protocol this requirement isn’t true. There are cases where a retransmission is required, which are indicated by a received message and thus gnutls_record_get_direction must be called to decide which direction to check prior to restoring a function call.

Function: int gnutls_record_get_direction (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function provides information about the internals of the record protocol and is only useful if a prior gnutls function call (e.g. gnutls_handshake() ) was interrupted for some reason, that is, if a function returned GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN . In such a case, you might want to call select() or poll() before calling the interrupted gnutls function again. To tell you whether a file descriptor should be selected for either reading or writing, gnutls_record_get_direction() returns 0 if the interrupted function was trying to read data, and 1 if it was trying to write data.

Returns: 0 if trying to read data, 1 if trying to write data.

Moreover, to prevent blocking from DTLS’ retransmission timers to block a handshake, the gnutls_init function should be called with the GNUTLS_NONBLOCK flag set (see Session initialization). In that case, in order to be able to use the DTLS handshake timers, the function gnutls_dtls_get_timeout should be used to estimate when to call gnutls_handshake if no packets have been received.


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6.5.2 DTLS sessions

Because datagram TLS can operate over connections where the client cannot be reliably verified, functionality in the form of cookies, is available to prevent denial of service attacks to servers. GnuTLS requires a server to generate a secret key that is used to sign a cookie10. That cookie is sent to the client using gnutls_dtls_cookie_send, and the client must reply using the correct cookie. The server side should verify the initial message sent by client using gnutls_dtls_cookie_verify. If successful the session should be initialized and associated with the cookie using gnutls_dtls_prestate_set, before proceeding to the handshake.

int gnutls_key_generate (gnutls_datum_t * key, unsigned int key_size)
int gnutls_dtls_cookie_send (gnutls_datum_t * key, void * client_data, size_t client_data_size, gnutls_dtls_prestate_st * prestate, gnutls_transport_ptr_t ptr, gnutls_push_func push_func)
int gnutls_dtls_cookie_verify (gnutls_datum_t * key, void * client_data, size_t client_data_size, void * _msg, size_t msg_size, gnutls_dtls_prestate_st * prestate)
void gnutls_dtls_prestate_set (gnutls_session_t session, gnutls_dtls_prestate_st * prestate)

Note that the above apply to server side only and they are not mandatory to be used. Not using them, however, allows denial of service attacks. The client side cookie handling is part of gnutls_handshake.

Datagrams are typically restricted by a maximum transfer unit (MTU). For that both client and server side should set the correct maximum transfer unit for the layer underneath GnuTLS. This will allow proper fragmentation of DTLS messages and prevent messages from being silently discarded by the transport layer. The “correct” maximum transfer unit can be obtained through a path MTU discovery mechanism [RFC4821].

void gnutls_dtls_set_mtu (gnutls_session_t session, unsigned int mtu)
unsigned int gnutls_dtls_get_mtu (gnutls_session_t session)
unsigned int gnutls_dtls_get_data_mtu (gnutls_session_t session)

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6.6 TLS handshake

Once a session has been initialized and a network connection has been set up, TLS and DTLS protocols perform a handshake. The handshake is the actual key exchange.

Function: int gnutls_handshake (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function does the handshake of the TLS/SSL protocol, and initializes the TLS connection.

This function will fail if any problem is encountered, and will return a negative error code. In case of a client, if the client has asked to resume a session, but the server couldn’t, then a full handshake will be performed.

The non-fatal errors such as GNUTLS_E_AGAIN and GNUTLS_E_INTERRUPTED interrupt the handshake procedure, which should be resumed later. Call this function again, until it returns 0; cf. gnutls_record_get_direction() and gnutls_error_is_fatal() .

If this function is called by a server after a rehandshake request then GNUTLS_E_GOT_APPLICATION_DATA or GNUTLS_E_WARNING_ALERT_RECEIVED may be returned. Note that these are non fatal errors, only in the specific case of a rehandshake. Their meaning is that the client rejected the rehandshake request or in the case of GNUTLS_E_GOT_APPLICATION_DATA it might also mean that some data were pending.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code.

Function: void gnutls_handshake_set_timeout (gnutls_session_t session, unsigned int ms)

session: is a gnutls_session_t structure.

ms: is a timeout value in milliseconds

This function sets the timeout for the handshake process to the provided value. Use an ms value of zero to disable timeout.

Note that in order for the timeout to be enforced gnutls_transport_set_pull_timeout_function() must be set (it is set by default in most systems).

The handshake process doesn’t ensure the verification of the peer’s identity. When certificates are in use, this can be done, either after the handshake is complete, or during the handshake if gnutls_certificate_set_verify_function has been used. In both cases the gnutls_certificate_verify_peers2 function can be used to verify the peer’s certificate (see Certificate authentication for more information).

int gnutls_certificate_verify_peers2 (gnutls_session_t session, unsigned int * status)

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6.7 Data transfer and termination

Once the handshake is complete and peer’s identity has been verified data can be exchanged. The available functions resemble the POSIX recv and send functions. It is suggested to use gnutls_error_is_fatal to check whether the error codes returned by these functions are fatal for the protocol or can be ignored.

Function: ssize_t gnutls_record_send (gnutls_session_t session, const void * data, size_t data_size)

session: is a gnutls_session_t structure.

data: contains the data to send

data_size: is the length of the data

This function has the similar semantics with send() . The only difference is that it accepts a GnuTLS session, and uses different error codes. Note that if the send buffer is full, send() will block this function. See the send() documentation for full information. You can replace the default push function by using gnutls_transport_set_ptr2() with a call to send() with a MSG_DONTWAIT flag if blocking is a problem. If the EINTR is returned by the internal push function (the default is send() ) then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again, with the same parameters; alternatively you could provide a NULL pointer for data, and 0 for size. cf. gnutls_record_get_direction() .

Note that in DTLS this function will return the GNUTLS_E_LARGE_PACKET error code if the send data exceed the data MTU value - as returned by gnutls_dtls_get_data_mtu() . The errno value EMSGSIZE also maps to GNUTLS_E_LARGE_PACKET .

Returns: The number of bytes sent, or a negative error code. The number of bytes sent might be less than data_size . The maximum number of bytes this function can send in a single call depends on the negotiated maximum record size.

Function: ssize_t gnutls_record_recv (gnutls_session_t session, void * data, size_t data_size)

session: is a gnutls_session_t structure.

data: the buffer that the data will be read into

data_size: the number of requested bytes

This function has the similar semantics with recv() . The only difference is that it accepts a GnuTLS session, and uses different error codes. In the special case that a server requests a renegotiation, the client may receive an error code of GNUTLS_E_REHANDSHAKE . This message may be simply ignored, replied with an alert GNUTLS_A_NO_RENEGOTIATION , or replied with a new handshake, depending on the client’s will. If EINTR is returned by the internal push function (the default is recv() ) then GNUTLS_E_INTERRUPTED will be returned. If GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN is returned, you must call this function again to get the data. See also gnutls_record_get_direction() . A server may also receive GNUTLS_E_REHANDSHAKE when a client has initiated a handshake. In that case the server can only initiate a handshake or terminate the connection.

Returns: The number of bytes received and zero on EOF (for stream connections). A negative error code is returned in case of an error. The number of bytes received might be less than the requested data_size .

Function: int gnutls_error_is_fatal (int error)

error: is a GnuTLS error code, a negative error code

If a GnuTLS function returns a negative error code you may feed that value to this function to see if the error condition is fatal to a TLS session (i.e., must be terminated).

Note that you may also want to check the error code manually, since some non-fatal errors to the protocol (such as a warning alert or a rehandshake request) may be fatal for your program.

This function is only useful if you are dealing with errors from functions that relate to a TLS session (e.g., record layer or handshake layer handling functions).

Returns: zero on non fatal errors or positive error values. Non-zero on fatal error codes.

Although, in the TLS protocol the receive function can be called at any time, when DTLS is used the GnuTLS receive functions must be called once a message is available for reading, even if no data are expected. This is because in DTLS various (internal) actions may be required due to retransmission timers. Moreover, an extended receive function is shown below, which allows the extraction of the message’s sequence number. Due to the unreliable nature of the protocol, this field allows distinguishing out-of-order messages.

Function: ssize_t gnutls_record_recv_seq (gnutls_session_t session, void * data, size_t data_size, unsigned char * seq)

session: is a gnutls_session_t structure.

data: the buffer that the data will be read into

data_size: the number of requested bytes

seq: is the packet’s 64-bit sequence number. Should have space for 8 bytes.

This function is the same as gnutls_record_recv() , except that it returns in addition to data, the sequence number of the data. This is useful in DTLS where record packets might be received out-of-order. The returned 8-byte sequence number is an integer in big-endian format and should be treated as a unique message identification.

Returns: The number of bytes received and zero on EOF. A negative error code is returned in case of an error. The number of bytes received might be less than data_size .

Since: 3.0

The gnutls_record_check_pending helper function is available to allow checking whether data are available to be read in a GnuTLS session buffers. Note that this function complements but does not replace select, i.e., gnutls_record_check_pending reports no data to be read, select should be called to check for data in the network buffers.

Function: size_t gnutls_record_check_pending (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function checks if there are unread data in the gnutls buffers. If the return value is non-zero the next call to gnutls_record_recv() is guaranteed not to block.

Returns: Returns the size of the data or zero.

int gnutls_record_get_direction (gnutls_session_t session)

Once a TLS or DTLS session is no longer needed, it is recommended to use gnutls_bye to terminate the session. That way the peer is notified securely about the intention of termination, which allows distinguishing it from a malicious connection termination. A session can be deinitialized with the gnutls_deinit function.

Function: int gnutls_bye (gnutls_session_t session, gnutls_close_request_t how)

session: is a gnutls_session_t structure.

how: is an integer

Terminates the current TLS/SSL connection. The connection should have been initiated using gnutls_handshake() . how should be one of GNUTLS_SHUT_RDWR , GNUTLS_SHUT_WR .

In case of GNUTLS_SHUT_RDWR the TLS session gets terminated and further receives and sends will be disallowed. If the return value is zero you may continue using the underlying transport layer. GNUTLS_SHUT_RDWR sends an alert containing a close request and waits for the peer to reply with the same message.

In case of GNUTLS_SHUT_WR the TLS session gets terminated and further sends will be disallowed. In order to reuse the connection you should wait for an EOF from the peer. GNUTLS_SHUT_WR sends an alert containing a close request.

Note that not all implementations will properly terminate a TLS connection. Some of them, usually for performance reasons, will terminate only the underlying transport layer, and thus not distinguishing between a malicious party prematurely terminating the connection and normal termination.

This function may also return GNUTLS_E_AGAIN or GNUTLS_E_INTERRUPTED ; cf. gnutls_record_get_direction() .

Returns: GNUTLS_E_SUCCESS on success, or an error code, see function documentation for entire semantics.

Function: void gnutls_deinit (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function clears all buffers associated with the session . This function will also remove session data from the session database if the session was terminated abnormally.


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6.8 Buffered data transfer

Although gnutls_record_send is sufficient to transmit data to the peer, when many small chunks of data are to be transmitted it is inefficient and wastes bandwidth due to the TLS record overhead. In that case it is preferrable to combine the small chunks before transmission. The following functions provide that functionality.

Function: void gnutls_record_cork (gnutls_session_t session)

session: is a gnutls_session_t structure.

If called gnutls_record_send() will no longer send partial records. All queued records will be sent when gnutls_uncork() is called, or when the maximum record size is reached.

Since: 3.1.9

Function: int gnutls_record_uncork (gnutls_session_t session, unsigned int flags)

session: is a gnutls_session_t structure.

flags: Could be zero or GNUTLS_RECORD_WAIT

This resets the effect of gnutls_cork() , and flushes any pending data. If the GNUTLS_RECORD_WAIT flag is specified then this function will block until the data is sent or a fatal error occurs (i.e., the function will retry on GNUTLS_E_AGAIN and GNUTLS_E_INTERRUPTED ).

If the flag GNUTLS_RECORD_WAIT is not specified and the function is interrupted then the GNUTLS_E_AGAIN or GNUTLS_E_INTERRUPTED errors will be returned. To obtain the data left in the corked buffer use gnutls_record_check_corked() .

Returns: On success the number of transmitted data is returned, or otherwise a negative error code.

Since: 3.1.9


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6.9 Handling alerts

During a TLS connection alert messages may be exchanged by the two peers. Those messages may be fatal, meaning the connection must be terminated afterwards, or warning when something needs to be reported to the peer, but without interrupting the session. The error codes GNUTLS_E_WARNING_ALERT_RECEIVED or GNUTLS_E_FATAL_ALERT_RECEIVED signal those alerts when received, and may be returned by all GnuTLS functions that receive data from the peer, being gnutls_handshake and gnutls_record_recv.

If those error codes are received the alert and its level should be logged or reported to the peer using the functions below.

Function: gnutls_alert_description_t gnutls_alert_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function will return the last alert number received. This function should be called when GNUTLS_E_WARNING_ALERT_RECEIVED or GNUTLS_E_FATAL_ALERT_RECEIVED errors are returned by a gnutls function. The peer may send alerts if he encounters an error. If no alert has been received the returned value is undefined.

Returns: the last alert received, a gnutls_alert_description_t value.

Function: const char * gnutls_alert_get_name (gnutls_alert_description_t alert)

alert: is an alert number.

This function will return a string that describes the given alert number, or NULL . See gnutls_alert_get() .

Returns: string corresponding to gnutls_alert_description_t value.

The peer may also be warned or notified of a fatal issue by using one of the functions below. All the available alerts are listed in The Alert Protocol.

Function: int gnutls_alert_send (gnutls_session_t session, gnutls_alert_level_t level, gnutls_alert_description_t desc)

session: is a gnutls_session_t structure.

level: is the level of the alert

desc: is the alert description

This function will send an alert to the peer in order to inform him of something important (eg. his Certificate could not be verified). If the alert level is Fatal then the peer is expected to close the connection, otherwise he may ignore the alert and continue.

The error code of the underlying record send function will be returned, so you may also receive GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN as well.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

Function: int gnutls_error_to_alert (int err, int * level)

err: is a negative integer

level: the alert level will be stored there

Get an alert depending on the error code returned by a gnutls function. All alerts sent by this function should be considered fatal. The only exception is when err is GNUTLS_E_REHANDSHAKE , where a warning alert should be sent to the peer indicating that no renegotiation will be performed.

If there is no mapping to a valid alert the alert to indicate internal error is returned.

Returns: the alert code to use for a particular error code.


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6.10 Priority strings

The GnuTLS priority string specifies the TLS session’s handshake algorithms and options in a compact, easy-to-use format. That string may contain a single initial keyword such as in Table 6.2 and may be followed by additional algorithm or special keywords.

int gnutls_priority_set_direct (gnutls_session_t session, const char * priorities, const char ** err_pos)
int gnutls_priority_set (gnutls_session_t session, gnutls_priority_t priority)
KeywordDescription
PERFORMANCEAll the known to be secure ciphersuites are enabled, limited to 128 bit ciphers and sorted by terms of speed performance. The message authenticity security level is of 64 bits or more.
NORMALMeans all the known to be secure ciphersuites. The ciphers are sorted by security margin, although the 256-bit ciphers are included as a fallback only. The message authenticity security level is of 64 bits or more.

This priority string implicitly enables DHE and ECDHE.

PFSMeans all the known to be secure ciphersuites that support perfect forward secrecy. The ciphers are sorted by security margin, although the 256-bit ciphers are included as a fallback only. The message authenticity security level is of 64 bits or more. This option is available since 3.2.4 or later.
SECURE128Means all known to be secure ciphersuites that offer a security level 128-bit or more and a message authenticity security level of 80 bits or more.
SECURE192Means all the known to be secure ciphersuites that offer a security level 192-bit or more and a message authenticity security level of 128 bits or more.
SECURE256Currently alias for SECURE192.
SUITEB128Means all the NSA Suite B cryptography (RFC5430) ciphersuites with an 128 bit security level.
SUITEB192Means all the NSA Suite B cryptography (RFC5430) ciphersuites with an 192 bit security level.
EXPORTMeans all ciphersuites are enabled, including the low-security 40 bit ciphers.
NONEMeans nothing is enabled. This disables even protocols and compression methods. It should be followed by the algorithms to be enabled.

Table 6.2: Supported initial keywords.

Unless the initial keyword is "NONE" the defaults (in preference order) are for TLS protocols TLS 1.2, TLS1.1, TLS1.0, SSL3.0; for compression NULL; for certificate types X.509. In key exchange algorithms when in NORMAL or SECURE levels the perfect forward secrecy algorithms take precedence of the other protocols. In all cases all the supported key exchange algorithms are enabled.

Note that the SECURE levels distinguish between overall security level and message authenticity security level. That is because the message authenticity security level requires the adversary to break the algorithms at real-time during the protocol run, whilst the overall security level refers to off-line adversaries (e.g. adversaries breaking the ciphertext years after it was captured).

The NONE keyword, if used, must followed by keywords specifying the algorithms and protocols to be enabled. The other initial keywords do not require, but may be followed by such keywords. All level keywords can be combined, and for example a level of "SECURE256:+SECURE128" is allowed.

The order with which every algorithm or protocol is specified is significant. Algorithms specified before others will take precedence. The supported algorithms and protocols are shown in Table 6.3. To avoid collisions in order to specify a compression algorithm in the priority string you have to prefix it with "COMP-", protocol versions with "VERS-", signature algorithms with "SIGN-" and certificate types with "CTYPE-". All other algorithms don’t need a prefix. Each specified keyword can be prefixed with any of the following characters.

’!’ or ’-’

appended with an algorithm will remove this algorithm.

"+"

appended with an algorithm will add this algorithm.

TypeKeywords
CiphersAES-128-CBC, AES-256-CBC, AES-128-GCM, CAMELLIA-128-CBC, CAMELLIA-256-CBC, ARCFOUR-128, 3DES-CBC ARCFOUR-40. Catch all name is CIPHER-ALL which will add all the algorithms from NORMAL priority.
Key exchangeRSA, DHE-RSA, DHE-DSS, SRP, SRP-RSA, SRP-DSS, PSK, DHE-PSK, ECDHE-RSA, ANON-ECDH, ANON-DH. The Catch all name is KX-ALL which will add all the algorithms from NORMAL priority.

Add !DHE-RSA:!DHE-DSS to the priority string to disable DHE.

MACMD5, SHA1, SHA256, SHA384, AEAD (used with GCM ciphers only). All algorithms from NORMAL priority can be accessed with MAC-ALL.
Compression algorithmsCOMP-NULL, COMP-DEFLATE. Catch all is COMP-ALL.
TLS versionsVERS-SSL3.0, VERS-TLS1.0, VERS-TLS1.1, VERS-TLS1.2, VERS-DTLS1.2, VERS-DTLS1.0. Catch all is VERS-TLS-ALL and VERS-DTLS-ALL.
Signature algorithmsSIGN-RSA-SHA1, SIGN-RSA-SHA224, SIGN-RSA-SHA256, SIGN-RSA-SHA384, SIGN-RSA-SHA512, SIGN-DSA-SHA1, SIGN-DSA-SHA224, SIGN-DSA-SHA256, SIGN-RSA-MD5. Catch all is SIGN-ALL. This is only valid for TLS 1.2 and later.
Elliptic curvesCURVE-SECP192R1, CURVE-SECP224R1, CURVE-SECP256R1, CURVE-SECP384R1, CURVE-SECP521R1. Catch all is CURVE-ALL.

Table 6.3: The supported algorithm keywords in priority strings.

Note that the DHE key exchange methods are generally slower11 than their elliptic curves counterpart (ECDHE). Moreover the plain Diffie-Hellman key exchange requires parameters to be generated and associated with a credentials structure by the server (see Parameter generation).

The available special keywords are shown in Table 6.4 and Table 6.5.

KeywordDescription
%COMPATwill enable compatibility mode. It might mean that violations of the protocols are allowed as long as maximum compatibility with problematic clients and servers is achieved. More specifically this string would disable TLS record random padding, tolerate packets over the maximum allowed TLS record, and add a padding to TLS Client Hello packet to prevent it being in the 256-512 range which is known to be causing issues with a commonly used firewall.
%DUMBFWwill add a private extension with bogus data that make the client hello exceed 512 bytes. This avoids a black hole behavior in some firewalls. This is a non-standard TLS extension, use with care.
%NO_EXTENSIONSwill prevent the sending of any TLS extensions in client side. Note that TLS 1.2 requires extensions to be used, as well as safe renegotiation thus this option must be used with care.
%SERVER_PRECEDENCEThe ciphersuite will be selected according to server priorities and not the client’s.
%SSL3_RECORD_VERSIONwill use SSL3.0 record version in client hello. This is the default.
%LATEST_RECORD_VERSIONwill use the latest TLS version record version in client hello.

Table 6.4: Special priority string keywords.

KeywordDescription
%STATELESS_COMPRESSIONwill disable keeping state across records when compressing. This may help to mitigate attacks when compression is used but an attacker is in control of input data. This has to be used only when the data that are possibly controlled by an attacker are placed in separate records.
%DISABLE_SAFE_RENEGOTIATIONwill completely disable safe renegotiation completely. Do not use unless you know what you are doing.
%UNSAFE_RENEGOTIATIONwill allow handshakes and re-handshakes without the safe renegotiation extension. Note that for clients this mode is insecure (you may be under attack), and for servers it will allow insecure clients to connect (which could be fooled by an attacker). Do not use unless you know what you are doing and want maximum compatibility.
%PARTIAL_RENEGOTIATIONwill allow initial handshakes to proceed, but not re-handshakes. This leaves the client vulnerable to attack, and servers will be compatible with non-upgraded clients for initial handshakes. This is currently the default for clients and servers, for compatibility reasons.
%SAFE_RENEGOTIATIONwill enforce safe renegotiation. Clients and servers will refuse to talk to an insecure peer. Currently this causes interoperability problems, but is required for full protection.
%VERIFY_ALLOW_SIGN_RSA_MD5will allow RSA-MD5 signatures in certificate chains.
%VERIFY_DISABLE_CRL_CHECKSwill disable CRL or OCSP checks in the verification of the certificate chain.
%VERIFY_ALLOW_X509_V1_CA_CRTwill allow V1 CAs in chains.

Table 6.5: More priority string keywords.

Finally the ciphersuites enabled by any priority string can be listed using the gnutls-cli application (see gnutls-cli Invocation), or by using the priority functions as in Listing the ciphersuites in a priority string.

Example priority strings are:

The default priority without the HMAC-MD5:
    "NORMAL:-MD5"

Specifying RSA with AES-128-CBC:
    "NONE:+VERS-TLS-ALL:+MAC-ALL:+RSA:+AES-128-CBC:+SIGN-ALL:+COMP-NULL"

Specifying the defaults except ARCFOUR-128:
    "NORMAL:-ARCFOUR-128"

Enabling the 128-bit secure ciphers, while disabling SSL 3.0 and enabling compression:
    "SECURE128:-VERS-SSL3.0:+COMP-DEFLATE"

Enabling the 128-bit and 192-bit secure ciphers, while disabling all TLS versions 
except TLS 1.2:
    "SECURE128:+SECURE192:-VERS-TLS-ALL:+VERS-TLS1.2"

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6.11 Selecting cryptographic key sizes

Because many algorithms are involved in TLS, it is not easy to set a consistent security level. For this reason in Table 6.6 we present some correspondence between key sizes of symmetric algorithms and public key algorithms based on [ECRYPT]. Those can be used to generate certificates with appropriate key sizes as well as select parameters for Diffie-Hellman and SRP authentication.

Security bitsRSA, DH and SRP parameter sizeECC key sizeSecurity parameterDescription
<72<1008<160INSECUREConsidered to be insecure
721008160WEAKShort term protection against small organizations
801248160LOWVery short term protection against agencies
961776192LEGACYLegacy standard level
1122432224NORMALMedium-term protection
1283248256HIGHLong term protection
25615424512ULTRAForeseeable future

Table 6.6: Key sizes and security parameters.

The first column provides a security parameter in a number of bits. This gives an indication of the number of combinations to be tried by an adversary to brute force a key. For example to test all possible keys in a 112 bit security parameter 2^{112} combinations have to be tried. For today’s technology this is infeasible. The next two columns correlate the security parameter with actual bit sizes of parameters for DH, RSA, SRP and ECC algorithms. A mapping to gnutls_sec_param_t value is given for each security parameter, on the next column, and finally a brief description of the level.

Note, however, that the values suggested here are nothing more than an educated guess that is valid today. There are no guarantees that an algorithm will remain unbreakable or that these values will remain constant in time. There could be scientific breakthroughs that cannot be predicted or total failure of the current public key systems by quantum computers. On the other hand though the cryptosystems used in TLS are selected in a conservative way and such catastrophic breakthroughs or failures are believed to be unlikely. The NIST publication SP 800-57 [NISTSP80057] contains a similar table.

When using GnuTLS and a decision on bit sizes for a public key algorithm is required, use of the following functions is recommended:

Function: unsigned int gnutls_sec_param_to_pk_bits (gnutls_pk_algorithm_t algo, gnutls_sec_param_t param)

algo: is a public key algorithm

param: is a security parameter

When generating private and public key pairs a difficult question is which size of "bits" the modulus will be in RSA and the group size in DSA. The easy answer is 1024, which is also wrong. This function will convert a human understandable security parameter to an appropriate size for the specific algorithm.

Returns: The number of bits, or (0).

Since: 2.12.0

Function: gnutls_sec_param_t gnutls_pk_bits_to_sec_param (gnutls_pk_algorithm_t algo, unsigned int bits)

algo: is a public key algorithm

bits: is the number of bits

This is the inverse of gnutls_sec_param_to_pk_bits() . Given an algorithm and the number of bits, it will return the security parameter. This is a rough indication.

Returns: The security parameter.

Since: 2.12.0

Those functions will convert a human understandable security parameter of gnutls_sec_param_t type, to a number of bits suitable for a public key algorithm.

const char * gnutls_sec_param_get_name (gnutls_sec_param_t param)

The following functions will set the minimum acceptable group size for Diffie-Hellman and SRP authentication.

void gnutls_dh_set_prime_bits (gnutls_session_t session, unsigned int bits)
void gnutls_srp_set_prime_bits (gnutls_session_t session, unsigned int bits)

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6.12 Advanced topics


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6.12.1 Session resumption

Client side

To reduce time and roundtrips spent in a handshake the client can request session resumption from a server that previously shared a session with the client. For that the client has to retrieve and store the session parameters. Before establishing a new session to the same server the parameters must be re-associated with the GnuTLS session using gnutls_session_set_data.

int gnutls_session_get_data2 (gnutls_session_t session, gnutls_datum_t * data)
int gnutls_session_get_id2 (gnutls_session_t session, gnutls_datum_t * session_id)
int gnutls_session_set_data (gnutls_session_t session, const void * session_data, size_t session_data_size)

Keep in mind that sessions will be expired after some time, depending on the server, and a server may choose not to resume a session even when requested to. The expiration is to prevent temporal session keys from becoming long-term keys. Also note that as a client you must enable, using the priority functions, at least the algorithms used in the last session.

Function: int gnutls_session_is_resumed (gnutls_session_t session)

session: is a gnutls_session_t structure.

Check whether session is resumed or not.

Returns: non zero if this session is resumed, or a zero if this is a new session.

Server side

In order to support resumption a server can store the session security parameters in a local database or by using session tickets (see Session tickets) to delegate storage to the client. Because session tickets might not be supported by all clients, servers could combine the two methods.

A storing server needs to specify callback functions to store, retrieve and delete session data. These can be registered with the functions below. The stored sessions in the database can be checked using gnutls_db_check_entry for expiration.

void gnutls_db_set_retrieve_function (gnutls_session_t session, gnutls_db_retr_func retr_func)
void gnutls_db_set_store_function (gnutls_session_t session, gnutls_db_store_func store_func)
void gnutls_db_set_ptr (gnutls_session_t session, void * ptr)
void gnutls_db_set_remove_function (gnutls_session_t session, gnutls_db_remove_func rem_func)
int gnutls_db_check_entry (gnutls_session_t session, gnutls_datum_t session_entry)

A server utilizing tickets should generate ticket encryption and authentication keys using gnutls_session_ticket_key_generate. Those keys should be associated with the GnuTLS session using gnutls_session_ticket_enable_server.

Function: int gnutls_session_ticket_enable_server (gnutls_session_t session, const gnutls_datum_t * key)

session: is a gnutls_session_t structure.

key: key to encrypt session parameters.

Request that the server should attempt session resumption using SessionTicket. key must be initialized with gnutls_session_ticket_key_generate() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, or an error code.

Since: 2.10.0

Function: int gnutls_session_ticket_key_generate (gnutls_datum_t * key)

key: is a pointer to a gnutls_datum_t which will contain a newly created key.

Generate a random key to encrypt security parameters within SessionTicket.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, or an error code.

Since: 2.10.0

Function: int gnutls_session_resumption_requested (gnutls_session_t session)

session: is a gnutls_session_t structure.

Check whether the client has asked for session resumption. This function is valid only on server side.

Returns: non zero if session resumption was asked, or a zero if not.

A server enabling both session tickets and a storage for session data would use session tickets when clients support it and the storage otherwise.


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6.12.2 Certificate verification

In this section the functionality for additional certificate verification methods is listed. These methods are intended to be used in addition to normal PKI verification, in order to reduce the risk of a compromised CA being undetected.

6.12.2.1 Trust on first use

The GnuTLS library includes functionlity to use an SSH-like trust on first use authentication. The available functions to store and verify public keys are listed below.

Function: int gnutls_verify_stored_pubkey (const char * db_name, gnutls_tdb_t tdb, const char * host, const char * service, gnutls_certificate_type_t cert_type, const gnutls_datum_t * cert, unsigned int flags)

db_name: A file specifying the stored keys (use NULL for the default)

tdb: A storage structure or NULL to use the default

host: The peer’s name

service: non-NULL if this key is specific to a service (e.g. http)

cert_type: The type of the certificate

cert: The raw (der) data of the certificate

flags: should be 0.

This function will try to verify the provided certificate using a list of stored public keys. The service field if non-NULL should be a port number.

The retrieve variable if non-null specifies a custom backend for the retrieval of entries. If it is NULL then the default file backend will be used. In POSIX-like systems the file backend uses the $HOME/.gnutls/known_hosts file.

Note that if the custom storage backend is provided the retrieval function should return GNUTLS_E_CERTIFICATE_KEY_MISMATCH if the host/service pair is found but key doesn’t match, GNUTLS_E_NO_CERTIFICATE_FOUND if no such host/service with the given key is found, and 0 if it was found. The storage function should return 0 on success.

Returns: If no associated public key is found then GNUTLS_E_NO_CERTIFICATE_FOUND will be returned. If a key is found but does not match GNUTLS_E_CERTIFICATE_KEY_MISMATCH is returned. On success, GNUTLS_E_SUCCESS (0) is returned, or a negative error value on other errors.

Since: 3.0

Function: int gnutls_store_pubkey (const char * db_name, gnutls_tdb_t tdb, const char * host, const char * service, gnutls_certificate_type_t cert_type, const gnutls_datum_t * cert, time_t expiration, unsigned int flags)

db_name: A file specifying the stored keys (use NULL for the default)

tdb: A storage structure or NULL to use the default

host: The peer’s name

service: non-NULL if this key is specific to a service (e.g. http)

cert_type: The type of the certificate

cert: The data of the certificate

expiration: The expiration time (use 0 to disable expiration)

flags: should be 0.

This function will store the provided certificate to the list of stored public keys. The key will be considered valid until the provided expiration time.

The store variable if non-null specifies a custom backend for the storage of entries. If it is NULL then the default file backend will be used.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

In addition to the above the gnutls_store_commitment can be used to implement a key-pinning architecture as in [KEYPIN]. This provides a way for web server to commit on a public key that is not yet active.

Function: int gnutls_store_commitment (const char * db_name, gnutls_tdb_t tdb, const char * host, const char * service, gnutls_digest_algorithm_t hash_algo, const gnutls_datum_t * hash, time_t expiration, unsigned int flags)

db_name: A file specifying the stored keys (use NULL for the default)

tdb: A storage structure or NULL to use the default

host: The peer’s name

service: non-NULL if this key is specific to a service (e.g. http)

hash_algo: The hash algorithm type

hash: The raw hash

expiration: The expiration time (use 0 to disable expiration)

flags: should be 0.

This function will store the provided hash commitment to the list of stored public keys. The key with the given hash will be considered valid until the provided expiration time.

The store variable if non-null specifies a custom backend for the storage of entries. If it is NULL then the default file backend will be used.

Note that this function is not thread safe with the default backend.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

The storage and verification functions may be used with the default text file based back-end, or another back-end may be specified. That should contain storage and retrieval functions and specified as below.

int gnutls_tdb_init (gnutls_tdb_t * tdb)
void gnutls_tdb_deinit (gnutls_tdb_t tdb)
void gnutls_tdb_set_verify_func (gnutls_tdb_t tdb, gnutls_tdb_verify_func verify)
void gnutls_tdb_set_store_func (gnutls_tdb_t tdb, gnutls_tdb_store_func store)
void gnutls_tdb_set_store_commitment_func (gnutls_tdb_t tdb, gnutls_tdb_store_commitment_func cstore)

6.12.2.2 DANE verification

Since the DANE library is not included in GnuTLS it requires programs to be linked against it. This can be achieved with the following commands.

gcc -o foo foo.c `pkg-config gnutls-dane --cflags --libs`

When a program uses the GNU autoconf system, then the following line or similar can be used to detect the presence of the library.

PKG_CHECK_MODULES([LIBDANE], [gnutls-dane >= 3.0.0])

AC_SUBST([LIBDANE_CFLAGS])
AC_SUBST([LIBDANE_LIBS])

The high level functionality provided by the DANE library is shown below.

Function: int dane_verify_crt (dane_state_t s, const gnutls_datum_t * chain, unsigned chain_size, gnutls_certificate_type_t chain_type, const char * hostname, const char * proto, unsigned int port, unsigned int sflags, unsigned int vflags, unsigned int * verify)

s: A DANE state structure (may be NULL)

chain: A certificate chain

chain_size: The size of the chain

chain_type: The type of the certificate chain

hostname: The hostname associated with the chain

proto: The protocol of the service connecting (e.g. tcp)

port: The port of the service connecting (e.g. 443)

sflags: Flags for the the initialization of s (if NULL)

vflags: Verification flags; an OR’ed list of dane_verify_flags_t .

verify: An OR’ed list of dane_verify_status_t .

This function will verify the given certificate chain against the CA constrains and/or the certificate available via DANE. If no information via DANE can be obtained the flag DANE_VERIFY_NO_DANE_INFO is set. If a DNSSEC signature is not available for the DANE record then the verify flag DANE_VERIFY_NO_DNSSEC_DATA is set.

Note that the CA constraint only applies for the directly certifying CA and does not account for long CA chains.

Due to the many possible options of DANE, there is no single threat model countered. When notifying the user about DANE verification results it may be better to mention: DANE verification did not reject the certificate, rather than mentioning a successful DANE verication.

If the q parameter is provided it will be used for caching entries.

Returns: On success, DANE_E_SUCCESS (0) is returned, otherwise a negative error value.

int dane_verify_session_crt (dane_state_t s, gnutls_session_t session, const char * hostname, const char * proto, unsigned int port, unsigned int sflags, unsigned int vflags, unsigned int * verify)
const char * dane_strerror (int error)

Note that the dane_state_t structure that is accepted by both verification functions is optional. It is required when many queries are performed to facilitate caching. The following flags are returned by the verify functions to indicate the status of the verification.

DANE_VERIFY_CA_CONSTRAINTS_VIOLATED

The CA constrains was violated.

DANE_VERIFY_CERT_DIFFERS

The certificate obtained via DNS differs.

DANE_VERIFY_NO_DANE_INFO

No DANE data were found in the DNS record.

Figure 6.2: The DANE verification status flags.

In order to generate a DANE TLSA entry to use in a DNS server you may use danetool (see danetool Invocation).


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6.12.3 Parameter generation

Several TLS ciphersuites require additional parameters that need to be generated or provided by the application. The Diffie-Hellman based ciphersuites (ANON-DH or DHE), require the group parameters to be provided. Those can either be be generated on the fly using gnutls_dh_params_generate2 or imported from pregenerated data using gnutls_dh_params_import_pkcs3. The parameters can be used in a TLS session by calling gnutls_certificate_set_dh_params or gnutls_anon_set_server_dh_params for anonymous sessions.

int gnutls_dh_params_generate2 (gnutls_dh_params_t params, unsigned int bits)
int gnutls_dh_params_import_pkcs3 (gnutls_dh_params_t params, const gnutls_datum_t * pkcs3_params, gnutls_x509_crt_fmt_t format)
void gnutls_certificate_set_dh_params (gnutls_certificate_credentials_t res, gnutls_dh_params_t dh_params)
void gnutls_anon_set_server_dh_params (gnutls_anon_server_credentials_t res, gnutls_dh_params_t dh_params)

Due to the time-consuming calculations required for the generation of Diffie-Hellman parameters we suggest against performing generation of them within an application. The certtool tool can be used to generate or export known safe values that can be stored in code or in a configuration file to provide the ability to replace. We also recommend the usage of gnutls_sec_param_to_pk_bits (see Selecting cryptographic key sizes) to determine the bit size of the generated parameters.

Note that the information stored in the generated PKCS #3 structure changed with GnuTLS 3.0.9. Since that version the privateValueLength member of the structure is set, allowing the server utilizing the parameters to use keys of the size of the security parameter. This provides better performance in key exchange.

To allow renewal of the parameters within an application without accessing the credentials, which are a shared structure, an alternative interface is available using a callback function.

Function: void gnutls_certificate_set_params_function (gnutls_certificate_credentials_t res, gnutls_params_function * func)

res: is a gnutls_certificate_credentials_t structure

func: is the function to be called

This function will set a callback in order for the server to get the Diffie-Hellman or RSA parameters for certificate authentication. The callback should return GNUTLS_E_SUCCESS (0) on success.


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6.12.4 Deriving keys for other applications/protocols

In several cases, after a TLS connection is established, it is desirable to derive keys to be used in another application or protocol (e.g., in an other TLS session using pre-shared keys). The following describe GnuTLS’ implementation of RFC5705 to extract keys based on a session’s master secret.

The API to use is gnutls_prf. The function needs to be provided with a label, and additional context data to mix in the extra parameter. Moreover, the API allows to switch the mix of the client and server random nonces, using the server_random_first parameter. In typical uses you don’t need it, so a zero value should be provided in server_random_first.

For example, after establishing a TLS session using gnutls_handshake, you can obtain 32-bytes to be used as key, using this call:

#define MYLABEL "EXPORTER-My-protocol-name"
#define MYCONTEXT "my-protocol's-1st-session"

char out[32];
rc = gnutls_prf (session, sizeof(MYLABEL)-1, MYLABEL, 0,
                 sizeof(MYCONTEXT)-1, MYCONTEXT, 32, out);

The output key depends on TLS’ master secret, and is the same on both client and server.

If you don’t want to use the RFC5705 interface and not mix in the client and server random nonces, there is a low-level TLS PRF interface called gnutls_prf_raw.


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6.12.5 Channel bindings

In user authentication protocols (e.g., EAP or SASL mechanisms) it is useful to have a unique string that identifies the secure channel that is used, to bind together the user authentication with the secure channel. This can protect against man-in-the-middle attacks in some situations. That unique string is called a “channel binding”. For background and discussion see [RFC5056].

In GnuTLS you can extract a channel binding using the gnutls_session_channel_binding function. Currently only the type GNUTLS_CB_TLS_UNIQUE is supported, which corresponds to the tls-unique channel binding for TLS defined in [RFC5929].

The following example describes how to print the channel binding data. Note that it must be run after a successful TLS handshake.

{
  gnutls_datum_t cb;
  int rc;

  rc = gnutls_session_channel_binding (session,
                                       GNUTLS_CB_TLS_UNIQUE,
                                       &cb);
  if (rc)
    fprintf (stderr, "Channel binding error: %s\n",
             gnutls_strerror (rc));
  else
    {
      size_t i;
      printf ("- Channel binding 'tls-unique': ");
      for (i = 0; i < cb.size; i++)
        printf ("%02x", cb.data[i]);
      printf ("\n");
    }
}

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6.12.6 Interoperability

The TLS protocols support many ciphersuites, extensions and version numbers. As a result, few implementations are not able to properly interoperate once faced with extensions or version protocols they do not support and understand. The TLS protocol allows for a graceful downgrade to the commonly supported options, but practice shows it is not always implemented correctly.

Because there is no way to achieve maximum interoperability with broken peers without sacrificing security, GnuTLS ignores such peers by default. This might not be acceptable in cases where maximum compatibility is required. Thus we allow enabling compatibility with broken peers using priority strings (see Priority Strings). A conservative priority string that would disable certain TLS protocol options that are known to cause compatibility problems, is shown below.

NORMAL:%COMPAT

For broken peers that do not tolerate TLS version numbers over TLS 1.0 another priority string is:

NORMAL:-VERS-TLS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:%COMPAT

This priority string will in addition to above, only enable SSL 3.0 and TLS 1.0 as protocols.


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6.12.7 Compatibility with the OpenSSL library

To ease GnuTLS’ integration with existing applications, a compatibility layer with the OpenSSL library is included in the gnutls-openssl library. This compatibility layer is not complete and it is not intended to completely re-implement the OpenSSL API with GnuTLS. It only provides limited source-level compatibility.

The prototypes for the compatibility functions are in the gnutls/openssl.h header file. The limitations imposed by the compatibility layer include:


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7 GnuTLS application examples

In this chapter several examples of real-world use cases are listed. The examples are simplified to promote readability and contain little or no error checking.


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7.1 Client examples

This section contains examples of TLS and SSL clients, using GnuTLS. Note that some of the examples require functions implemented by another example.


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7.1.1 Simple client example with X.509 certificate support

Let’s assume now that we want to create a TCP client which communicates with servers that use X.509 or OpenPGP certificate authentication. The following client is a very simple TLS client, which uses the high level verification functions for certificates, but does not support session resumption.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include "examples.h"

/* A very basic TLS client, with X.509 authentication and server certificate
 * verification. Note that error checking for missing files etc. is omitted
 * for simplicity.
 */

#define MAX_BUF 1024
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int tcp_connect(void);
extern void tcp_close(int sd);
static int _verify_certificate_callback(gnutls_session_t session);

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        const char *err;
        gnutls_certificate_credentials_t xcred;

        gnutls_global_init();

        /* X509 stuff */
        gnutls_certificate_allocate_credentials(&xcred);

        /* sets the trusted cas file
         */
        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);
        gnutls_certificate_set_verify_function(xcred,
                                               _verify_certificate_callback);

        /* If client holds a certificate it can be set using the following:
         *
         gnutls_certificate_set_x509_key_file (xcred, 
         "cert.pem", "key.pem", 
         GNUTLS_X509_FMT_PEM); 
         */

        /* Initialize TLS session 
         */
        gnutls_init(&session, GNUTLS_CLIENT);

        gnutls_session_set_ptr(session, (void *) "my_host_name");

        gnutls_server_name_set(session, GNUTLS_NAME_DNS, "my_host_name",
                               strlen("my_host_name"));

        /* Use default priorities */
        ret = gnutls_priority_set_direct(session, "NORMAL", &err);
        if (ret < 0) {
                if (ret == GNUTLS_E_INVALID_REQUEST) {
                        fprintf(stderr, "Syntax error at: %s\n", err);
                }
                exit(1);
        }

        /* put the x509 credentials to the current session
         */
        gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);

        /* connect to the peer
         */
        sd = tcp_connect();

        gnutls_transport_set_int(session, sd);
        gnutls_handshake_set_timeout(session,
                                     GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

        /* Perform the TLS handshake
         */
        do {
                ret = gnutls_handshake(session);
        }
        while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                fprintf(stderr, "*** Warning: %s\n", gnutls_strerror(ret));
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        if (ret > 0) {
                printf("- Received %d bytes: ", ret);
                for (ii = 0; ii < ret; ii++) {
                        fputc(buffer[ii], stdout);
                }
                fputs("\n", stdout);
        }

        gnutls_bye(session, GNUTLS_SHUT_RDWR);

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}

/* This function will verify the peer's certificate, and check
 * if the hostname matches, as well as the activation, expiration dates.
 */
static int _verify_certificate_callback(gnutls_session_t session)
{
        unsigned int status;
        int ret, type;
        const char *hostname;
        gnutls_datum_t out;

        /* read hostname */
        hostname = gnutls_session_get_ptr(session);

        /* This verification function uses the trusted CAs in the credentials
         * structure. So you must have installed one or more CA certificates.
         */
        ret = gnutls_certificate_verify_peers3(session, hostname, &status);
        if (ret < 0) {
                printf("Error\n");
                return GNUTLS_E_CERTIFICATE_ERROR;
        }

        type = gnutls_certificate_type_get(session);

        ret =
            gnutls_certificate_verification_status_print(status, type,
                                                         &out, 0);
        if (ret < 0) {
                printf("Error\n");
                return GNUTLS_E_CERTIFICATE_ERROR;
        }

        printf("%s", out.data);

        gnutls_free(out.data);

        if (status != 0)        /* Certificate is not trusted */
                return GNUTLS_E_CERTIFICATE_ERROR;

        /* notify gnutls to continue handshake normally */
        return 0;
}

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7.1.2 Simple client example with SSH-style certificate verification

This is an alternative verification function that will use the X.509 certificate authorities for verification, but also assume an trust on first use (SSH-like) authentication system. That is the user is prompted on unknown public keys and known public keys are considered trusted.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include "examples.h"

/* This function will verify the peer's certificate, check
 * if the hostname matches. In addition it will perform an
 * SSH-style authentication, where ultimately trusted keys
 * are only the keys that have been seen before.
 */
int _ssh_verify_certificate_callback(gnutls_session_t session)
{
        unsigned int status;
        const gnutls_datum_t *cert_list;
        unsigned int cert_list_size;
        int ret, type;
        gnutls_datum_t out;
        const char *hostname;

        /* read hostname */
        hostname = gnutls_session_get_ptr(session);

        /* This verification function uses the trusted CAs in the credentials
         * structure. So you must have installed one or more CA certificates.
         */
        ret = gnutls_certificate_verify_peers3(session, hostname, &status);
        if (ret < 0) {
                printf("Error\n");
                return GNUTLS_E_CERTIFICATE_ERROR;
        }

        type = gnutls_certificate_type_get(session);

        ret =
            gnutls_certificate_verification_status_print(status, type,
                                                         &out, 0);
        if (ret < 0) {
                printf("Error\n");
                return GNUTLS_E_CERTIFICATE_ERROR;
        }

        printf("%s", out.data);

        gnutls_free(out.data);

        if (status != 0)        /* Certificate is not trusted */
                return GNUTLS_E_CERTIFICATE_ERROR;

        /* Do SSH verification */
        cert_list = gnutls_certificate_get_peers(session, &cert_list_size);
        if (cert_list == NULL) {
                printf("No certificate was found!\n");
                return GNUTLS_E_CERTIFICATE_ERROR;
        }

        /* service may be obtained alternatively using getservbyport() */
        ret = gnutls_verify_stored_pubkey(NULL, NULL, hostname, "https",
                                          type, &cert_list[0], 0);
        if (ret == GNUTLS_E_NO_CERTIFICATE_FOUND) {
                printf("Host %s is not known.", hostname);
                if (status == 0)
                        printf("Its certificate is valid for %s.\n",
                               hostname);

                /* the certificate must be printed and user must be asked on
                 * whether it is trustworthy. --see gnutls_x509_crt_print() */

                /* if not trusted */
                return GNUTLS_E_CERTIFICATE_ERROR;
        } else if (ret == GNUTLS_E_CERTIFICATE_KEY_MISMATCH) {
                printf
                    ("Warning: host %s is known but has another key associated.",
                     hostname);
                printf
                    ("It might be that the server has multiple keys, or you are under attack\n");
                if (status == 0)
                        printf("Its certificate is valid for %s.\n",
                               hostname);

                /* the certificate must be printed and user must be asked on
                 * whether it is trustworthy. --see gnutls_x509_crt_print() */

                /* if not trusted */
                return GNUTLS_E_CERTIFICATE_ERROR;
        } else if (ret < 0) {
                printf("gnutls_verify_stored_pubkey: %s\n",
                       gnutls_strerror(ret));
                return ret;
        }

        /* user trusts the key -> store it */
        if (ret != 0) {
                ret = gnutls_store_pubkey(NULL, NULL, hostname, "https",
                                          type, &cert_list[0], 0, 0);
                if (ret < 0)
                        printf("gnutls_store_pubkey: %s\n",
                               gnutls_strerror(ret));
        }

        /* notify gnutls to continue handshake normally */
        return 0;
}

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7.1.3 Simple client example with anonymous authentication

The simplest client using TLS is the one that doesn’t do any authentication. This means no external certificates or passwords are needed to set up the connection. As could be expected, the connection is vulnerable to man-in-the-middle (active or redirection) attacks. However, the data are integrity protected and encrypted from passive eavesdroppers.

Note that due to the vulnerable nature of this method very few public servers support it.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

/* A very basic TLS client, with anonymous authentication.
 */

#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int tcp_connect(void);
extern void tcp_close(int sd);

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        gnutls_anon_client_credentials_t anoncred;
        /* Need to enable anonymous KX specifically. */

        gnutls_global_init();

        gnutls_anon_allocate_client_credentials(&anoncred);

        /* Initialize TLS session 
         */
        gnutls_init(&session, GNUTLS_CLIENT);

        /* Use default priorities */
        gnutls_priority_set_direct(session,
                                   "PERFORMANCE:+ANON-ECDH:+ANON-DH",
                                   NULL);

        /* put the anonymous credentials to the current session
         */
        gnutls_credentials_set(session, GNUTLS_CRD_ANON, anoncred);

        /* connect to the peer
         */
        sd = tcp_connect();

        gnutls_transport_set_int(session, sd);
        gnutls_handshake_set_timeout(session,
                                     GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

        /* Perform the TLS handshake
         */
        do {
                ret = gnutls_handshake(session);
        }
        while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                fprintf(stderr, "*** Warning: %s\n", gnutls_strerror(ret));
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        if (ret > 0) {
                printf("- Received %d bytes: ", ret);
                for (ii = 0; ii < ret; ii++) {
                        fputc(buffer[ii], stdout);
                }
                fputs("\n", stdout);
        }

        gnutls_bye(session, GNUTLS_SHUT_RDWR);

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_anon_free_client_credentials(anoncred);

        gnutls_global_deinit();

        return 0;
}

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7.1.4 Simple datagram TLS client example

This is a client that uses UDP to connect to a server. This is the DTLS equivalent to the TLS example with X.509 certificates.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/dtls.h>

/* A very basic Datagram TLS client, over UDP with X.509 authentication.
 */

#define MAX_BUF 1024
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int udp_connect(void);
extern void udp_close(int sd);
extern int verify_certificate_callback(gnutls_session_t session);

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        const char *err;
        gnutls_certificate_credentials_t xcred;

        gnutls_global_init();

        /* X509 stuff */
        gnutls_certificate_allocate_credentials(&xcred);

        /* sets the trusted cas file */
        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);
        gnutls_certificate_set_verify_function(xcred,
                                               verify_certificate_callback);

        /* Initialize TLS session */
        gnutls_init(&session, GNUTLS_CLIENT | GNUTLS_DATAGRAM);

        /* Use default priorities */
        ret = gnutls_priority_set_direct(session, "NORMAL", &err);
        if (ret < 0) {
                if (ret == GNUTLS_E_INVALID_REQUEST) {
                        fprintf(stderr, "Syntax error at: %s\n", err);
                }
                exit(1);
        }

        /* put the x509 credentials to the current session */
        gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);
        gnutls_server_name_set(session, GNUTLS_NAME_DNS, "my_host_name",
                               strlen("my_host_name"));

        /* connect to the peer */
        sd = udp_connect();

        gnutls_transport_set_int(session, sd);

        /* set the connection MTU */
        gnutls_dtls_set_mtu(session, 1000);
        gnutls_handshake_set_timeout(session,
                                     GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

        /* Perform the TLS handshake */
        do {
                ret = gnutls_handshake(session);
        }
        while (ret == GNUTLS_E_INTERRUPTED || ret == GNUTLS_E_AGAIN);
        /* Note that DTLS may also receive GNUTLS_E_LARGE_PACKET */

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                fprintf(stderr, "*** Warning: %s\n", gnutls_strerror(ret));
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        if (ret > 0) {
                printf("- Received %d bytes: ", ret);
                for (ii = 0; ii < ret; ii++) {
                        fputc(buffer[ii], stdout);
                }
                fputs("\n", stdout);
        }

        /* It is suggested not to use GNUTLS_SHUT_RDWR in DTLS
         * connections because the peer's closure message might
         * be lost */
        gnutls_bye(session, GNUTLS_SHUT_WR);

      end:

        udp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}

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7.1.5 Obtaining session information

Most of the times it is desirable to know the security properties of the current established session. This includes the underlying ciphers and the protocols involved. That is the purpose of the following function. Note that this function will print meaningful values only if called after a successful gnutls_handshake.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

#include "examples.h"

/* This function will print some details of the
 * given session.
 */
int print_info(gnutls_session_t session)
{
        const char *tmp;
        gnutls_credentials_type_t cred;
        gnutls_kx_algorithm_t kx;
        int dhe, ecdh;

        dhe = ecdh = 0;

        /* print the key exchange's algorithm name
         */
        kx = gnutls_kx_get(session);
        tmp = gnutls_kx_get_name(kx);
        printf("- Key Exchange: %s\n", tmp);

        /* Check the authentication type used and switch
         * to the appropriate.
         */
        cred = gnutls_auth_get_type(session);
        switch (cred) {
        case GNUTLS_CRD_IA:
                printf("- TLS/IA session\n");
                break;


#ifdef ENABLE_SRP
        case GNUTLS_CRD_SRP:
                printf("- SRP session with username %s\n",
                       gnutls_srp_server_get_username(session));
                break;
#endif

        case GNUTLS_CRD_PSK:
                /* This returns NULL in server side.
                 */
                if (gnutls_psk_client_get_hint(session) != NULL)
                        printf("- PSK authentication. PSK hint '%s'\n",
                               gnutls_psk_client_get_hint(session));
                /* This returns NULL in client side.
                 */
                if (gnutls_psk_server_get_username(session) != NULL)
                        printf("- PSK authentication. Connected as '%s'\n",
                               gnutls_psk_server_get_username(session));

                if (kx == GNUTLS_KX_ECDHE_PSK)
                        ecdh = 1;
                else if (kx == GNUTLS_KX_DHE_PSK)
                        dhe = 1;
                break;

        case GNUTLS_CRD_ANON:  /* anonymous authentication */

                printf("- Anonymous authentication.\n");
                if (kx == GNUTLS_KX_ANON_ECDH)
                        ecdh = 1;
                else if (kx == GNUTLS_KX_ANON_DH)
                        dhe = 1;
                break;

        case GNUTLS_CRD_CERTIFICATE:   /* certificate authentication */

                /* Check if we have been using ephemeral Diffie-Hellman.
                 */
                if (kx == GNUTLS_KX_DHE_RSA || kx == GNUTLS_KX_DHE_DSS)
                        dhe = 1;
                else if (kx == GNUTLS_KX_ECDHE_RSA
                         || kx == GNUTLS_KX_ECDHE_ECDSA)
                        ecdh = 1;

                /* if the certificate list is available, then
                 * print some information about it.
                 */
                print_x509_certificate_info(session);

        }                       /* switch */

        if (ecdh != 0)
                printf("- Ephemeral ECDH using curve %s\n",
                       gnutls_ecc_curve_get_name(gnutls_ecc_curve_get
                                                 (session)));
        else if (dhe != 0)
                printf("- Ephemeral DH using prime of %d bits\n",
                       gnutls_dh_get_prime_bits(session));

        /* print the protocol's name (ie TLS 1.0) 
         */
        tmp =
            gnutls_protocol_get_name(gnutls_protocol_get_version(session));
        printf("- Protocol: %s\n", tmp);

        /* print the certificate type of the peer.
         * ie X.509
         */
        tmp =
            gnutls_certificate_type_get_name(gnutls_certificate_type_get
                                             (session));

        printf("- Certificate Type: %s\n", tmp);

        /* print the compression algorithm (if any)
         */
        tmp = gnutls_compression_get_name(gnutls_compression_get(session));
        printf("- Compression: %s\n", tmp);

        /* print the name of the cipher used.
         * ie 3DES.
         */
        tmp = gnutls_cipher_get_name(gnutls_cipher_get(session));
        printf("- Cipher: %s\n", tmp);

        /* Print the MAC algorithms name.
         * ie SHA1
         */
        tmp = gnutls_mac_get_name(gnutls_mac_get(session));
        printf("- MAC: %s\n", tmp);

        return 0;
}

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7.1.6 Using a callback to select the certificate to use

There are cases where a client holds several certificate and key pairs, and may not want to load all of them in the credentials structure. The following example demonstrates the use of the certificate selection callback.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <gnutls/abstract.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>

/* A TLS client that loads the certificate and key.
 */

#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"

#define CERT_FILE "cert.pem"
#define KEY_FILE "key.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"

extern int tcp_connect(void);
extern void tcp_close(int sd);

static int
cert_callback(gnutls_session_t session,
              const gnutls_datum_t * req_ca_rdn, int nreqs,
              const gnutls_pk_algorithm_t * sign_algos,
              int sign_algos_length, gnutls_pcert_st ** pcert,
              unsigned int *pcert_length, gnutls_privkey_t * pkey);

gnutls_pcert_st pcrt;
gnutls_privkey_t key;

/* Load the certificate and the private key.
 */
static void load_keys(void)
{
        int ret;
        gnutls_datum_t data;

        ret = gnutls_load_file(CERT_FILE, &data);
        if (ret < 0) {
                fprintf(stderr, "*** Error loading certificate file.\n");
                exit(1);
        }

        ret =
            gnutls_pcert_import_x509_raw(&pcrt, &data, GNUTLS_X509_FMT_PEM,
                                         0);
        if (ret < 0) {
                fprintf(stderr, "*** Error loading certificate file: %s\n",
                        gnutls_strerror(ret));
                exit(1);
        }

        gnutls_free(data.data);

        ret = gnutls_load_file(KEY_FILE, &data);
        if (ret < 0) {
                fprintf(stderr, "*** Error loading key file.\n");
                exit(1);
        }

        gnutls_privkey_init(&key);

        ret =
            gnutls_privkey_import_x509_raw(key, &data, GNUTLS_X509_FMT_PEM,
                                           NULL, 0);
        if (ret < 0) {
                fprintf(stderr, "*** Error loading key file: %s\n",
                        gnutls_strerror(ret));
                exit(1);
        }

        gnutls_free(data.data);
}

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        gnutls_priority_t priorities_cache;
        char buffer[MAX_BUF + 1];
        gnutls_certificate_credentials_t xcred;
        /* Allow connections to servers that have OpenPGP keys as well.
         */

        gnutls_global_init();

        load_keys();

        /* X509 stuff */
        gnutls_certificate_allocate_credentials(&xcred);

        /* priorities */
        gnutls_priority_init(&priorities_cache, "NORMAL", NULL);


        /* sets the trusted cas file
         */
        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);

        gnutls_certificate_set_retrieve_function2(xcred, cert_callback);

        /* Initialize TLS session 
         */
        gnutls_init(&session, GNUTLS_CLIENT);

        /* Use default priorities */
        gnutls_priority_set(session, priorities_cache);

        /* put the x509 credentials to the current session
         */
        gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);

        /* connect to the peer
         */
        sd = tcp_connect();

        gnutls_transport_set_int(session, sd);

        /* Perform the TLS handshake
         */
        ret = gnutls_handshake(session);

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        printf("- Received %d bytes: ", ret);
        for (ii = 0; ii < ret; ii++) {
                fputc(buffer[ii], stdout);
        }
        fputs("\n", stdout);

        gnutls_bye(session, GNUTLS_SHUT_RDWR);

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);
        gnutls_priority_deinit(priorities_cache);

        gnutls_global_deinit();

        return 0;
}



/* This callback should be associated with a session by calling
 * gnutls_certificate_client_set_retrieve_function( session, cert_callback),
 * before a handshake.
 */

static int
cert_callback(gnutls_session_t session,
              const gnutls_datum_t * req_ca_rdn, int nreqs,
              const gnutls_pk_algorithm_t * sign_algos,
              int sign_algos_length, gnutls_pcert_st ** pcert,
              unsigned int *pcert_length, gnutls_privkey_t * pkey)
{
        char issuer_dn[256];
        int i, ret;
        size_t len;
        gnutls_certificate_type_t type;

        /* Print the server's trusted CAs
         */
        if (nreqs > 0)
                printf("- Server's trusted authorities:\n");
        else
                printf
                    ("- Server did not send us any trusted authorities names.\n");

        /* print the names (if any) */
        for (i = 0; i < nreqs; i++) {
                len = sizeof(issuer_dn);
                ret = gnutls_x509_rdn_get(&req_ca_rdn[i], issuer_dn, &len);
                if (ret >= 0) {
                        printf("   [%d]: ", i);
                        printf("%s\n", issuer_dn);
                }
        }

        /* Select a certificate and return it.
         * The certificate must be of any of the "sign algorithms"
         * supported by the server.
         */
        type = gnutls_certificate_type_get(session);
        if (type == GNUTLS_CRT_X509) {
                *pcert_length = 1;
                *pcert = &pcrt;
                *pkey = key;
        } else {
                return -1;
        }

        return 0;

}

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7.1.7 Verifying a certificate

An example is listed below which uses the high level verification functions to verify a given certificate list.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

#include "examples.h"

/* All the available CRLs
 */
gnutls_x509_crl_t *crl_list;
int crl_list_size;

/* All the available trusted CAs
 */
gnutls_x509_crt_t *ca_list;
int ca_list_size;

static int print_details_func(gnutls_x509_crt_t cert,
                              gnutls_x509_crt_t issuer,
                              gnutls_x509_crl_t crl,
                              unsigned int verification_output);

/* This function will try to verify the peer's certificate chain, and
 * also check if the hostname matches.
 */
void
verify_certificate_chain(const char *hostname,
                         const gnutls_datum_t * cert_chain,
                         int cert_chain_length)
{
        int i;
        gnutls_x509_trust_list_t tlist;
        gnutls_x509_crt_t *cert;

        unsigned int output;

        /* Initialize the trusted certificate list. This should be done
         * once on initialization. gnutls_x509_crt_list_import2() and
         * gnutls_x509_crl_list_import2() can be used to load them.
         */
        gnutls_x509_trust_list_init(&tlist, 0);

        gnutls_x509_trust_list_add_cas(tlist, ca_list, ca_list_size, 0);
        gnutls_x509_trust_list_add_crls(tlist, crl_list, crl_list_size,
                                        GNUTLS_TL_VERIFY_CRL, 0);

        cert = malloc(sizeof(*cert) * cert_chain_length);

        /* Import all the certificates in the chain to
         * native certificate format.
         */
        for (i = 0; i < cert_chain_length; i++) {
                gnutls_x509_crt_init(&cert[i]);
                gnutls_x509_crt_import(cert[i], &cert_chain[i],
                                       GNUTLS_X509_FMT_DER);
        }

        gnutls_x509_trust_list_verify_named_crt(tlist, cert[0], hostname,
                                                strlen(hostname),
                                                GNUTLS_VERIFY_DISABLE_CRL_CHECKS,
                                                &output,
                                                print_details_func);

        /* if this certificate is not explicitly trusted verify against CAs 
         */
        if (output != 0) {
                gnutls_x509_trust_list_verify_crt(tlist, cert,
                                                  cert_chain_length, 0,
                                                  &output,
                                                  print_details_func);
        }

        if (output & GNUTLS_CERT_INVALID) {
                fprintf(stderr, "Not trusted");

                if (output & GNUTLS_CERT_SIGNER_NOT_FOUND)
                        fprintf(stderr, ": no issuer was found");
                if (output & GNUTLS_CERT_SIGNER_NOT_CA)
                        fprintf(stderr, ": issuer is not a CA");
                if (output & GNUTLS_CERT_NOT_ACTIVATED)
                        fprintf(stderr, ": not yet activated\n");
                if (output & GNUTLS_CERT_EXPIRED)
                        fprintf(stderr, ": expired\n");

                fprintf(stderr, "\n");
        } else
                fprintf(stderr, "Trusted\n");

        /* Check if the name in the first certificate matches our destination!
         */
        if (!gnutls_x509_crt_check_hostname(cert[0], hostname)) {
                printf
                    ("The certificate's owner does not match hostname '%s'\n",
                     hostname);
        }

        gnutls_x509_trust_list_deinit(tlist, 1);

        return;
}

static int
print_details_func(gnutls_x509_crt_t cert,
                   gnutls_x509_crt_t issuer, gnutls_x509_crl_t crl,
                   unsigned int verification_output)
{
        char name[512];
        char issuer_name[512];
        size_t name_size;
        size_t issuer_name_size;

        issuer_name_size = sizeof(issuer_name);
        gnutls_x509_crt_get_issuer_dn(cert, issuer_name,
                                      &issuer_name_size);

        name_size = sizeof(name);
        gnutls_x509_crt_get_dn(cert, name, &name_size);

        fprintf(stdout, "\tSubject: %s\n", name);
        fprintf(stdout, "\tIssuer: %s\n", issuer_name);

        if (issuer != NULL) {
                issuer_name_size = sizeof(issuer_name);
                gnutls_x509_crt_get_dn(issuer, issuer_name,
                                       &issuer_name_size);

                fprintf(stdout, "\tVerified against: %s\n", issuer_name);
        }

        if (crl != NULL) {
                issuer_name_size = sizeof(issuer_name);
                gnutls_x509_crl_get_issuer_dn(crl, issuer_name,
                                              &issuer_name_size);

                fprintf(stdout, "\tVerified against CRL of: %s\n",
                        issuer_name);
        }

        fprintf(stdout, "\tVerification output: %x\n\n",
                verification_output);

        return 0;
}

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7.1.8 Using a smart card with TLS

This example will demonstrate how to load keys and certificates from a smart-card or any other PKCS #11 token, and use it in a TLS connection.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <gnutls/pkcs11.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <getpass.h>            /* for getpass() */

/* A TLS client that loads the certificate and key.
 */

#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"
#define MIN(x,y) (((x)<(y))?(x):(y))

#define CAFILE "/etc/ssl/certs/ca-certificates.crt"

/* The URLs of the objects can be obtained
 * using p11tool --list-all --login
 */
#define KEY_URL "pkcs11:manufacturer=SomeManufacturer;object=Private%20Key" \
  ";objecttype=private;id=%db%5b%3e%b5%72%33"
#define CERT_URL "pkcs11:manufacturer=SomeManufacturer;object=Certificate;" \
  "objecttype=cert;id=db%5b%3e%b5%72%33"

extern int tcp_connect(void);
extern void tcp_close(int sd);

static int
pin_callback(void *user, int attempt, const char *token_url,
             const char *token_label, unsigned int flags, char *pin,
             size_t pin_max)
{
        const char *password;
        int len;

        printf("PIN required for token '%s' with URL '%s'\n", token_label,
               token_url);
        if (flags & GNUTLS_PIN_FINAL_TRY)
                printf("*** This is the final try before locking!\n");
        if (flags & GNUTLS_PIN_COUNT_LOW)
                printf("*** Only few tries left before locking!\n");
        if (flags & GNUTLS_PIN_WRONG)
                printf("*** Wrong PIN\n");

        password = getpass("Enter pin: ");
        if (password == NULL || password[0] == 0) {
                fprintf(stderr, "No password given\n");
                exit(1);
        }

        len = MIN(pin_max - 1, strlen(password));
        memcpy(pin, password, len);
        pin[len] = 0;

        return 0;
}

int main(void)
{
        int ret, sd, ii;
        gnutls_session_t session;
        gnutls_priority_t priorities_cache;
        char buffer[MAX_BUF + 1];
        gnutls_certificate_credentials_t xcred;
        /* Allow connections to servers that have OpenPGP keys as well.
         */

        gnutls_global_init();
        /* PKCS11 private key operations might require PIN.
         * Register a callback.
         */
        gnutls_pkcs11_set_pin_function(pin_callback, NULL);

        /* X509 stuff */
        gnutls_certificate_allocate_credentials(&xcred);

        /* priorities */
        gnutls_priority_init(&priorities_cache, "NORMAL", NULL);

        /* sets the trusted cas file
         */
        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);

        gnutls_certificate_set_x509_key_file(xcred, CERT_URL, KEY_URL,
                                             GNUTLS_X509_FMT_DER);

        /* Initialize TLS session
         */
        gnutls_init(&session, GNUTLS_CLIENT);

        /* Use default priorities */
        gnutls_priority_set(session, priorities_cache);

        /* put the x509 credentials to the current session
         */
        gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, xcred);

        /* connect to the peer
         */
        sd = tcp_connect();

        gnutls_transport_set_int(session, sd);

        /* Perform the TLS handshake
         */
        ret = gnutls_handshake(session);

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (ret == 0) {
                printf("- Peer has closed the TLS connection\n");
                goto end;
        } else if (ret < 0) {
                fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
                goto end;
        }

        printf("- Received %d bytes: ", ret);
        for (ii = 0; ii < ret; ii++) {
                fputc(buffer[ii], stdout);
        }
        fputs("\n", stdout);

        gnutls_bye(session, GNUTLS_SHUT_RDWR);

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);
        gnutls_priority_deinit(priorities_cache);

        gnutls_global_deinit();

        return 0;
}

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7.1.9 Client with resume capability example

This is a modification of the simple client example. Here we demonstrate the use of session resumption. The client tries to connect once using TLS, close the connection and then try to establish a new connection using the previously negotiated data.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>

/* Those functions are defined in other examples.
 */
extern void check_alert(gnutls_session_t session, int ret);
extern int tcp_connect(void);
extern void tcp_close(int sd);

#define MAX_BUF 1024
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define MSG "GET / HTTP/1.0\r\n\r\n"

int main(void)
{
        int ret;
        int sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        gnutls_certificate_credentials_t xcred;

        /* variables used in session resuming 
         */
        int t;
        char *session_data = NULL;
        size_t session_data_size = 0;

        gnutls_global_init();

        /* X509 stuff */
        gnutls_certificate_allocate_credentials(&xcred);

        gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);

        for (t = 0; t < 2; t++) {       /* connect 2 times to the server */

                sd = tcp_connect();

                gnutls_init(&session, GNUTLS_CLIENT);

                gnutls_priority_set_direct(session,
                                           "PERFORMANCE:!ARCFOUR-128",
                                           NULL);

                gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE,
                                       xcred);

                if (t > 0) {
                        /* if this is not the first time we connect */
                        gnutls_session_set_data(session, session_data,
                                                session_data_size);
                        free(session_data);
                }

                gnutls_transport_set_int(session, sd);
                gnutls_handshake_set_timeout(session,
                                             GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

                /* Perform the TLS handshake
                 */
                do {
                        ret = gnutls_handshake(session);
                }
                while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

                if (ret < 0) {
                        fprintf(stderr, "*** Handshake failed\n");
                        gnutls_perror(ret);
                        goto end;
                } else {
                        printf("- Handshake was completed\n");
                }

                if (t == 0) {   /* the first time we connect */
                        /* get the session data size */
                        gnutls_session_get_data(session, NULL,
                                                &session_data_size);
                        session_data = malloc(session_data_size);

                        /* put session data to the session variable */
                        gnutls_session_get_data(session, session_data,
                                                &session_data_size);

                } else {        /* the second time we connect */

                        /* check if we actually resumed the previous session */
                        if (gnutls_session_is_resumed(session) != 0) {
                                printf("- Previous session was resumed\n");
                        } else {
                                fprintf(stderr,
                                        "*** Previous session was NOT resumed\n");
                        }
                }

                /* This function was defined in a previous example
                 */
                /* print_info(session); */

                gnutls_record_send(session, MSG, strlen(MSG));

                ret = gnutls_record_recv(session, buffer, MAX_BUF);
                if (ret == 0) {
                        printf("- Peer has closed the TLS connection\n");
                        goto end;
                } else if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                        fprintf(stderr, "*** Warning: %s\n",
                                gnutls_strerror(ret));
                } else if (ret < 0) {
                        fprintf(stderr, "*** Error: %s\n",
                                gnutls_strerror(ret));
                        goto end;
                }

                if (ret > 0) {
                        printf("- Received %d bytes: ", ret);
                        for (ii = 0; ii < ret; ii++) {
                                fputc(buffer[ii], stdout);
                        }
                        fputs("\n", stdout);
                }

                gnutls_bye(session, GNUTLS_SHUT_RDWR);

              end:

                tcp_close(sd);

                gnutls_deinit(session);

        }                       /* for() */

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}

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7.1.10 Simple client example with SRP authentication

The following client is a very simple SRP TLS client which connects to a server and authenticates using a username and a password. The server may authenticate itself using a certificate, and in that case it has to be verified.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>

/* Those functions are defined in other examples.
 */
extern void check_alert(gnutls_session_t session, int ret);
extern int tcp_connect(void);
extern void tcp_close(int sd);

#define MAX_BUF 1024
#define USERNAME "user"
#define PASSWORD "pass"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define MSG "GET / HTTP/1.0\r\n\r\n"

int main(void)
{
        int ret;
        int sd, ii;
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        gnutls_srp_client_credentials_t srp_cred;
        gnutls_certificate_credentials_t cert_cred;

        gnutls_global_init();

        gnutls_srp_allocate_client_credentials(&srp_cred);
        gnutls_certificate_allocate_credentials(&cert_cred);

        gnutls_certificate_set_x509_trust_file(cert_cred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);
        gnutls_srp_set_client_credentials(srp_cred, USERNAME, PASSWORD);

        /* connects to server
         */
        sd = tcp_connect();

        /* Initialize TLS session
         */
        gnutls_init(&session, GNUTLS_CLIENT);


        /* Set the priorities.
         */
        gnutls_priority_set_direct(session,
                                   "NORMAL:+SRP:+SRP-RSA:+SRP-DSS", NULL);

        /* put the SRP credentials to the current session
         */
        gnutls_credentials_set(session, GNUTLS_CRD_SRP, srp_cred);
        gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, cert_cred);

        gnutls_transport_set_int(session, sd);
        gnutls_handshake_set_timeout(session,
                                     GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

        /* Perform the TLS handshake
         */
        do {
                ret = gnutls_handshake(session);
        }
        while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

        if (ret < 0) {
                fprintf(stderr, "*** Handshake failed\n");
                gnutls_perror(ret);
                goto end;
        } else {
                char *desc;

                desc = gnutls_session_get_desc(session);
                printf("- Session info: %s\n", desc);
                gnutls_free(desc);
        }

        gnutls_record_send(session, MSG, strlen(MSG));

        ret = gnutls_record_recv(session, buffer, MAX_BUF);
        if (gnutls_error_is_fatal(ret) != 0 || ret == 0) {
                if (ret == 0) {
                        printf
                            ("- Peer has closed the GnuTLS connection\n");
                        goto end;
                } else {
                        fprintf(stderr, "*** Error: %s\n",
                                gnutls_strerror(ret));
                        goto end;
                }
        } else
                check_alert(session, ret);

        if (ret > 0) {
                printf("- Received %d bytes: ", ret);
                for (ii = 0; ii < ret; ii++) {
                        fputc(buffer[ii], stdout);
                }
                fputs("\n", stdout);
        }
        gnutls_bye(session, GNUTLS_SHUT_RDWR);

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_srp_free_client_credentials(srp_cred);
        gnutls_certificate_free_credentials(cert_cred);

        gnutls_global_deinit();

        return 0;
}

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7.1.11 Simple client example using the C++ API

The following client is a simple example of a client client utilizing the GnuTLS C++ API.

#include <config.h>
#include <iostream>
#include <stdexcept>
#include <gnutls/gnutls.h>
#include <gnutls/gnutlsxx.h>
#include <cstring> /* for strlen */

/* A very basic TLS client, with anonymous authentication.
 * written by Eduardo Villanueva Che.
 */

#define MAX_BUF 1024
#define SA struct sockaddr

#define CAFILE "ca.pem"
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern "C"
{
    int tcp_connect(void);
    void tcp_close(int sd);
}


int main(void)
{
    int sd = -1;
    gnutls_global_init();

    try
    {

        /* Allow connections to servers that have OpenPGP keys as well.
         */
        gnutls::client_session session;

        /* X509 stuff */
        gnutls::certificate_credentials credentials;


        /* sets the trusted cas file
         */
        credentials.set_x509_trust_file(CAFILE, GNUTLS_X509_FMT_PEM);
        /* put the x509 credentials to the current session
         */
        session.set_credentials(credentials);

        /* Use default priorities */
        session.set_priority ("NORMAL", NULL);

        /* connect to the peer
         */
        sd = tcp_connect();
        session.set_transport_ptr((gnutls_transport_ptr_t) (ptrdiff_t)sd);

        /* Perform the TLS handshake
         */
        int ret = session.handshake();
        if (ret < 0)
        {
            throw std::runtime_error("Handshake failed");
        }
        else
        {
            std::cout << "- Handshake was completed" << std::endl;
        }

        session.send(MSG, strlen(MSG));
        char buffer[MAX_BUF + 1];
        ret = session.recv(buffer, MAX_BUF);
        if (ret == 0)
        {
            throw std::runtime_error("Peer has closed the TLS connection");
        }
        else if (ret < 0)
        {
            throw std::runtime_error(gnutls_strerror(ret));
        }

        std::cout << "- Received " << ret << " bytes:" << std::endl;
        std::cout.write(buffer, ret);
        std::cout << std::endl;

        session.bye(GNUTLS_SHUT_RDWR);
    }
    catch (std::exception &ex)
    {
        std::cerr << "Exception caught: " << ex.what() << std::endl;
    }

    if (sd != -1)
        tcp_close(sd);

    gnutls_global_deinit();

    return 0;
}

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7.1.12 Helper functions for TCP connections

Those helper function abstract away TCP connection handling from the other examples. It is required to build some examples.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>

/* tcp.c */
int tcp_connect(void);
void tcp_close(int sd);

/* Connects to the peer and returns a socket
 * descriptor.
 */
extern int tcp_connect(void)
{
        const char *PORT = "5556";
        const char *SERVER = "127.0.0.1";
        int err, sd;
        struct sockaddr_in sa;

        /* connects to server
         */
        sd = socket(AF_INET, SOCK_STREAM, 0);

        memset(&sa, '\0', sizeof(sa));
        sa.sin_family = AF_INET;
        sa.sin_port = htons(atoi(PORT));
        inet_pton(AF_INET, SERVER, &sa.sin_addr);

        err = connect(sd, (struct sockaddr *) &sa, sizeof(sa));
        if (err < 0) {
                fprintf(stderr, "Connect error\n");
                exit(1);
        }

        return sd;
}

/* closes the given socket descriptor.
 */
extern void tcp_close(int sd)
{
        shutdown(sd, SHUT_RDWR);        /* no more receptions */
        close(sd);
}

Previous: , Up: Client examples   [Contents][Index]

7.1.13 Helper functions for UDP connections

The UDP helper functions abstract away UDP connection handling from the other examples. It is required to build the examples using UDP.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <unistd.h>

/* udp.c */
int udp_connect(void);
void udp_close(int sd);

/* Connects to the peer and returns a socket
 * descriptor.
 */
extern int udp_connect(void)
{
        const char *PORT = "5557";
        const char *SERVER = "127.0.0.1";
        int err, sd, optval;
        struct sockaddr_in sa;

        /* connects to server
         */
        sd = socket(AF_INET, SOCK_DGRAM, 0);

        memset(&sa, '\0', sizeof(sa));
        sa.sin_family = AF_INET;
        sa.sin_port = htons(atoi(PORT));
        inet_pton(AF_INET, SERVER, &sa.sin_addr);

#if defined(IP_DONTFRAG)
        optval = 1;
        setsockopt(sd, IPPROTO_IP, IP_DONTFRAG,
                   (const void *) &optval, sizeof(optval));
#elif defined(IP_MTU_DISCOVER)
        optval = IP_PMTUDISC_DO;
        setsockopt(sd, IPPROTO_IP, IP_MTU_DISCOVER,
                   (const void *) &optval, sizeof(optval));
#endif

        err = connect(sd, (struct sockaddr *) &sa, sizeof(sa));
        if (err < 0) {
                fprintf(stderr, "Connect error\n");
                exit(1);
        }

        return sd;
}

/* closes the given socket descriptor.
 */
extern void udp_close(int sd)
{
        close(sd);
}

Next: , Previous: , Up: GnuTLS application examples   [Contents][Index]

7.2 Server examples

This section contains examples of TLS and SSL servers, using GnuTLS.


Next: , Up: Server examples   [Contents][Index]

7.2.1 Echo server with X.509 authentication

This example is a very simple echo server which supports X.509 authentication.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define CRLFILE "crl.pem"

/* The OCSP status file contains up to date information about revocation
 * of the server's certificate. That can be periodically be updated
 * using:
 * $ ocsptool --ask --load-cert your_cert.pem --load-issuer your_issuer.pem
 *            --load-signer your_issuer.pem --outfile ocsp-status.der
 */
#define OCSP_STATUS_FILE "ocsp-status.der"

/* This is a sample TLS 1.0 echo server, using X.509 authentication and
 * OCSP stapling support.
 */

#define MAX_BUF 1024
#define PORT 5556               /* listen to 5556 port */

/* These are global */
static gnutls_dh_params_t dh_params;

static int generate_dh_params(void)
{
        unsigned int bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH,
                                                        GNUTLS_SEC_PARAM_LEGACY);

        /* Generate Diffie-Hellman parameters - for use with DHE
         * kx algorithms. When short bit length is used, it might
         * be wise to regenerate parameters often.
         */
        gnutls_dh_params_init(&dh_params);
        gnutls_dh_params_generate2(dh_params, bits);

        return 0;
}

int main(void)
{
        int listen_sd;
        int sd, ret;
        gnutls_certificate_credentials_t x509_cred;
        gnutls_priority_t priority_cache;
        struct sockaddr_in sa_serv;
        struct sockaddr_in sa_cli;
        socklen_t client_len;
        char topbuf[512];
        gnutls_session_t session;
        char buffer[MAX_BUF + 1];
        int optval = 1;

        /* this must be called once in the program
         */
        gnutls_global_init();

        gnutls_certificate_allocate_credentials(&x509_cred);
        /* gnutls_certificate_set_x509_system_trust(xcred); */
        gnutls_certificate_set_x509_trust_file(x509_cred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);

        gnutls_certificate_set_x509_crl_file(x509_cred, CRLFILE,
                                             GNUTLS_X509_FMT_PEM);

        ret =
            gnutls_certificate_set_x509_key_file(x509_cred, CERTFILE,
                                                 KEYFILE,
                                                 GNUTLS_X509_FMT_PEM);
        if (ret < 0) {
                printf("No certificate or key were found\n");
                exit(1);
        }

        /* loads an OCSP status request if available */
        gnutls_certificate_set_ocsp_status_request_file(x509_cred,
                                                        OCSP_STATUS_FILE,
                                                        0);

        generate_dh_params();

        gnutls_priority_init(&priority_cache,
                             "PERFORMANCE:%SERVER_PRECEDENCE", NULL);


        gnutls_certificate_set_dh_params(x509_cred, dh_params);

        /* Socket operations
         */
        listen_sd = socket(AF_INET, SOCK_STREAM, 0);

        memset(&sa_serv, '\0', sizeof(sa_serv));
        sa_serv.sin_family = AF_INET;
        sa_serv.sin_addr.s_addr = INADDR_ANY;
        sa_serv.sin_port = htons(PORT); /* Server Port number */

        setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (void *) &optval,
                   sizeof(int));

        bind(listen_sd, (struct sockaddr *) &sa_serv, sizeof(sa_serv));

        listen(listen_sd, 1024);

        printf("Server ready. Listening to port '%d'.\n\n", PORT);

        client_len = sizeof(sa_cli);
        for (;;) {
                gnutls_init(&session, GNUTLS_SERVER);
                gnutls_priority_set(session, priority_cache);
                gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE,
                                       x509_cred);
                /* We don't request any certificate from the client.
                 * If we did we would need to verify it.
                 */
                gnutls_certificate_server_set_request(session,
                                                      GNUTLS_CERT_IGNORE);

                sd = accept(listen_sd, (struct sockaddr *) &sa_cli,
                            &client_len);

                printf("- connection from %s, port %d\n",
                       inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
                                 sizeof(topbuf)), ntohs(sa_cli.sin_port));

                gnutls_transport_set_int(session, sd);

                do {
                        ret = gnutls_handshake(session);
                }
                while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

                if (ret < 0) {
                        close(sd);
                        gnutls_deinit(session);
                        fprintf(stderr,
                                "*** Handshake has failed (%s)\n\n",
                                gnutls_strerror(ret));
                        continue;
                }
                printf("- Handshake was completed\n");

                /* see the Getting peer's information example */
                /* print_info(session); */

                for (;;) {
                        ret = gnutls_record_recv(session, buffer, MAX_BUF);

                        if (ret == 0) {
                                printf
                                    ("\n- Peer has closed the GnuTLS connection\n");
                                break;
                        } else if (ret < 0
                                   && gnutls_error_is_fatal(ret) == 0) {
                                fprintf(stderr, "*** Warning: %s\n",
                                        gnutls_strerror(ret));
                        } else if (ret < 0) {
                                fprintf(stderr, "\n*** Received corrupted "
                                        "data(%d). Closing the connection.\n\n",
                                        ret);
                                break;
                        } else if (ret > 0) {
                                /* echo data back to the client
                                 */
                                gnutls_record_send(session, buffer, ret);
                        }
                }
                printf("\n");
                /* do not wait for the peer to close the connection.
                 */
                gnutls_bye(session, GNUTLS_SHUT_WR);

                close(sd);
                gnutls_deinit(session);

        }
        close(listen_sd);

        gnutls_certificate_free_credentials(x509_cred);
        gnutls_priority_deinit(priority_cache);

        gnutls_global_deinit();

        return 0;

}

Next: , Previous: , Up: Server examples   [Contents][Index]

7.2.2 Echo server with OpenPGP authentication

The following example is an echo server which supports OpenPGP key authentication. You can easily combine this functionality —that is have a server that supports both X.509 and OpenPGP certificates— but we separated them to keep these examples as simple as possible.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/openpgp.h>

#define KEYFILE "secret.asc"
#define CERTFILE "public.asc"
#define RINGFILE "ring.gpg"

/* This is a sample TLS 1.0-OpenPGP echo server.
 */


#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556               /* listen to 5556 port */

/* These are global */
gnutls_dh_params_t dh_params;

static int generate_dh_params(void)
{
        unsigned int bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH,
                                                        GNUTLS_SEC_PARAM_LEGACY);

        /* Generate Diffie-Hellman parameters - for use with DHE
         * kx algorithms. These should be discarded and regenerated
         * once a day, once a week or once a month. Depending on the
         * security requirements.
         */
        gnutls_dh_params_init(&dh_params);
        gnutls_dh_params_generate2(dh_params, bits);

        return 0;
}

int main(void)
{
        int err, listen_sd;
        int sd, ret;
        struct sockaddr_in sa_serv;
        struct sockaddr_in sa_cli;
        socklen_t client_len;
        char topbuf[512];
        gnutls_session_t session;
        gnutls_certificate_credentials_t cred;
        char buffer[MAX_BUF + 1];
        int optval = 1;
        char name[256];

        strcpy(name, "Echo Server");

        /* this must be called once in the program
         */
        gnutls_global_init();

        gnutls_certificate_allocate_credentials(&cred);
        gnutls_certificate_set_openpgp_keyring_file(cred, RINGFILE,
                                                    GNUTLS_OPENPGP_FMT_BASE64);

        gnutls_certificate_set_openpgp_key_file(cred, CERTFILE, KEYFILE,
                                                GNUTLS_OPENPGP_FMT_BASE64);

        generate_dh_params();

        gnutls_certificate_set_dh_params(cred, dh_params);

        /* Socket operations
         */
        listen_sd = socket(AF_INET, SOCK_STREAM, 0);
        SOCKET_ERR(listen_sd, "socket");

        memset(&sa_serv, '\0', sizeof(sa_serv));
        sa_serv.sin_family = AF_INET;
        sa_serv.sin_addr.s_addr = INADDR_ANY;
        sa_serv.sin_port = htons(PORT); /* Server Port number */

        setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (void *) &optval,
                   sizeof(int));

        err =
            bind(listen_sd, (struct sockaddr *) &sa_serv, sizeof(sa_serv));
        SOCKET_ERR(err, "bind");
        err = listen(listen_sd, 1024);
        SOCKET_ERR(err, "listen");

        printf("%s ready. Listening to port '%d'.\n\n", name, PORT);

        client_len = sizeof(sa_cli);
        for (;;) {
                gnutls_init(&session, GNUTLS_SERVER);
                gnutls_priority_set_direct(session,
                                           "NORMAL:+CTYPE-OPENPGP", NULL);

                /* request client certificate if any.
                 */
                gnutls_certificate_server_set_request(session,
                                                      GNUTLS_CERT_REQUEST);

                sd = accept(listen_sd, (struct sockaddr *) &sa_cli,
                            &client_len);

                printf("- connection from %s, port %d\n",
                       inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
                                 sizeof(topbuf)), ntohs(sa_cli.sin_port));

                gnutls_transport_set_int(session, sd);
                ret = gnutls_handshake(session);
                if (ret < 0) {
                        close(sd);
                        gnutls_deinit(session);
                        fprintf(stderr,
                                "*** Handshake has failed (%s)\n\n",
                                gnutls_strerror(ret));
                        continue;
                }
                printf("- Handshake was completed\n");

                /* see the Getting peer's information example */
                /* print_info(session); */

                for (;;) {
                        ret = gnutls_record_recv(session, buffer, MAX_BUF);

                        if (ret == 0) {
                                printf
                                    ("\n- Peer has closed the GnuTLS connection\n");
                                break;
                        } else if (ret < 0
                                   && gnutls_error_is_fatal(ret) == 0) {
                                fprintf(stderr, "*** Warning: %s\n",
                                        gnutls_strerror(ret));
                        } else if (ret < 0) {
                                fprintf(stderr, "\n*** Received corrupted "
                                        "data(%d). Closing the connection.\n\n",
                                        ret);
                                break;
                        } else if (ret > 0) {
                                /* echo data back to the client
                                 */
                                gnutls_record_send(session, buffer, ret);
                        }
                }
                printf("\n");
                /* do not wait for the peer to close the connection.
                 */
                gnutls_bye(session, GNUTLS_SHUT_WR);

                close(sd);
                gnutls_deinit(session);

        }
        close(listen_sd);

        gnutls_certificate_free_credentials(cred);

        gnutls_global_deinit();

        return 0;

}

Next: , Previous: , Up: Server examples   [Contents][Index]

7.2.3 Echo server with SRP authentication

This is a server which supports SRP authentication. It is also possible to combine this functionality with a certificate server. Here it is separate for simplicity.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

#define SRP_PASSWD "tpasswd"
#define SRP_PASSWD_CONF "tpasswd.conf"

#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"

/* This is a sample TLS-SRP echo server.
 */

#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556               /* listen to 5556 port */

int main(void)
{
        int err, listen_sd;
        int sd, ret;
        struct sockaddr_in sa_serv;
        struct sockaddr_in sa_cli;
        socklen_t client_len;
        char topbuf[512];
        gnutls_session_t session;
        gnutls_srp_server_credentials_t srp_cred;
        gnutls_certificate_credentials_t cert_cred;
        char buffer[MAX_BUF + 1];
        int optval = 1;
        char name[256];

        strcpy(name, "Echo Server");

        gnutls_global_init();

        /* SRP_PASSWD a password file (created with the included srptool utility) 
         */
        gnutls_srp_allocate_server_credentials(&srp_cred);
        gnutls_srp_set_server_credentials_file(srp_cred, SRP_PASSWD,
                                               SRP_PASSWD_CONF);

        gnutls_certificate_allocate_credentials(&cert_cred);
        gnutls_certificate_set_x509_trust_file(cert_cred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);
        gnutls_certificate_set_x509_key_file(cert_cred, CERTFILE, KEYFILE,
                                             GNUTLS_X509_FMT_PEM);

        /* TCP socket operations
         */
        listen_sd = socket(AF_INET, SOCK_STREAM, 0);
        SOCKET_ERR(listen_sd, "socket");

        memset(&sa_serv, '\0', sizeof(sa_serv));
        sa_serv.sin_family = AF_INET;
        sa_serv.sin_addr.s_addr = INADDR_ANY;
        sa_serv.sin_port = htons(PORT); /* Server Port number */

        setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (void *) &optval,
                   sizeof(int));

        err =
            bind(listen_sd, (struct sockaddr *) &sa_serv, sizeof(sa_serv));
        SOCKET_ERR(err, "bind");
        err = listen(listen_sd, 1024);
        SOCKET_ERR(err, "listen");

        printf("%s ready. Listening to port '%d'.\n\n", name, PORT);

        client_len = sizeof(sa_cli);
        for (;;) {
                gnutls_init(&session, GNUTLS_SERVER);
                gnutls_priority_set_direct(session,
                                           "NORMAL:-KX-ALL:+SRP:+SRP-DSS:+SRP-RSA",
                                           NULL);
                gnutls_credentials_set(session, GNUTLS_CRD_SRP, srp_cred);
                /* for the certificate authenticated ciphersuites.
                 */
                gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE,
                                       cert_cred);

                /* request client certificate if any.
                 */
                gnutls_certificate_server_set_request(session,
                                                      GNUTLS_CERT_IGNORE);

                sd = accept(listen_sd, (struct sockaddr *) &sa_cli,
                            &client_len);

                printf("- connection from %s, port %d\n",
                       inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
                                 sizeof(topbuf)), ntohs(sa_cli.sin_port));

                gnutls_transport_set_int(session, sd);

                do {
                        ret = gnutls_handshake(session);
                }
                while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

                if (ret < 0) {
                        close(sd);
                        gnutls_deinit(session);
                        fprintf(stderr,
                                "*** Handshake has failed (%s)\n\n",
                                gnutls_strerror(ret));
                        continue;
                }
                printf("- Handshake was completed\n");
                printf("- User %s was connected\n",
                       gnutls_srp_server_get_username(session));

                /* print_info(session); */

                for (;;) {
                        ret = gnutls_record_recv(session, buffer, MAX_BUF);

                        if (ret == 0) {
                                printf
                                    ("\n- Peer has closed the GnuTLS connection\n");
                                break;
                        } else if (ret < 0
                                   && gnutls_error_is_fatal(ret) == 0) {
                                fprintf(stderr, "*** Warning: %s\n",
                                        gnutls_strerror(ret));
                        } else if (ret < 0) {
                                fprintf(stderr, "\n*** Received corrupted "
                                        "data(%d). Closing the connection.\n\n",
                                        ret);
                                break;
                        } else if (ret > 0) {
                                /* echo data back to the client
                                 */
                                gnutls_record_send(session, buffer, ret);
                        }
                }
                printf("\n");
                /* do not wait for the peer to close the connection. */
                gnutls_bye(session, GNUTLS_SHUT_WR);

                close(sd);
                gnutls_deinit(session);

        }
        close(listen_sd);

        gnutls_srp_free_server_credentials(srp_cred);
        gnutls_certificate_free_credentials(cert_cred);

        gnutls_global_deinit();

        return 0;

}

Next: , Previous: , Up: Server examples   [Contents][Index]

7.2.4 Echo server with anonymous authentication

This example server supports anonymous authentication, and could be used to serve the example client for anonymous authentication.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

/* This is a sample TLS 1.0 echo server, for anonymous authentication only.
 */


#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556               /* listen to 5556 port */

/* These are global */
static gnutls_dh_params_t dh_params;

static int generate_dh_params(void)
{
        unsigned int bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH,
                                                        GNUTLS_SEC_PARAM_LEGACY);
        /* Generate Diffie-Hellman parameters - for use with DHE
         * kx algorithms. These should be discarded and regenerated
         * once a day, once a week or once a month. Depending on the
         * security requirements.
         */
        gnutls_dh_params_init(&dh_params);
        gnutls_dh_params_generate2(dh_params, bits);

        return 0;
}

int main(void)
{
        int err, listen_sd;
        int sd, ret;
        struct sockaddr_in sa_serv;
        struct sockaddr_in sa_cli;
        socklen_t client_len;
        char topbuf[512];
        gnutls_session_t session;
        gnutls_anon_server_credentials_t anoncred;
        char buffer[MAX_BUF + 1];
        int optval = 1;

        /* this must be called once in the program
         */
        gnutls_global_init();

        gnutls_anon_allocate_server_credentials(&anoncred);

        generate_dh_params();

        gnutls_anon_set_server_dh_params(anoncred, dh_params);

        /* Socket operations
         */
        listen_sd = socket(AF_INET, SOCK_STREAM, 0);
        SOCKET_ERR(listen_sd, "socket");

        memset(&sa_serv, '\0', sizeof(sa_serv));
        sa_serv.sin_family = AF_INET;
        sa_serv.sin_addr.s_addr = INADDR_ANY;
        sa_serv.sin_port = htons(PORT); /* Server Port number */

        setsockopt(listen_sd, SOL_SOCKET, SO_REUSEADDR, (void *) &optval,
                   sizeof(int));

        err =
            bind(listen_sd, (struct sockaddr *) &sa_serv, sizeof(sa_serv));
        SOCKET_ERR(err, "bind");
        err = listen(listen_sd, 1024);
        SOCKET_ERR(err, "listen");

        printf("Server ready. Listening to port '%d'.\n\n", PORT);

        client_len = sizeof(sa_cli);
        for (;;) {
                gnutls_init(&session, GNUTLS_SERVER);
                gnutls_priority_set_direct(session,
                                           "NORMAL:+ANON-ECDH:+ANON-DH",
                                           NULL);
                gnutls_credentials_set(session, GNUTLS_CRD_ANON, anoncred);

                sd = accept(listen_sd, (struct sockaddr *) &sa_cli,
                            &client_len);

                printf("- connection from %s, port %d\n",
                       inet_ntop(AF_INET, &sa_cli.sin_addr, topbuf,
                                 sizeof(topbuf)), ntohs(sa_cli.sin_port));

                gnutls_transport_set_int(session, sd);

                do {
                        ret = gnutls_handshake(session);
                }
                while (ret < 0 && gnutls_error_is_fatal(ret) == 0);

                if (ret < 0) {
                        close(sd);
                        gnutls_deinit(session);
                        fprintf(stderr,
                                "*** Handshake has failed (%s)\n\n",
                                gnutls_strerror(ret));
                        continue;
                }
                printf("- Handshake was completed\n");

                /* see the Getting peer's information example */
                /* print_info(session); */

                for (;;) {
                        ret = gnutls_record_recv(session, buffer, MAX_BUF);

                        if (ret == 0) {
                                printf
                                    ("\n- Peer has closed the GnuTLS connection\n");
                                break;
                        } else if (ret < 0
                                   && gnutls_error_is_fatal(ret) == 0) {
                                fprintf(stderr, "*** Warning: %s\n",
                                        gnutls_strerror(ret));
                        } else if (ret < 0) {
                                fprintf(stderr, "\n*** Received corrupted "
                                        "data(%d). Closing the connection.\n\n",
                                        ret);
                                break;
                        } else if (ret > 0) {
                                /* echo data back to the client
                                 */
                                gnutls_record_send(session, buffer, ret);
                        }
                }
                printf("\n");
                /* do not wait for the peer to close the connection.
                 */
                gnutls_bye(session, GNUTLS_SHUT_WR);

                close(sd);
                gnutls_deinit(session);

        }
        close(listen_sd);

        gnutls_anon_free_server_credentials(anoncred);

        gnutls_global_deinit();

        return 0;

}

Previous: , Up: Server examples   [Contents][Index]

7.2.5 DTLS echo server with X.509 authentication

This example is a very simple echo server using Datagram TLS and X.509 authentication.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <sys/select.h>
#include <netdb.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>
#include <gnutls/dtls.h>

#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define CRLFILE "crl.pem"

/* This is a sample DTLS echo server, using X.509 authentication.
 * Note that error checking is minimal to simplify the example.
 */

#define MAX_BUFFER 1024
#define PORT 5556

typedef struct {
        gnutls_session_t session;
        int fd;
        struct sockaddr *cli_addr;
        socklen_t cli_addr_size;
} priv_data_st;

static int pull_timeout_func(gnutls_transport_ptr_t ptr, unsigned int ms);
static ssize_t push_func(gnutls_transport_ptr_t p, const void *data,
                         size_t size);
static ssize_t pull_func(gnutls_transport_ptr_t p, void *data,
                         size_t size);
static const char *human_addr(const struct sockaddr *sa, socklen_t salen,
                              char *buf, size_t buflen);
static int wait_for_connection(int fd);
static int generate_dh_params(void);

/* Use global credentials and parameters to simplify
 * the example. */
static gnutls_certificate_credentials_t x509_cred;
static gnutls_priority_t priority_cache;
static gnutls_dh_params_t dh_params;

int main(void)
{
        int listen_sd;
        int sock, ret;
        struct sockaddr_in sa_serv;
        struct sockaddr_in cli_addr;
        socklen_t cli_addr_size;
        gnutls_session_t session;
        char buffer[MAX_BUFFER];
        priv_data_st priv;
        gnutls_datum_t cookie_key;
        gnutls_dtls_prestate_st prestate;
        int mtu = 1400;
        unsigned char sequence[8];

        /* this must be called once in the program
         */
        gnutls_global_init();

        gnutls_certificate_allocate_credentials(&x509_cred);
        gnutls_certificate_set_x509_trust_file(x509_cred, CAFILE,
                                               GNUTLS_X509_FMT_PEM);

        gnutls_certificate_set_x509_crl_file(x509_cred, CRLFILE,
                                             GNUTLS_X509_FMT_PEM);

        ret =
            gnutls_certificate_set_x509_key_file(x509_cred, CERTFILE,
                                                 KEYFILE,
                                                 GNUTLS_X509_FMT_PEM);
        if (ret < 0) {
                printf("No certificate or key were found\n");
                exit(1);
        }

        generate_dh_params();

        gnutls_certificate_set_dh_params(x509_cred, dh_params);

        gnutls_priority_init(&priority_cache,
                             "PERFORMANCE:-VERS-TLS-ALL:+VERS-DTLS1.0:%SERVER_PRECEDENCE",
                             NULL);

        gnutls_key_generate(&cookie_key, GNUTLS_COOKIE_KEY_SIZE);

        /* Socket operations
         */
        listen_sd = socket(AF_INET, SOCK_DGRAM, 0);

        memset(&sa_serv, '\0', sizeof(sa_serv));
        sa_serv.sin_family = AF_INET;
        sa_serv.sin_addr.s_addr = INADDR_ANY;
        sa_serv.sin_port = htons(PORT);

        {                       /* DTLS requires the IP don't fragment (DF) bit to be set */
#if defined(IP_DONTFRAG)
                int optval = 1;
                setsockopt(listen_sd, IPPROTO_IP, IP_DONTFRAG,
                           (const void *) &optval, sizeof(optval));
#elif defined(IP_MTU_DISCOVER)
                int optval = IP_PMTUDISC_DO;
                setsockopt(listen_sd, IPPROTO_IP, IP_MTU_DISCOVER,
                           (const void *) &optval, sizeof(optval));
#endif
        }

        bind(listen_sd, (struct sockaddr *) &sa_serv, sizeof(sa_serv));

        printf("UDP server ready. Listening to port '%d'.\n\n", PORT);

        for (;;) {
                printf("Waiting for connection...\n");
                sock = wait_for_connection(listen_sd);
                if (sock < 0)
                        continue;

                cli_addr_size = sizeof(cli_addr);
                ret = recvfrom(sock, buffer, sizeof(buffer), MSG_PEEK,
                               (struct sockaddr *) &cli_addr,
                               &cli_addr_size);
                if (ret > 0) {
                        memset(&prestate, 0, sizeof(prestate));
                        ret =
                            gnutls_dtls_cookie_verify(&cookie_key,
                                                      &cli_addr,
                                                      sizeof(cli_addr),
                                                      buffer, ret,
                                                      &prestate);
                        if (ret < 0) {  /* cookie not valid */
                                priv_data_st s;

                                memset(&s, 0, sizeof(s));
                                s.fd = sock;
                                s.cli_addr = (void *) &cli_addr;
                                s.cli_addr_size = sizeof(cli_addr);

                                printf
                                    ("Sending hello verify request to %s\n",
                                     human_addr((struct sockaddr *)
                                                &cli_addr,
                                                sizeof(cli_addr), buffer,
                                                sizeof(buffer)));

                                gnutls_dtls_cookie_send(&cookie_key,
                                                        &cli_addr,
                                                        sizeof(cli_addr),
                                                        &prestate,
                                                        (gnutls_transport_ptr_t)
                                                        & s, push_func);

                                /* discard peeked data */
                                recvfrom(sock, buffer, sizeof(buffer), 0,
                                         (struct sockaddr *) &cli_addr,
                                         &cli_addr_size);
                                usleep(100);
                                continue;
                        }
                        printf("Accepted connection from %s\n",
                               human_addr((struct sockaddr *)
                                          &cli_addr, sizeof(cli_addr),
                                          buffer, sizeof(buffer)));
                } else
                        continue;

                gnutls_init(&session, GNUTLS_SERVER | GNUTLS_DATAGRAM);
                gnutls_priority_set(session, priority_cache);
                gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE,
                                       x509_cred);

                gnutls_dtls_prestate_set(session, &prestate);
                gnutls_dtls_set_mtu(session, mtu);

                priv.session = session;
                priv.fd = sock;
                priv.cli_addr = (struct sockaddr *) &cli_addr;
                priv.cli_addr_size = sizeof(cli_addr);

                gnutls_transport_set_ptr(session, &priv);
                gnutls_transport_set_push_function(session, push_func);
                gnutls_transport_set_pull_function(session, pull_func);
                gnutls_transport_set_pull_timeout_function(session,
                                                           pull_timeout_func);

                do {
                        ret = gnutls_handshake(session);
                }
                while (ret == GNUTLS_E_INTERRUPTED
                       || ret == GNUTLS_E_AGAIN);
                /* Note that DTLS may also receive GNUTLS_E_LARGE_PACKET.
                 * In that case the MTU should be adjusted.
                 */

                if (ret < 0) {
                        fprintf(stderr, "Error in handshake(): %s\n",
                                gnutls_strerror(ret));
                        gnutls_deinit(session);
                        continue;
                }

                printf("- Handshake was completed\n");

                for (;;) {
                        do {
                                ret =
                                    gnutls_record_recv_seq(session, buffer,
                                                           MAX_BUFFER,
                                                           sequence);
                        }
                        while (ret == GNUTLS_E_AGAIN
                               || ret == GNUTLS_E_INTERRUPTED);

                        if (ret < 0 && gnutls_error_is_fatal(ret) == 0) {
                                fprintf(stderr, "*** Warning: %s\n",
                                        gnutls_strerror(ret));
                                continue;
                        } else if (ret < 0) {
                                fprintf(stderr, "Error in recv(): %s\n",
                                        gnutls_strerror(ret));
                                break;
                        }

                        if (ret == 0) {
                                printf("EOF\n\n");
                                break;
                        }

                        buffer[ret] = 0;
                        printf
                            ("received[%.2x%.2x%.2x%.2x%.2x%.2x%.2x%.2x]: %s\n",
                             sequence[0], sequence[1], sequence[2],
                             sequence[3], sequence[4], sequence[5],
                             sequence[6], sequence[7], buffer);

                        /* reply back */
                        ret = gnutls_record_send(session, buffer, ret);
                        if (ret < 0) {
                                fprintf(stderr, "Error in send(): %s\n",
                                        gnutls_strerror(ret));
                                break;
                        }
                }

                gnutls_bye(session, GNUTLS_SHUT_WR);
                gnutls_deinit(session);

        }
        close(listen_sd);

        gnutls_certificate_free_credentials(x509_cred);
        gnutls_priority_deinit(priority_cache);

        gnutls_global_deinit();

        return 0;

}

static int wait_for_connection(int fd)
{
        fd_set rd, wr;
        int n;

        FD_ZERO(&rd);
        FD_ZERO(&wr);

        FD_SET(fd, &rd);

        /* waiting part */
        n = select(fd + 1, &rd, &wr, NULL, NULL);
        if (n == -1 && errno == EINTR)
                return -1;
        if (n < 0) {
                perror("select()");
                exit(1);
        }

        return fd;
}

/* Wait for data to be received within a timeout period in milliseconds
 */
static int pull_timeout_func(gnutls_transport_ptr_t ptr, unsigned int ms)
{
        fd_set rfds;
        struct timeval tv;
        priv_data_st *priv = ptr;
        struct sockaddr_in cli_addr;
        socklen_t cli_addr_size;
        int ret;
        char c;

        FD_ZERO(&rfds);
        FD_SET(priv->fd, &rfds);

        tv.tv_sec = 0;
        tv.tv_usec = ms * 1000;

        while (tv.tv_usec >= 1000000) {
                tv.tv_usec -= 1000000;
                tv.tv_sec++;
        }

        ret = select(priv->fd + 1, &rfds, NULL, NULL, &tv);

        if (ret <= 0)
                return ret;

        /* only report ok if the next message is from the peer we expect
         * from 
         */
        cli_addr_size = sizeof(cli_addr);
        ret =
            recvfrom(priv->fd, &c, 1, MSG_PEEK,
                     (struct sockaddr *) &cli_addr, &cli_addr_size);
        if (ret > 0) {
                if (cli_addr_size == priv->cli_addr_size
                    && memcmp(&cli_addr, priv->cli_addr,
                              sizeof(cli_addr)) == 0)
                        return 1;
        }

        return 0;
}

static ssize_t
push_func(gnutls_transport_ptr_t p, const void *data, size_t size)
{
        priv_data_st *priv = p;

        return sendto(priv->fd, data, size, 0, priv->cli_addr,
                      priv->cli_addr_size);
}

static ssize_t pull_func(gnutls_transport_ptr_t p, void *data, size_t size)
{
        priv_data_st *priv = p;
        struct sockaddr_in cli_addr;
        socklen_t cli_addr_size;
        char buffer[64];
        int ret;

        cli_addr_size = sizeof(cli_addr);
        ret =
            recvfrom(priv->fd, data, size, 0,
                     (struct sockaddr *) &cli_addr, &cli_addr_size);
        if (ret == -1)
                return ret;

        if (cli_addr_size == priv->cli_addr_size
            && memcmp(&cli_addr, priv->cli_addr, sizeof(cli_addr)) == 0)
                return ret;

        printf("Denied connection from %s\n",
               human_addr((struct sockaddr *)
                          &cli_addr, sizeof(cli_addr), buffer,
                          sizeof(buffer)));

        gnutls_transport_set_errno(priv->session, EAGAIN);
        return -1;
}

static const char *human_addr(const struct sockaddr *sa, socklen_t salen,
                              char *buf, size_t buflen)
{
        const char *save_buf = buf;
        size_t l;

        if (!buf || !buflen)
                return NULL;

        *buf = '\0';

        switch (sa->sa_family) {
#if HAVE_IPV6
        case AF_INET6:
                snprintf(buf, buflen, "IPv6 ");
                break;
#endif

        case AF_INET:
                snprintf(buf, buflen, "IPv4 ");
                break;
        }

        l = strlen(buf);
        buf += l;
        buflen -= l;

        if (getnameinfo(sa, salen, buf, buflen, NULL, 0, NI_NUMERICHOST) !=
            0)
                return NULL;

        l = strlen(buf);
        buf += l;
        buflen -= l;

        strncat(buf, " port ", buflen);

        l = strlen(buf);
        buf += l;
        buflen -= l;

        if (getnameinfo(sa, salen, NULL, 0, buf, buflen, NI_NUMERICSERV) !=
            0)
                return NULL;

        return save_buf;
}

static int generate_dh_params(void)
{
        int bits = gnutls_sec_param_to_pk_bits(GNUTLS_PK_DH,
                                               GNUTLS_SEC_PARAM_LEGACY);

        /* Generate Diffie-Hellman parameters - for use with DHE
         * kx algorithms. When short bit length is used, it might
         * be wise to regenerate parameters often.
         */
        gnutls_dh_params_init(&dh_params);
        gnutls_dh_params_generate2(dh_params, bits);

        return 0;
}

Next: , Previous: , Up: GnuTLS application examples   [Contents][Index]

7.3 OCSP example

Generate OCSP request

A small tool to generate OCSP requests.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/crypto.h>
#include <gnutls/ocsp.h>
#ifndef NO_LIBCURL
#include <curl/curl.h>
#endif
#include "read-file.h"

size_t get_data(void *buffer, size_t size, size_t nmemb, void *userp);
static gnutls_x509_crt_t load_cert(const char *cert_file);
static void _response_info(const gnutls_datum_t * data);
static void
_generate_request(gnutls_datum_t * rdata, gnutls_x509_crt_t cert,
                  gnutls_x509_crt_t issuer, gnutls_datum_t *nonce);
static int
_verify_response(gnutls_datum_t * data, gnutls_x509_crt_t cert,
                 gnutls_x509_crt_t signer, gnutls_datum_t *nonce);

/* This program queries an OCSP server.
   It expects three files. argv[1] containing the certificate to
   be checked, argv[2] holding the issuer for this certificate,
   and argv[3] holding a trusted certificate to verify OCSP's response.
   argv[4] is optional and should hold the server host name.
   
   For simplicity the libcurl library is used.
 */

int main(int argc, char *argv[])
{
        gnutls_datum_t ud, tmp;
        int ret;
        gnutls_datum_t req;
        gnutls_x509_crt_t cert, issuer, signer;
#ifndef NO_LIBCURL
        CURL *handle;
        struct curl_slist *headers = NULL;
#endif
        int v, seq;
        const char *cert_file = argv[1];
        const char *issuer_file = argv[2];
        const char *signer_file = argv[3];
        char *hostname = NULL;
        unsigned char noncebuf[23];
        gnutls_datum_t nonce = { noncebuf, sizeof(noncebuf) };

        gnutls_global_init();

        if (argc > 4)
                hostname = argv[4];

        ret = gnutls_rnd(GNUTLS_RND_NONCE, nonce.data, nonce.size);
        if (ret < 0)
                exit(1);

        cert = load_cert(cert_file);
        issuer = load_cert(issuer_file);
        signer = load_cert(signer_file);

        if (hostname == NULL) {

                for (seq = 0;; seq++) {
                        ret =
                            gnutls_x509_crt_get_authority_info_access(cert,
                                                                      seq,
                                                                      GNUTLS_IA_OCSP_URI,
                                                                      &tmp,
                                                                      NULL);
                        if (ret == GNUTLS_E_UNKNOWN_ALGORITHM)
                                continue;
                        if (ret == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE) {
                                fprintf(stderr,
                                        "No URI was found in the certificate.\n");
                                exit(1);
                        }
                        if (ret < 0) {
                                fprintf(stderr, "error: %s\n",
                                        gnutls_strerror(ret));
                                exit(1);
                        }

                        printf("CA issuers URI: %.*s\n", tmp.size,
                               tmp.data);

                        hostname = malloc(tmp.size + 1);
                        memcpy(hostname, tmp.data, tmp.size);
                        hostname[tmp.size] = 0;

                        gnutls_free(tmp.data);
                        break;
                }

        }

        /* Note that the OCSP servers hostname might be available
         * using gnutls_x509_crt_get_authority_info_access() in the issuer's
         * certificate */

        memset(&ud, 0, sizeof(ud));
        fprintf(stderr, "Connecting to %s\n", hostname);

        _generate_request(&req, cert, issuer, &nonce);

#ifndef NO_LIBCURL
        curl_global_init(CURL_GLOBAL_ALL);

        handle = curl_easy_init();
        if (handle == NULL)
                exit(1);

        headers =
            curl_slist_append(headers,
                              "Content-Type: application/ocsp-request");

        curl_easy_setopt(handle, CURLOPT_HTTPHEADER, headers);
        curl_easy_setopt(handle, CURLOPT_POSTFIELDS, (void *) req.data);
        curl_easy_setopt(handle, CURLOPT_POSTFIELDSIZE, req.size);
        curl_easy_setopt(handle, CURLOPT_URL, hostname);
        curl_easy_setopt(handle, CURLOPT_WRITEFUNCTION, get_data);
        curl_easy_setopt(handle, CURLOPT_WRITEDATA, &ud);

        ret = curl_easy_perform(handle);
        if (ret != 0) {
                fprintf(stderr, "curl[%d] error %d\n", __LINE__, ret);
                exit(1);
        }

        curl_easy_cleanup(handle);
#endif

        _response_info(&ud);

        v = _verify_response(&ud, cert, signer, &nonce);

        gnutls_x509_crt_deinit(cert);
        gnutls_x509_crt_deinit(issuer);
        gnutls_x509_crt_deinit(signer);
        gnutls_global_deinit();

        return v;
}

static void _response_info(const gnutls_datum_t * data)
{
        gnutls_ocsp_resp_t resp;
        int ret;
        gnutls_datum buf;

        ret = gnutls_ocsp_resp_init(&resp);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_resp_import(resp, data);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_resp_print(resp, GNUTLS_OCSP_PRINT_FULL, &buf);
        if (ret != 0)
                exit(1);

        printf("%.*s", buf.size, buf.data);
        gnutls_free(buf.data);

        gnutls_ocsp_resp_deinit(resp);
}

static gnutls_x509_crt_t load_cert(const char *cert_file)
{
        gnutls_x509_crt_t crt;
        int ret;
        gnutls_datum_t data;
        size_t size;

        ret = gnutls_x509_crt_init(&crt);
        if (ret < 0)
                exit(1);

        data.data = (void *) read_binary_file(cert_file, &size);
        data.size = size;

        if (!data.data) {
                fprintf(stderr, "Cannot open file: %s\n", cert_file);
                exit(1);
        }

        ret = gnutls_x509_crt_import(crt, &data, GNUTLS_X509_FMT_PEM);
        free(data.data);
        if (ret < 0) {
                fprintf(stderr, "Cannot import certificate in %s: %s\n",
                        cert_file, gnutls_strerror(ret));
                exit(1);
        }

        return crt;
}

static void
_generate_request(gnutls_datum_t * rdata, gnutls_x509_crt_t cert,
                  gnutls_x509_crt_t issuer, gnutls_datum_t *nonce)
{
        gnutls_ocsp_req_t req;
        int ret;

        ret = gnutls_ocsp_req_init(&req);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_req_add_cert(req, GNUTLS_DIG_SHA1, issuer, cert);
        if (ret < 0)
                exit(1);


        ret = gnutls_ocsp_req_set_nonce(req, 0, nonce);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_req_export(req, rdata);
        if (ret != 0)
                exit(1);

        gnutls_ocsp_req_deinit(req);

        return;
}

static int
_verify_response(gnutls_datum_t * data, gnutls_x509_crt_t cert,
                 gnutls_x509_crt_t signer, gnutls_datum_t *nonce)
{
        gnutls_ocsp_resp_t resp;
        int ret;
        unsigned verify;
        gnutls_datum_t rnonce;

        ret = gnutls_ocsp_resp_init(&resp);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_resp_import(resp, data);
        if (ret < 0)
                exit(1);

        ret = gnutls_ocsp_resp_check_crt(resp, 0, cert);
        if (ret < 0)
                exit(1);

	ret = gnutls_ocsp_resp_get_nonce(resp, NULL, &rnonce);
	if (ret < 0)
		exit(1);

	if (rnonce.size != nonce->size || memcmp(nonce->data, rnonce.data,
		nonce->size) != 0) {
		exit(1);
	}

        ret = gnutls_ocsp_resp_verify_direct(resp, signer, &verify, 0);
        if (ret < 0)
                exit(1);

        printf("Verifying OCSP Response: ");
        if (verify == 0)
                printf("Verification success!\n");
        else
                printf("Verification error!\n");

        if (verify & GNUTLS_OCSP_VERIFY_SIGNER_NOT_FOUND)
                printf("Signer cert not found\n");

        if (verify & GNUTLS_OCSP_VERIFY_SIGNER_KEYUSAGE_ERROR)
                printf("Signer cert keyusage error\n");

        if (verify & GNUTLS_OCSP_VERIFY_UNTRUSTED_SIGNER)
                printf("Signer cert is not trusted\n");

        if (verify & GNUTLS_OCSP_VERIFY_INSECURE_ALGORITHM)
                printf("Insecure algorithm\n");

        if (verify & GNUTLS_OCSP_VERIFY_SIGNATURE_FAILURE)
                printf("Signature failure\n");

        if (verify & GNUTLS_OCSP_VERIFY_CERT_NOT_ACTIVATED)
                printf("Signer cert not yet activated\n");

        if (verify & GNUTLS_OCSP_VERIFY_CERT_EXPIRED)
                printf("Signer cert expired\n");

        gnutls_free(rnonce.data);
        gnutls_ocsp_resp_deinit(resp);

        return verify;
}

size_t get_data(void *buffer, size_t size, size_t nmemb, void *userp)
{
        gnutls_datum_t *ud = userp;

        size *= nmemb;

        ud->data = realloc(ud->data, size + ud->size);
        if (ud->data == NULL) {
                fprintf(stderr, "Not enough memory for the request\n");
                exit(1);
        }

        memcpy(&ud->data[ud->size], buffer, size);
        ud->size += size;

        return size;
}

Next: , Previous: , Up: GnuTLS application examples   [Contents][Index]

7.4 Miscellaneous examples


Next: , Up: Miscellaneous examples   [Contents][Index]

7.4.1 Checking for an alert

This is a function that checks if an alert has been received in the current session.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>

#include "examples.h"

/* This function will check whether the given return code from
 * a gnutls function (recv/send), is an alert, and will print
 * that alert.
 */
void check_alert(gnutls_session_t session, int ret)
{
        int last_alert;

        if (ret == GNUTLS_E_WARNING_ALERT_RECEIVED
            || ret == GNUTLS_E_FATAL_ALERT_RECEIVED) {
                last_alert = gnutls_alert_get(session);

                /* The check for renegotiation is only useful if we are 
                 * a server, and we had requested a rehandshake.
                 */
                if (last_alert == GNUTLS_A_NO_RENEGOTIATION &&
                    ret == GNUTLS_E_WARNING_ALERT_RECEIVED)
                        printf("* Received NO_RENEGOTIATION alert. "
                               "Client Does not support renegotiation.\n");
                else
                        printf("* Received alert '%d': %s.\n", last_alert,
                               gnutls_alert_get_name(last_alert));
        }
}

Next: , Previous: , Up: Miscellaneous examples   [Contents][Index]

7.4.2 X.509 certificate parsing example

To demonstrate the X.509 parsing capabilities an example program is listed below. That program reads the peer’s certificate, and prints information about it.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

#include "examples.h"

static const char *bin2hex(const void *bin, size_t bin_size)
{
        static char printable[110];
        const unsigned char *_bin = bin;
        char *print;
        size_t i;

        if (bin_size > 50)
                bin_size = 50;

        print = printable;
        for (i = 0; i < bin_size; i++) {
                sprintf(print, "%.2x ", _bin[i]);
                print += 2;
        }

        return printable;
}

/* This function will print information about this session's peer
 * certificate.
 */
void print_x509_certificate_info(gnutls_session_t session)
{
        char serial[40];
        char dn[256];
        size_t size;
        unsigned int algo, bits;
        time_t expiration_time, activation_time;
        const gnutls_datum_t *cert_list;
        unsigned int cert_list_size = 0;
        gnutls_x509_crt_t cert;
        gnutls_datum_t cinfo;

        /* This function only works for X.509 certificates.
         */
        if (gnutls_certificate_type_get(session) != GNUTLS_CRT_X509)
                return;

        cert_list = gnutls_certificate_get_peers(session, &cert_list_size);

        printf("Peer provided %d certificates.\n", cert_list_size);

        if (cert_list_size > 0) {
                int ret;

                /* we only print information about the first certificate.
                 */
                gnutls_x509_crt_init(&cert);

                gnutls_x509_crt_import(cert, &cert_list[0],
                                       GNUTLS_X509_FMT_DER);

                printf("Certificate info:\n");

                /* This is the preferred way of printing short information about
                   a certificate. */

                ret =
                    gnutls_x509_crt_print(cert, GNUTLS_CRT_PRINT_ONELINE,
                                          &cinfo);
                if (ret == 0) {
                        printf("\t%s\n", cinfo.data);
                        gnutls_free(cinfo.data);
                }

                /* If you want to extract fields manually for some other reason,
                   below are popular example calls. */

                expiration_time =
                    gnutls_x509_crt_get_expiration_time(cert);
                activation_time =
                    gnutls_x509_crt_get_activation_time(cert);

                printf("\tCertificate is valid since: %s",
                       ctime(&activation_time));
                printf("\tCertificate expires: %s",
                       ctime(&expiration_time));

                /* Print the serial number of the certificate.
                 */
                size = sizeof(serial);
                gnutls_x509_crt_get_serial(cert, serial, &size);

                printf("\tCertificate serial number: %s\n",
                       bin2hex(serial, size));

                /* Extract some of the public key algorithm's parameters
                 */
                algo = gnutls_x509_crt_get_pk_algorithm(cert, &bits);

                printf("Certificate public key: %s",
                       gnutls_pk_algorithm_get_name(algo));

                /* Print the version of the X.509
                 * certificate.
                 */
                printf("\tCertificate version: #%d\n",
                       gnutls_x509_crt_get_version(cert));

                size = sizeof(dn);
                gnutls_x509_crt_get_dn(cert, dn, &size);
                printf("\tDN: %s\n", dn);

                size = sizeof(dn);
                gnutls_x509_crt_get_issuer_dn(cert, dn, &size);
                printf("\tIssuer's DN: %s\n", dn);

                gnutls_x509_crt_deinit(cert);

        }
}

Next: , Previous: , Up: Miscellaneous examples   [Contents][Index]

7.4.3 Listing the ciphersuites in a priority string

This is a small program to list the enabled ciphersuites by a priority string.

/* This example code is placed in the public domain. */

#include <config.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>

static void print_cipher_suite_list(const char *priorities)
{
        size_t i;
        int ret;
        unsigned int idx;
        const char *name;
        const char *err;
        unsigned char id[2];
        gnutls_protocol_t version;
        gnutls_priority_t pcache;

        if (priorities != NULL) {
                printf("Cipher suites for %s\n", priorities);

                ret = gnutls_priority_init(&pcache, priorities, &err);
                if (ret < 0) {
                        fprintf(stderr, "Syntax error at: %s\n", err);
                        exit(1);
                }

                for (i = 0;; i++) {
                        ret =
                            gnutls_priority_get_cipher_suite_index(pcache,
                                                                   i,
                                                                   &idx);
                        if (ret == GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE)
                                break;
                        if (ret == GNUTLS_E_UNKNOWN_CIPHER_SUITE)
                                continue;

                        name =
                            gnutls_cipher_suite_info(idx, id, NULL, NULL,
                                                     NULL, &version);

                        if (name != NULL)
                                printf("%-50s\t0x%02x, 0x%02x\t%s\n",
                                       name, (unsigned char) id[0],
                                       (unsigned char) id[1],
                                       gnutls_protocol_get_name(version));
                }

                return;
        }
}

int main(int argc, char **argv)
{
        if (argc > 1)
                print_cipher_suite_list(argv[1]);
        return 0;
}

Previous: , Up: Miscellaneous examples   [Contents][Index]

7.4.4 PKCS #12 structure generation example

This small program demonstrates the usage of the PKCS #12 API, by generating such a structure.

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <gnutls/gnutls.h>
#include <gnutls/pkcs12.h>

#include "examples.h"

#define OUTFILE "out.p12"

/* This function will write a pkcs12 structure into a file.
 * cert: is a DER encoded certificate
 * pkcs8_key: is a PKCS #8 encrypted key (note that this must be
 *  encrypted using a PKCS #12 cipher, or some browsers will crash)
 * password: is the password used to encrypt the PKCS #12 packet.
 */
int
write_pkcs12(const gnutls_datum_t * cert,
             const gnutls_datum_t * pkcs8_key, const char *password)
{
        gnutls_pkcs12_t pkcs12;
        int ret, bag_index;
        gnutls_pkcs12_bag_t bag, key_bag;
        char pkcs12_struct[10 * 1024];
        size_t pkcs12_struct_size;
        FILE *fd;

        /* A good idea might be to use gnutls_x509_privkey_get_key_id()
         * to obtain a unique ID.
         */
        gnutls_datum_t key_id = { (void *) "\x00\x00\x07", 3 };

        gnutls_global_init();

        /* Firstly we create two helper bags, which hold the certificate,
         * and the (encrypted) key.
         */

        gnutls_pkcs12_bag_init(&bag);
        gnutls_pkcs12_bag_init(&key_bag);

        ret =
            gnutls_pkcs12_bag_set_data(bag, GNUTLS_BAG_CERTIFICATE, cert);
        if (ret < 0) {
                fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
                return 1;
        }

        /* ret now holds the bag's index.
         */
        bag_index = ret;

        /* Associate a friendly name with the given certificate. Used
         * by browsers.
         */
        gnutls_pkcs12_bag_set_friendly_name(bag, bag_index, "My name");

        /* Associate the certificate with the key using a unique key
         * ID.
         */
        gnutls_pkcs12_bag_set_key_id(bag, bag_index, &key_id);

        /* use weak encryption for the certificate. 
         */
        gnutls_pkcs12_bag_encrypt(bag, password,
                                  GNUTLS_PKCS_USE_PKCS12_RC2_40);

        /* Now the key.
         */

        ret = gnutls_pkcs12_bag_set_data(key_bag,
                                         GNUTLS_BAG_PKCS8_ENCRYPTED_KEY,
                                         pkcs8_key);
        if (ret < 0) {
                fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
                return 1;
        }

        /* Note that since the PKCS #8 key is already encrypted we don't
         * bother encrypting that bag.
         */
        bag_index = ret;

        gnutls_pkcs12_bag_set_friendly_name(key_bag, bag_index, "My name");

        gnutls_pkcs12_bag_set_key_id(key_bag, bag_index, &key_id);


        /* The bags were filled. Now create the PKCS #12 structure.
         */
        gnutls_pkcs12_init(&pkcs12);

        /* Insert the two bags in the PKCS #12 structure.
         */

        gnutls_pkcs12_set_bag(pkcs12, bag);
        gnutls_pkcs12_set_bag(pkcs12, key_bag);


        /* Generate a message authentication code for the PKCS #12
         * structure.
         */
        gnutls_pkcs12_generate_mac(pkcs12, password);

        pkcs12_struct_size = sizeof(pkcs12_struct);
        ret =
            gnutls_pkcs12_export(pkcs12, GNUTLS_X509_FMT_DER,
                                 pkcs12_struct, &pkcs12_struct_size);
        if (ret < 0) {
                fprintf(stderr, "ret: %s\n", gnutls_strerror(ret));
                return 1;
        }

        fd = fopen(OUTFILE, "w");
        if (fd == NULL) {
                fprintf(stderr, "cannot open file\n");
                return 1;
        }
        fwrite(pkcs12_struct, 1, pkcs12_struct_size, fd);
        fclose(fd);

        gnutls_pkcs12_bag_deinit(bag);
        gnutls_pkcs12_bag_deinit(key_bag);
        gnutls_pkcs12_deinit(pkcs12);

        return 0;
}

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7.5 XSSL examples

XSSL is an experimental API available in the gnutls-xssl library and in gnutls/xssl.h header. It is intended to be a very simple to use API avoid the GnuTLS API. The API however has the following limitations


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7.5.1 Example client with X.509 certificate authentication

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/xssl.h>
#include "examples.h"

/* A simple TLS client, with X.509 authentication. Certificate verification
 * is explicit.
 */

extern int tcp_connect(void);
extern void tcp_close(int sd);

int main(void)
{
        int ret;
        char *line = NULL;
        size_t line_len;
        xssl_cred_t cred;
        xssl_t sb;
        unsigned int status;
        int fd;

        gnutls_global_init();

        fd = tcp_connect();

        ret = xssl_cred_init(&cred, GNUTLS_VMETHOD_SYSTEM_CAS, NULL, 0);
        if (ret < 0)
                exit(1);

        /* Initialize TLS session
         */
        ret = xssl_client_init(&sb, "www.example.com", NULL,
                               (gnutls_transport_ptr_t) fd,
                               NULL, cred, &status, 0);
        if (ret < 0) {
                if (ret == GNUTLS_E_AUTH_ERROR) {
                        gnutls_datum_t txt;

                        gnutls_certificate_verification_status_print
                            (status, GNUTLS_CRT_X509, &txt, 0);

                        fprintf(stderr, "Verification error (%x): %s\n",
                                status, txt.data);
                        gnutls_free(txt.data);
                }
                exit(1);
        }
#define REQ "GET / HTTP/1.0\r\n"
        ret = xssl_write(sb, REQ, sizeof(REQ) - 1);
        if (ret < 0)
                exit(1);

        do {
                ret = xssl_getline(sb, &line, &line_len);
                if (ret < 0)
                        exit(1);

                fprintf(stderr, "received: %s\n", line);
        }
        while (ret >= 0);

        gnutls_free(line);

        xssl_deinit(sb);

        tcp_close(fd);

        xssl_cred_deinit(cred);

        gnutls_global_deinit();

        return 0;
}

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7.5.2 Example client with X.509 certificate authentication and TOFU

/* This example code is placed in the public domain. */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <gnutls/gnutls.h>
#include <gnutls/xssl.h>
#include "examples.h"

/* A simple TLS client, with X.509 authentication. Certificate verification
 * with a fixed CA, and trust on first use. 
 */

extern int tcp_connect(void);
extern void tcp_close(int sd);

int main(void)
{
        int ret;
        char *line = NULL;
        size_t line_len;
        xssl_cred_t cred;
        xssl_t sb;
        gnutls_cinput_st aux[2];
        unsigned aux_size = 0;
        unsigned int status;
        int fd;

        gnutls_global_init();

        fd = tcp_connect();

        aux[aux_size].type = GNUTLS_CINPUT_TYPE_FILE;
        aux[aux_size].contents = GNUTLS_CINPUT_CAS;
        aux[aux_size].fmt = GNUTLS_X509_FMT_PEM;
        aux[aux_size].i1.file = "/path/to/ca/file";
        aux_size++;

        /* This may be skipped to use the default DB file */
        aux[aux_size].type = GNUTLS_CINPUT_TYPE_FILE;
        aux[aux_size].contents = GNUTLS_CINPUT_TOFU_DB;
        aux[aux_size].i1.file = "/path/to/trust/db/file";
        aux_size++;

        ret =
            xssl_cred_init(&cred,
                           GNUTLS_VMETHOD_GIVEN_CAS | GNUTLS_VMETHOD_TOFU,
                           aux, aux_size);
        if (ret < 0)
                exit(1);

        /* Initialize TLS session
         */
        ret = xssl_client_init(&sb, "www.example.com", NULL,
                               (gnutls_transport_ptr_t) fd,
                               NULL, cred, &status, 0);
        if (ret < 0) {
                if (ret == GNUTLS_E_AUTH_ERROR) {
                        gnutls_datum_t txt;

                        gnutls_certificate_verification_status_print
                            (status, GNUTLS_CRT_X509, &txt, 0);

                        fprintf(stderr, "Verification error (%x): %s\n",
                                status, txt.data);
                        gnutls_free(txt.data);
                }
                exit(1);
        }
#define REQ "GET / HTTP/1.0\r\n"
        ret = xssl_write(sb, REQ, sizeof(REQ) - 1);
        if (ret < 0)
                exit(1);

        do {
                ret = xssl_getline(sb, &line, &line_len);
                if (ret < 0)
                        exit(1);

                fprintf(stderr, "received: %s\n", line);
        }
        while (ret >= 0);

        gnutls_free(line);

        xssl_deinit(sb);

        tcp_close(fd);

        xssl_cred_deinit(cred);

        gnutls_global_deinit();

        return 0;
}

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8 Using GnuTLS as a cryptographic library

GnuTLS is not a low-level cryptographic library, i.e., it does not provide access to basic cryptographic primitives. However it abstracts the internal cryptographic back-end (see Cryptographic Backend), providing symmetric crypto, hash and HMAC algorithms, as well access to the random number generation.


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8.1 Symmetric algorithms

The available functions to access symmetric crypto algorithms operations are shown below. The supported algorithms are the algorithms required by the TLS protocol. They are listed in Table 3.1.

int gnutls_cipher_init (gnutls_cipher_hd_t * handle, gnutls_cipher_algorithm_t cipher, const gnutls_datum_t * key, const gnutls_datum_t * iv)
int gnutls_cipher_encrypt2 (gnutls_cipher_hd_t handle, const void * text, size_t textlen, void * ciphertext, size_t ciphertextlen)
int gnutls_cipher_decrypt2 (gnutls_cipher_hd_t handle, const void * ciphertext, size_t ciphertextlen, void * text, size_t textlen)
void gnutls_cipher_set_iv (gnutls_cipher_hd_t handle, void * iv, size_t ivlen)
void gnutls_cipher_deinit (gnutls_cipher_hd_t handle)

In order to support authenticated encryption with associated data (AEAD) algorithms the following functions are provided to set the associated data and retrieve the authentication tag.

int gnutls_cipher_add_auth (gnutls_cipher_hd_t handle, const void * text, size_t text_size)
int gnutls_cipher_tag (gnutls_cipher_hd_t handle, void * tag, size_t tag_size)

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8.2 Public key algorithms

Public key cryptography algorithms such as RSA, DSA and ECDSA, can be accessed using the abstract key API in Abstract key types. This is a high level API with the advantage of transparently handling keys in memory and keys present in smart cards.


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8.3 Hash and HMAC functions

The available operations to access hash functions and hash-MAC (HMAC) algorithms are shown below. HMAC algorithms provided keyed hash functionality. They supported HMAC algorithms are listed in Table 3.2.

int gnutls_hmac_init (gnutls_hmac_hd_t * dig, gnutls_mac_algorithm_t algorithm, const void * key, size_t keylen)
int gnutls_hmac (gnutls_hmac_hd_t handle, const void * text, size_t textlen)
void gnutls_hmac_output (gnutls_hmac_hd_t handle, void * digest)
void gnutls_hmac_deinit (gnutls_hmac_hd_t handle, void * digest)
int gnutls_hmac_get_len (gnutls_mac_algorithm_t algorithm)
int gnutls_hmac_fast (gnutls_mac_algorithm_t algorithm, const void * key, size_t keylen, const void * text, size_t textlen, void * digest)

The available functions to access hash functions are shown below. The supported hash functions are the same as the HMAC algorithms.

int gnutls_hash_init (gnutls_hash_hd_t * dig, gnutls_digest_algorithm_t algorithm)
int gnutls_hash (gnutls_hash_hd_t handle, const void * text, size_t textlen)
void gnutls_hash_output (gnutls_hash_hd_t handle, void * digest)
void gnutls_hash_deinit (gnutls_hash_hd_t handle, void * digest)
int gnutls_hash_get_len (gnutls_digest_algorithm_t algorithm)
int gnutls_hash_fast (gnutls_digest_algorithm_t algorithm, const void * text, size_t textlen, void * digest)
int gnutls_fingerprint (gnutls_digest_algorithm_t algo, const gnutls_datum_t * data, void * result, size_t * result_size)

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8.4 Random number generation

Access to the random number generator is provided using the gnutls_rnd function. It allows obtaining random data of various levels.

GNUTLS_RND_NONCE

Non-predictable random number. Fatal in parts of session if broken, i.e., vulnerable to statistical analysis.

GNUTLS_RND_RANDOM

Pseudo-random cryptographic random number. Fatal in session if broken.

GNUTLS_RND_KEY

Fatal in many sessions if broken.

Figure 8.1: The random number levels.

Function: int gnutls_rnd (gnutls_rnd_level_t level, void * data, size_t len)

level: a security level

data: place to store random bytes

len: The requested size

This function will generate random data and store it to output buffer.

Returns: Zero or a negative error code on error.

Since: 2.12.0


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9 Other included programs

Included with GnuTLS are also a few command line tools that let you use the library for common tasks without writing an application. The applications are discussed in this chapter.


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9.1 Invoking gnutls-cli

Simple client program to set up a TLS connection to some other computer. It sets up a TLS connection and forwards data from the standard input to the secured socket and vice versa.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the gnutls-cli program. This software is released under the GNU General Public License, version 3 or later.

gnutls-cli help/usage (--help)

This is the automatically generated usage text for gnutls-cli.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

gnutls-cli - GnuTLS client
Usage:  gnutls-cli [ -<flag> [<val>] | --<name>[{=| }<val>] ]... [hostname]

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -V, --verbose              More verbose output
                                - may appear multiple times
       --tofu                 Enable trust on first use authentication
                                - disabled as '--no-tofu'
       --strict-tofu          Fail to connect if a known certificate has changed
                                - disabled as '--no-strict-tofu'
       --dane                 Enable DANE certificate verification (DNSSEC)
                                - disabled as '--no-dane'
       --local-dns            Use the local DNS server for DNSSEC resolving
                                - disabled as '--no-local-dns'
       --ca-verification      Disable CA certificate verification
                                - disabled as '--no-ca-verification'
                                - enabled by default
       --ocsp                 Enable OCSP certificate verification
                                - disabled as '--no-ocsp'
   -r, --resume               Establish a session and resume
   -e, --rehandshake          Establish a session and rehandshake
   -s, --starttls             Connect, establish a plain session and start TLS
   -u, --udp                  Use DTLS (datagram TLS) over UDP
       --mtu=num              Set MTU for datagram TLS
                                - it must be in the range:
                                  0 to 17000
       --crlf                 Send CR LF instead of LF
       --x509fmtder           Use DER format for certificates to read from
   -f, --fingerprint          Send the openpgp fingerprint, instead of the key
       --print-cert           Print peer's certificate in PEM format
       --dh-bits=num          The minimum number of bits allowed for DH
       --priority=str         Priorities string
       --x509cafile=str       Certificate file or PKCS #11 URL to use
       --x509crlfile=file     CRL file to use
                                - file must pre-exist
       --pgpkeyfile=file      PGP Key file to use
                                - file must pre-exist
       --pgpkeyring=file      PGP Key ring file to use
                                - file must pre-exist
       --pgpcertfile=file     PGP Public Key (certificate) file to use
                                - file must pre-exist
       --x509keyfile=str      X.509 key file or PKCS #11 URL to use
       --x509certfile=str     X.509 Certificate file or PKCS #11 URL to use
       --pgpsubkey=str        PGP subkey to use (hex or auto)
       --srpusername=str      SRP username to use
       --srppasswd=str        SRP password to use
       --pskusername=str      PSK username to use
       --pskkey=str           PSK key (in hex) to use
   -p, --port=str             The port or service to connect to
       --insecure             Don't abort program if server certificate can't be validated
       --ranges               Use length-hiding padding to prevent traffic analysis
       --benchmark-ciphers    Benchmark individual ciphers
       --benchmark-soft-ciphers  Benchmark individual software ciphers (no hw acceleration)
       --benchmark-tls-kx     Benchmark TLS key exchange methods
       --benchmark-tls-ciphers  Benchmark TLS ciphers
   -l, --list                 Print a list of the supported algorithms and modes
       --noticket             Don't allow session tickets
       --srtp-profiles=str    Offer SRTP profiles
   -!, --alpn=str             Application layer protocol
                                - may appear multiple times
   -b, --heartbeat            Activate heartbeat support
   -", --recordsize=num       The maximum record size to advertize
                                - it must be in the range:
                                  0 to 4096
   -#, --disable-sni          Do not send a Server Name Indication (SNI)
   -$, --disable-extensions   Disable all the TLS extensions
   -%, --inline-commands      Inline commands of the form ^<cmd>^
   -&, --inline-commands-prefix=str Change the default (^) used as a delimiter for inline commands.  The
value is a single US-ASCII character (octets 0 - 127).
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.
Operands and options may be intermixed.  They will be reordered.

Simple client program to set up a TLS connection to some other computer.  It
sets up a TLS connection and forwards data from the standard input to the
secured socket and vice versa.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

tofu option

This is the “enable trust on first use authentication” option.

This option has some usage constraints. It:

This option will, in addition to certificate authentication, perform authentication based on previously seen public keys, a model similar to SSH authentication.

strict-tofu option

This is the “fail to connect if a known certificate has changed” option.

This option has some usage constraints. It:

This option will perform authentication as with option –tofu; however, while –tofu asks whether to trust a changed certificate, this option will fail in case of certificate changes.

dane option

This is the “enable dane certificate verification (dnssec)” option.

This option has some usage constraints. It:

This option will, in addition to certificate authentication using the trusted CAs, verify the server certificates using on the DANE information available via DNSSEC.

local-dns option

This is the “use the local dns server for dnssec resolving” option.

This option has some usage constraints. It:

This option will use the local DNS server for DNSSEC. This is disabled by default due to many servers not allowing DNSSEC.

ca-verification option

This is the “disable ca certificate verification” option.

This option has some usage constraints. It:

This option will disable CA certificate verification. It is to be used with the –dane or –tofu options.

ocsp option

This is the “enable ocsp certificate verification” option.

This option has some usage constraints. It:

This option will enable verification of the peer’s certificate using ocsp

resume option (-r)

This is the “establish a session and resume” option. Connect, establish a session, reconnect and resume.

rehandshake option (-e)

This is the “establish a session and rehandshake” option. Connect, establish a session and rehandshake immediately.

starttls option (-s)

This is the “connect, establish a plain session and start tls” option. The TLS session will be initiated when EOF or a SIGALRM is received.

dh-bits option

This is the “the minimum number of bits allowed for dh” option. This option takes a number argument. This option sets the minimum number of bits allowed for a Diffie-Hellman key exchange. You may want to lower the default value if the peer sends a weak prime and you get an connection error with unacceptable prime.

priority option

This is the “priorities string” option. This option takes a string argument. TLS algorithms and protocols to enable. You can use predefined sets of ciphersuites such as PERFORMANCE, NORMAL, SECURE128, SECURE256. The default is NORMAL.

Check the GnuTLS manual on section “Priority strings” for more information on allowed keywords

ranges option

This is the “use length-hiding padding to prevent traffic analysis” option. When possible (e.g., when %NEW_PADDING is specified), use length-hiding padding to prevent traffic analysis.

list option (-l)

This is the “print a list of the supported algorithms and modes” option. Print a list of the supported algorithms and modes. If a priority string is given then only the enabled ciphersuites are shown.

alpn option

This is the “application layer protocol” option. This option takes a string argument.

This option has some usage constraints. It:

This option will set and enable the Application Layer Protocol Negotiation (ALPN) in the TLS protocol.

disable-extensions option

This is the “disable all the tls extensions” option. This option disables all TLS extensions. Deprecated option. Use the priority string.

inline-commands option

This is the “inline commands of the form ^<cmd>^” option. Enable inline commands of the form ^<cmd>^. The inline commands are expected to be in a line by themselves. The available commands are: resume and renegotiate.

inline-commands-prefix option

This is the “change the default (^) used as a delimiter for inline commands. the value is a single us-ascii character (octets 0 - 127).” option. This option takes a string argument. Change the default (^) delimiter used for inline commands. The delimiter is expected to be a single US-ASCII character (octets 0 - 127). This option is only relevant if inline commands are enabled via the inline-commands option

gnutls-cli exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

gnutls-cli See Also

gnutls-cli-debug(1), gnutls-serv(1)

gnutls-cli Examples

Connecting using PSK authentication

To connect to a server using PSK authentication, you need to enable the choice of PSK by using a cipher priority parameter such as in the example below.

$ ./gnutls-cli -p 5556 localhost --pskusername psk_identity \
    --pskkey 88f3824b3e5659f52d00e959bacab954b6540344 \
    --priority NORMAL:-KX-ALL:+ECDHE-PSK:+DHE-PSK:+PSK
Resolving 'localhost'...
Connecting to '127.0.0.1:5556'...
- PSK authentication.
- Version: TLS1.1
- Key Exchange: PSK
- Cipher: AES-128-CBC
- MAC: SHA1
- Compression: NULL
- Handshake was completed
    
- Simple Client Mode:

By keeping the –pskusername parameter and removing the –pskkey parameter, it will query only for the password during the handshake.

Listing ciphersuites in a priority string

To list the ciphersuites in a priority string:

$ ./gnutls-cli --priority SECURE192 -l
Cipher suites for SECURE192
TLS_ECDHE_ECDSA_AES_256_CBC_SHA384         0xc0, 0x24	TLS1.2
TLS_ECDHE_ECDSA_AES_256_GCM_SHA384         0xc0, 0x2e	TLS1.2
TLS_ECDHE_RSA_AES_256_GCM_SHA384           0xc0, 0x30	TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA256             0x00, 0x6b	TLS1.2
TLS_DHE_DSS_AES_256_CBC_SHA256             0x00, 0x6a	TLS1.2
TLS_RSA_AES_256_CBC_SHA256                 0x00, 0x3d	TLS1.2

Certificate types: CTYPE-X.509
Protocols: VERS-TLS1.2, VERS-TLS1.1, VERS-TLS1.0, VERS-SSL3.0, VERS-DTLS1.0
Compression: COMP-NULL
Elliptic curves: CURVE-SECP384R1, CURVE-SECP521R1
PK-signatures: SIGN-RSA-SHA384, SIGN-ECDSA-SHA384, SIGN-RSA-SHA512, SIGN-ECDSA-SHA512

Connecting using a PKCS #11 token

To connect to a server using a certificate and a private key present in a PKCS #11 token you need to substitute the PKCS 11 URLs in the x509certfile and x509keyfile parameters.

Those can be found using "p11tool –list-tokens" and then listing all the objects in the needed token, and using the appropriate.

$ p11tool --list-tokens

Token 0:
URL: pkcs11:model=PKCS15;manufacturer=MyMan;serial=1234;token=Test
Label: Test
Manufacturer: EnterSafe
Model: PKCS15
Serial: 1234

$ p11tool --login --list-certs "pkcs11:model=PKCS15;manufacturer=MyMan;serial=1234;token=Test"

Object 0:
URL: pkcs11:model=PKCS15;manufacturer=MyMan;serial=1234;token=Test;object=client;object-type=cert
Type: X.509 Certificate
Label: client
ID: 2a:97:0d:58:d1:51:3c:23:07:ae:4e:0d:72:26:03:7d:99:06:02:6a

$ export MYCERT="pkcs11:model=PKCS15;manufacturer=MyMan;serial=1234;token=Test;object=client;object-type=cert"
$ export MYKEY="pkcs11:model=PKCS15;manufacturer=MyMan;serial=1234;token=Test;object=client;object-type=private"

$ gnutls-cli www.example.com --x509keyfile $MYKEY --x509certfile MYCERT

Notice that the private key only differs from the certificate in the object-type.


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9.2 Invoking gnutls-serv

Server program that listens to incoming TLS connections.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the gnutls-serv program. This software is released under the GNU General Public License, version 3 or later.

gnutls-serv help/usage (--help)

This is the automatically generated usage text for gnutls-serv.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

gnutls-serv - GnuTLS server
Usage:  gnutls-serv [ -<flag> [<val>] | --<name>[{=| }<val>] ]...

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
       --noticket             Don't accept session tickets
   -g, --generate             Generate Diffie-Hellman and RSA-export parameters
   -q, --quiet                Suppress some messages
       --nodb                 Do not use a resumption database
       --http                 Act as an HTTP server
       --echo                 Act as an Echo server
   -u, --udp                  Use DTLS (datagram TLS) over UDP
       --mtu=num              Set MTU for datagram TLS
                                - it must be in the range:
                                  0 to 17000
       --srtp-profiles=str    Offer SRTP profiles
   -a, --disable-client-cert  Do not request a client certificate
   -r, --require-client-cert  Require a client certificate
   -b, --heartbeat            Activate heartbeat support
       --x509fmtder           Use DER format for certificates to read from
       --priority=str         Priorities string
       --dhparams=file        DH params file to use
                                - file must pre-exist
       --x509cafile=str       Certificate file or PKCS #11 URL to use
       --x509crlfile=file     CRL file to use
                                - file must pre-exist
       --pgpkeyfile=file      PGP Key file to use
                                - file must pre-exist
       --pgpkeyring=file      PGP Key ring file to use
                                - file must pre-exist
       --pgpcertfile=file     PGP Public Key (certificate) file to use
                                - file must pre-exist
       --x509keyfile=str      X.509 key file or PKCS #11 URL to use
       --x509certfile=str     X.509 Certificate file or PKCS #11 URL to use
       --x509dsakeyfile=str   Alternative X.509 key file or PKCS #11 URL to use
       --x509dsacertfile=str  Alternative X.509 Certificate file or PKCS #11 URL to use
       --x509ecckeyfile=str   Alternative X.509 key file or PKCS #11 URL to use
       --x509ecccertfile=str  Alternative X.509 Certificate file or PKCS #11 URL to use
       --pgpsubkey=str        PGP subkey to use (hex or auto)
       --srppasswd=file       SRP password file to use
                                - file must pre-exist
       --srppasswdconf=file   SRP password configuration file to use
                                - file must pre-exist
       --pskpasswd=file       PSK password file to use
                                - file must pre-exist
       --pskhint=str          PSK identity hint to use
       --ocsp-response=file   The OCSP response to send to client
                                - file must pre-exist
   -p, --port=num             The port to connect to
   -l, --list                 Print a list of the supported algorithms and modes
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.

Server program that listens to incoming TLS connections.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

heartbeat option (-b)

This is the “activate heartbeat support” option. Regularly ping client via heartbeat extension messages

priority option

This is the “priorities string” option. This option takes a string argument. TLS algorithms and protocols to enable. You can use predefined sets of ciphersuites such as PERFORMANCE, NORMAL, SECURE128, SECURE256. The default is NORMAL.

Check the GnuTLS manual on section “Priority strings” for more information on allowed keywords

ocsp-response option

This is the “the ocsp response to send to client” option. This option takes a file argument. If the client requested an OCSP response, return data from this file to the client.

list option (-l)

This is the “print a list of the supported algorithms and modes” option. Print a list of the supported algorithms and modes. If a priority string is given then only the enabled ciphersuites are shown.

gnutls-serv exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

gnutls-serv See Also

gnutls-cli-debug(1), gnutls-cli(1)

gnutls-serv Examples

Running your own TLS server based on GnuTLS can be useful when debugging clients and/or GnuTLS itself. This section describes how to use gnutls-serv as a simple HTTPS server.

The most basic server can be started as:

gnutls-serv --http --priority "NORMAL:+ANON-ECDH:+ANON-DH"

It will only support anonymous ciphersuites, which many TLS clients refuse to use.

The next step is to add support for X.509. First we generate a CA:

$ certtool --generate-privkey > x509-ca-key.pem
$ echo 'cn = GnuTLS test CA' > ca.tmpl
$ echo 'ca' >> ca.tmpl
$ echo 'cert_signing_key' >> ca.tmpl
$ certtool --generate-self-signed --load-privkey x509-ca-key.pem \
  --template ca.tmpl --outfile x509-ca.pem
...

Then generate a server certificate. Remember to change the dns_name value to the name of your server host, or skip that command to avoid the field.

$ certtool --generate-privkey > x509-server-key.pem
$ echo 'organization = GnuTLS test server' > server.tmpl
$ echo 'cn = test.gnutls.org' >> server.tmpl
$ echo 'tls_www_server' >> server.tmpl
$ echo 'encryption_key' >> server.tmpl
$ echo 'signing_key' >> server.tmpl
$ echo 'dns_name = test.gnutls.org' >> server.tmpl
$ certtool --generate-certificate --load-privkey x509-server-key.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template server.tmpl --outfile x509-server.pem
...

For use in the client, you may want to generate a client certificate as well.

$ certtool --generate-privkey > x509-client-key.pem
$ echo 'cn = GnuTLS test client' > client.tmpl
$ echo 'tls_www_client' >> client.tmpl
$ echo 'encryption_key' >> client.tmpl
$ echo 'signing_key' >> client.tmpl
$ certtool --generate-certificate --load-privkey x509-client-key.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template client.tmpl --outfile x509-client.pem
...

To be able to import the client key/certificate into some applications, you will need to convert them into a PKCS#12 structure. This also encrypts the security sensitive key with a password.

$ certtool --to-p12 --load-ca-certificate x509-ca.pem \
  --load-privkey x509-client-key.pem --load-certificate x509-client.pem \
  --outder --outfile x509-client.p12

For icing, we’ll create a proxy certificate for the client too.

$ certtool --generate-privkey > x509-proxy-key.pem
$ echo 'cn = GnuTLS test client proxy' > proxy.tmpl
$ certtool --generate-proxy --load-privkey x509-proxy-key.pem \
  --load-ca-certificate x509-client.pem --load-ca-privkey x509-client-key.pem \
  --load-certificate x509-client.pem --template proxy.tmpl \
  --outfile x509-proxy.pem
...

Then start the server again:

$ gnutls-serv --http \
            --x509cafile x509-ca.pem \
            --x509keyfile x509-server-key.pem \
            --x509certfile x509-server.pem

Try connecting to the server using your web browser. Note that the server listens to port 5556 by default.

While you are at it, to allow connections using DSA, you can also create a DSA key and certificate for the server. These credentials will be used in the final example below.

$ certtool --generate-privkey --dsa > x509-server-key-dsa.pem
$ certtool --generate-certificate --load-privkey x509-server-key-dsa.pem \
  --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
  --template server.tmpl --outfile x509-server-dsa.pem
...

The next step is to create OpenPGP credentials for the server.

gpg --gen-key
...enter whatever details you want, use 'test.gnutls.org' as name...

Make a note of the OpenPGP key identifier of the newly generated key, here it was 5D1D14D8. You will need to export the key for GnuTLS to be able to use it.

gpg -a --export 5D1D14D8 > openpgp-server.txt
gpg --export 5D1D14D8 > openpgp-server.bin
gpg --export-secret-keys 5D1D14D8 > openpgp-server-key.bin
gpg -a --export-secret-keys 5D1D14D8 > openpgp-server-key.txt

Let’s start the server with support for OpenPGP credentials:

gnutls-serv --http --priority NORMAL:+CTYPE-OPENPGP \
            --pgpkeyfile openpgp-server-key.txt \
            --pgpcertfile openpgp-server.txt

The next step is to add support for SRP authentication. This requires an SRP password file created with srptool. To start the server with SRP support:

gnutls-serv --http --priority NORMAL:+SRP-RSA:+SRP \
            --srppasswdconf srp-tpasswd.conf \
            --srppasswd srp-passwd.txt

Let’s also start a server with support for PSK. This would require a password file created with psktool.

gnutls-serv --http --priority NORMAL:+ECDHE-PSK:+PSK \
            --pskpasswd psk-passwd.txt

Finally, we start the server with all the earlier parameters and you get this command:

gnutls-serv --http --priority NORMAL:+PSK:+SRP:+CTYPE-OPENPGP \
            --x509cafile x509-ca.pem \
            --x509keyfile x509-server-key.pem \
            --x509certfile x509-server.pem \
            --x509dsakeyfile x509-server-key-dsa.pem \
            --x509dsacertfile x509-server-dsa.pem \
            --pgpkeyfile openpgp-server-key.txt \
            --pgpcertfile openpgp-server.txt \
            --srppasswdconf srp-tpasswd.conf \
            --srppasswd srp-passwd.txt \
            --pskpasswd psk-passwd.txt

Previous: , Up: Other included programs   [Contents][Index]

9.3 Invoking gnutls-cli-debug

TLS debug client. It sets up multiple TLS connections to a server and queries its capabilities. It was created to assist in debugging GnuTLS, but it might be useful to extract a TLS server’s capabilities. It connects to a TLS server, performs tests and print the server’s capabilities. If called with the ‘-v’ parameter more checks will be performed. Can be used to check for servers with special needs or bugs.

This section was generated by AutoGen, using the agtexi-cmd template and the option descriptions for the gnutls-cli-debug program. This software is released under the GNU General Public License, version 3 or later.

gnutls-cli-debug help/usage (--help)

This is the automatically generated usage text for gnutls-cli-debug.

The text printed is the same whether selected with the help option (--help) or the more-help option (--more-help). more-help will print the usage text by passing it through a pager program. more-help is disabled on platforms without a working fork(2) function. The PAGER environment variable is used to select the program, defaulting to more. Both will exit with a status code of 0.

gnutls-cli-debug - GnuTLS debug client
Usage:  gnutls-cli-debug [ -<flag> [<val>] | --<name>[{=| }<val>] ]... 

   -d, --debug=num            Enable debugging
                                - it must be in the range:
                                  0 to 9999
   -V, --verbose              More verbose output
                                - may appear multiple times
   -p, --port=num             The port to connect to
                                - it must be in the range:
                                  0 to 65536
   -v, --version[=arg]        output version information and exit
   -h, --help                 display extended usage information and exit
   -!, --more-help            extended usage information passed thru pager

Options are specified by doubled hyphens and their name or by a single
hyphen and the flag character.
Operands and options may be intermixed.  They will be reordered.

TLS debug client.  It sets up multiple TLS connections to a server and
queries its capabilities.  It was created to assist in debugging GnuTLS,
but it might be useful to extract a TLS server's capabilities.  It connects
to a TLS server, performs tests and print the server's capabilities.  If
called with the `-v' parameter more checks will be performed.  Can be used
to check for servers with special needs or bugs.

Please send bug reports to:  <@PACKAGE_BUGREPORT@>

debug option (-d)

This is the “enable debugging” option. This option takes a number argument. Specifies the debug level.

gnutls-cli-debug exit status

One of the following exit values will be returned:

0 (EXIT_SUCCESS)

Successful program execution.

1 (EXIT_FAILURE)

The operation failed or the command syntax was not valid.

gnutls-cli-debug See Also

gnutls-cli(1), gnutls-serv(1)

gnutls-cli-debug Examples

$ ../src/gnutls-cli-debug localhost
Resolving 'localhost'...
Connecting to '127.0.0.1:443'...
Checking for SSL 3.0 support... yes
Checking whether %COMPAT is required... no
Checking for TLS 1.0 support... yes
Checking for TLS 1.1 support... no
Checking fallback from TLS 1.1 to... TLS 1.0
Checking for TLS 1.2 support... no
Checking whether we need to disable TLS 1.0... N/A
Checking for Safe renegotiation support... yes
Checking for Safe renegotiation support (SCSV)... yes
Checking for HTTPS server name... not checked
Checking for version rollback bug in RSA PMS... no
Checking for version rollback bug in Client Hello... no
Checking whether the server ignores the RSA PMS version... no
Checking whether the server can accept Hello Extensions... yes
Checking whether the server can accept small records (512 bytes)... yes
Checking whether the server can accept cipher suites not in SSL 3.0 spec... yes
Checking whether the server can accept a bogus TLS record version in the client hello... yes
Checking for certificate information... N/A
Checking for trusted CAs... N/A
Checking whether the server understands TLS closure alerts... partially
Checking whether the server supports session resumption... yes
Checking for export-grade ciphersuite support... no
Checking RSA-export ciphersuite info... N/A
Checking for anonymous authentication support... no
Checking anonymous Diffie-Hellman group info... N/A
Checking for ephemeral Diffie-Hellman support... no
Checking ephemeral Diffie-Hellman group info... N/A
Checking for ephemeral EC Diffie-Hellman support... yes
Checking ephemeral EC Diffie-Hellman group info...
 Curve SECP256R1 
Checking for AES-GCM cipher support... no
Checking for AES-CBC cipher support... yes
Checking for CAMELLIA cipher support... no
Checking for 3DES-CBC cipher support... yes
Checking for ARCFOUR 128 cipher support... yes
Checking for ARCFOUR 40 cipher support... no
Checking for MD5 MAC support... yes
Checking for SHA1 MAC support... yes
Checking for SHA256 MAC support... no
Checking for ZLIB compression support... no
Checking for max record size... no
Checking for OpenPGP authentication support... no

Next: , Previous: , Up: Top   [Contents][Index]

10 Internal Architecture of GnuTLS

This chapter is to give a brief description of the way GnuTLS works. The focus is to give an idea to potential developers and those who want to know what happens inside the black box.


Next: , Up: Internal architecture of GnuTLS   [Contents][Index]

10.1 The TLS Protocol

The main use case for the TLS protocol is shown in Figure 10.1. A user of a library implementing the protocol expects no less than this functionality, i.e., to be able to set parameters such as the accepted security level, perform a negotiation with the peer and be able to exchange data.

gnutls-client-server-use-case

Figure 10.1: TLS protocol use case.


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10.2 TLS Handshake Protocol

The GnuTLS handshake protocol is implemented as a state machine that waits for input or returns immediately when the non-blocking transport layer functions are used. The main idea is shown in Figure 10.2.

gnutls-handshake-state

Figure 10.2: GnuTLS handshake state machine.

Also the way the input is processed varies per ciphersuite. Several implementations of the internal handlers are available and gnutls_handshake only multiplexes the input to the appropriate handler. For example a PSK ciphersuite has a different implementation of the process_client_key_exchange than a certificate ciphersuite. We illustrate the idea in Figure 10.3.

gnutls-handshake-sequence

Figure 10.3: GnuTLS handshake process sequence.


Next: , Previous: , Up: Internal architecture of GnuTLS   [Contents][Index]

10.3 TLS Authentication Methods

In GnuTLS authentication methods can be implemented quite easily. Since the required changes to add a new authentication method affect only the handshake protocol, a simple interface is used. An authentication method needs to implement the functions shown below.

typedef struct 
{
  const char *name;
  int (*gnutls_generate_server_certificate) (gnutls_session_t, gnutls_buffer_st*);
  int (*gnutls_generate_client_certificate) (gnutls_session_t, gnutls_buffer_st*);
  int (*gnutls_generate_server_kx) (gnutls_session_t, gnutls_buffer_st*);
  int (*gnutls_generate_client_kx) (gnutls_session_t, gnutls_buffer_st*);
  int (*gnutls_generate_client_cert_vrfy) (gnutls_session_t, gnutls_buffer_st *);
  int (*gnutls_generate_server_certificate_request) (gnutls_session_t,
                                                     gnutls_buffer_st *);

  int (*gnutls_process_server_certificate) (gnutls_session_t, opaque *,
                                            size_t);
  int (*gnutls_process_client_certificate) (gnutls_session_t, opaque *,
                                            size_t);
  int (*gnutls_process_server_kx) (gnutls_session_t, opaque *, size_t);
  int (*gnutls_process_client_kx) (gnutls_session_t, opaque *, size_t);
  int (*gnutls_process_client_cert_vrfy) (gnutls_session_t, opaque *, size_t);
  int (*gnutls_process_server_certificate_request) (gnutls_session_t,
                                                    opaque *, size_t);
} mod_auth_st;

Those functions are responsible for the interpretation of the handshake protocol messages. It is common for such functions to read data from one or more credentials_t structures12 and write data, such as certificates, usernames etc. to auth_info_t structures.

Simple examples of existing authentication methods can be seen in auth/psk.c for PSK ciphersuites and auth/srp.c for SRP ciphersuites. After implementing these functions the structure holding its pointers has to be registered in gnutls_algorithms.c in the _gnutls_kx_algorithms structure.


Next: , Previous: , Up: Internal architecture of GnuTLS   [Contents][Index]

10.4 TLS Extension Handling

As with authentication methods, the TLS extensions handlers can be implemented using the interface shown below.

typedef int (*gnutls_ext_recv_func) (gnutls_session_t session,
                                     const unsigned char *data, size_t len);
typedef int (*gnutls_ext_send_func) (gnutls_session_t session,
                                     gnutls_buffer_st *extdata);

Here there are two functions, one for receiving the extension data and one for sending. These functions have to check internally whether they operate in client or server side.

A simple example of an extension handler can be seen in ext/srp.c in GnuTLS’ source code. After implementing these functions, together with the extension number they handle, they have to be registered using _gnutls_ext_register in gnutls_extensions.c typically within _gnutls_ext_init.

Adding a new TLS extension

Adding support for a new TLS extension is done from time to time, and the process to do so is not difficult. Here are the steps you need to follow if you wish to do this yourself. For sake of discussion, let’s consider adding support for the hypothetical TLS extension foobar.

Add configure option like --enable-foobar or --disable-foobar.

This step is useful when the extension code is large and it might be desirable to disable the extension under some circumstances. Otherwise it can be safely skipped.

Whether to chose enable or disable depends on whether you intend to make the extension be enabled by default. Look at existing checks (i.e., SRP, authz) for how to model the code. For example:

AC_MSG_CHECKING([whether to disable foobar support])
AC_ARG_ENABLE(foobar,
	AS_HELP_STRING([--disable-foobar],
		[disable foobar support]),
	ac_enable_foobar=no)
if test x$ac_enable_foobar != xno; then
 AC_MSG_RESULT(no)
 AC_DEFINE(ENABLE_FOOBAR, 1, [enable foobar])
else
 ac_full=0
 AC_MSG_RESULT(yes)
fi
AM_CONDITIONAL(ENABLE_FOOBAR, test "$ac_enable_foobar" != "no")

These lines should go in m4/hooks.m4.

Add IANA extension value to extensions_t in gnutls_int.h.

A good name for the value would be GNUTLS_EXTENSION_FOOBAR. Check with http://www.iana.org/assignments/tls-extensiontype-values for allocated values. For experiments, you could pick a number but remember that some consider it a bad idea to deploy such modified version since it will lead to interoperability problems in the future when the IANA allocates that number to someone else, or when the foobar protocol is allocated another number.

Add an entry to _gnutls_extensions in gnutls_extensions.c.

A typical entry would be:

  int ret;

#if ENABLE_FOOBAR
  ret = _gnutls_ext_register (&foobar_ext);
  if (ret != GNUTLS_E_SUCCESS)
    return ret;
#endif

Most likely you’ll need to add an #include "ext/foobar.h", that will contain something like like:

  extension_entry_st foobar_ext = {
    .name = "FOOBAR",
    .type = GNUTLS_EXTENSION_FOOBAR,
    .parse_type = GNUTLS_EXT_TLS,
    .recv_func = _foobar_recv_params,
    .send_func = _foobar_send_params,
    .pack_func = _foobar_pack,
    .unpack_func = _foobar_unpack,
    .deinit_func = NULL
  }

The GNUTLS_EXTENSION_FOOBAR is the integer value you added to gnutls_int.h earlier. In this structure you specify the functions to read the extension from the hello message, the function to send the reply to, and two more functions to pack and unpack from stored session data (e.g. when resumming a session). The deinit function will be called to deinitialize the extension’s private parameters, if any.

Note that the conditional ENABLE_FOOBAR definition should only be used if step 1 with the configure options has taken place.

Add new files that implement the extension.

The functions you are responsible to add are those mentioned in the previous step. They should be added in a file such as ext/foobar.c and headers should be placed in ext/foobar.h. As a starter, you could add this:

int
_foobar_recv_params (gnutls_session_t session, const opaque * data,
                     size_t data_size)
{
  return 0;
}

int
_foobar_send_params (gnutls_session_t session, gnutls_buffer_st* data)
{
  return 0;
}

int
_foobar_pack (extension_priv_data_t epriv, gnutls_buffer_st * ps)
{
   /* Append the extension's internal state to buffer */
   return 0;
}

int
_foobar_unpack (gnutls_buffer_st * ps, extension_priv_data_t * epriv)
{
   /* Read the internal state from buffer */
   return 0;
}

The _foobar_recv_params function is responsible for parsing incoming extension data (both in the client and server).

The _foobar_send_params function is responsible for sending extension data (both in the client and server).

If you receive length fields that don’t match, return GNUTLS_E_UNEXPECTED_PACKET_LENGTH. If you receive invalid data, return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER. You can use other error codes from the list in Error codes. Return 0 on success.

An extension typically stores private information in the session data for later usage. That can be done using the functions _gnutls_ext_set_session_data and _gnutls_ext_get_session_data. You can check simple examples at ext/max_record.c and ext/server_name.c extensions. That private information can be saved and restored across session resumption if the following functions are set:

The _foobar_pack function is responsible for packing internal extension data to save them in the session resumption storage.

The _foobar_unpack function is responsible for restoring session data from the session resumption storage.

Recall that both the client and server, send and receive parameters, and your code most likely will need to do different things depending on which mode it is in. It may be useful to make this distinction explicit in the code. Thus, for example, a better template than above would be:

int
_gnutls_foobar_recv_params (gnutls_session_t session,
                            const opaque * data,
                            size_t data_size)
{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_recv_client (session, data, data_size);
  else
    return foobar_recv_server (session, data, data_size);
}

int
_gnutls_foobar_send_params (gnutls_session_t session,
                            gnutls_buffer_st * data)
{
  if (session->security_parameters.entity == GNUTLS_CLIENT)
    return foobar_send_client (session, data);
  else
    return foobar_send_server (session, data);
}

The functions used would be declared as static functions, of the appropriate prototype, in the same file. When adding the files, you’ll need to add them to ext/Makefile.am as well, for example:

if ENABLE_FOOBAR
libgnutls_ext_la_SOURCES += ext/foobar.c ext/foobar.h
endif

Add API functions to enable/disable the extension.

It might be desirable to allow users of the extension to request use of the extension, or set extension specific data. This can be implemented by adding extension specific function calls that can be added to includes/gnutls/gnutls.h, as long as the LGPLv2.1+ applies. The implementation of the function should lie in the ext/foobar.c file.

To make the API available in the shared library you need to add the symbol in lib/libgnutls.map, so that the symbol is exported properly.

When writing GTK-DOC style documentation for your new APIs, don’t forget to add Since: tags to indicate the GnuTLS version the API was introduced in.

Adding a new Supplemental Data Handshake Message

TLS handshake extensions allow to send so called supplemental data handshake messages [RFC4680]. This short section explains how to implement a supplemental data handshake message for a given TLS extension.

First of all, modify your extension foobar in the way, the that flags session->security_parameters.do_send_supplemental and session->security_parameters.do_recv_supplemental are set:

int
_gnutls_foobar_recv_params (gnutls_session_t session, const opaque * data,
                                 size_t _data_size)
{
   ...
   session->security_parameters.do_recv_supplemental=1;
   ...
}

int
_gnutls_foobar_send_params (gnutls_session_t session, gnutls_buffer_st *extdata)
{
   ...
   session->security_parameters.do_send_supplemental=1;
   ...
}

Furthermore add the functions _foobar_supp_recv_params and _foobar_supp_send_params to _foobar.h and _foobar.c. The following example code shows how to send a “Hello World” string in the supplemental data handshake message:

int 
_foobar_supp_recv_params(gnutls_session_t session, const opaque *data, size_t _data_size)
{
   uint8_t len = _data_size;
   unsigned char *msg;

   msg = gnutls_malloc(len);
   if (msg == NULL) return GNUTLS_E_MEMORY_ERROR;

   memcpy(msg, data, len);
   msg[len]='\0';

   /* do something with msg */
   gnutls_free(msg);

   return len;
}

int 
_foobar_supp_send_params(gnutls_session_t session, gnutls_buffer_st *buf)
{
   unsigned char *msg = "hello world";
   int len = strlen(msg);

   _gnutls_buffer_append_data_prefix(buf, 8, msg, len);

   return len;
}

Afterwards, add the new supplemental data handshake message to lib/gnutls_supplemental.c by adding a new entry to the _gnutls_supplemental[] structure:

gnutls_supplemental_entry _gnutls_supplemental[] = 
{
  {"foobar",
   GNUTLS_SUPPLEMENTAL_FOOBAR_DATA,
   _foobar_supp_recv_params,
   _foobar_supp_send_params},
  {0, 0, 0, 0}
};

You have to include your foobar.h header file as well:

#include "foobar.h"

Lastly, add the new supplemental data type to lib/includes/gnutls/gnutls.h:

typedef enum
{
    GNUTLS_SUPPLEMENTAL_USER_MAPPING_DATA = 0,
    GNUTLS_SUPPLEMENTAL_FOOBAR_DATA = 1
} gnutls_supplemental_data_format_type_t;

Heartbeat extension.

One such extension is HeartBeat protocol (RFC6520: https://tools.ietf.org/html/rfc6520) implementation. To enable it use option –heartbeat with example client and server supplied with gnutls:

./doc/credentials/gnutls-http-serv --priority "NORMAL:-CIPHER-ALL:+NULL" -d 100 \
    --heartbeat --echo
./src/gnutls-cli --priority "NORMAL:-CIPHER-ALL:+NULL" -d 100 localhost -p 5556 \
    --insecure --heartbeat

After that pasting

**HEARTBEAT**

command into gnutls-cli will trigger corresponding command on the server and it will send HeartBeat Request with random length to client.

Another way is to run capabilities check with:

./doc/credentials/gnutls-http-serv -d 100 --heartbeat
./src/gnutls-cli-debug localhost -p 5556

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10.5 Cryptographic Backend

Today most new processors, either for embedded or desktop systems include either instructions intended to speed up cryptographic operations, or a co-processor with cryptographic capabilities. Taking advantage of those is a challenging task for every cryptographic application or library. Unfortunately the cryptographic library that GnuTLS is based on takes no advantage of these capabilities. For this reason GnuTLS handles this internally by following a layered approach to accessing cryptographic operations as in Figure 10.4.

gnutls-crypto-layers

Figure 10.4: GnuTLS cryptographic back-end design.

The TLS layer uses a cryptographic provider layer, that will in turn either use the default crypto provider – a software crypto library, or use an external crypto provider, if available in the local system. The reason of handling the external cryptographic provider in GnuTLS and not delegating it to the cryptographic libraries, is that none of the supported cryptographic libraries support /dev/crypto or CPU-optimized cryptography in an efficient way.

Cryptographic library layer

The Cryptographic library layer, currently supports only libnettle. Older versions of GnuTLS used to support libgcrypt, but it was switched with nettle mainly for performance reasons13 and secondary because it is a simpler library to use. In the future other cryptographic libraries might be supported as well.

External cryptography provider

Systems that include a cryptographic co-processor, typically come with kernel drivers to utilize the operations from software. For this reason GnuTLS provides a layer where each individual algorithm used can be replaced by another implementation, i.e., the one provided by the driver. The FreeBSD, OpenBSD and Linux kernels14 include already a number of hardware assisted implementations, and also provide an interface to access them, called /dev/crypto. GnuTLS will take advantage of this interface if compiled with special options. That is because in most systems where hardware-assisted cryptographic operations are not available, using this interface might actually harm performance.

In systems that include cryptographic instructions with the CPU’s instructions set, using the kernel interface will introduce an unneeded layer. For this reason GnuTLS includes such optimizations found in popular processors such as the AES-NI or VIA PADLOCK instruction sets. This is achieved using a mechanism that detects CPU capabilities and overrides parts of crypto back-end at runtime. The next section discusses the registration of a detected algorithm optimization. For more information please consult the GnuTLS source code in lib/accelerated/.

Overriding specific algorithms

When an optimized implementation of a single algorithm is available, say a hardware assisted version of AES-CBC then the following (internal) functions, from crypto-backend.h, can be used to register those algorithms.

Those registration functions will only replace the specified algorithm and leave the rest of subsystem intact.

Overriding the cryptographic library

In some systems, that might contain a broad acceleration engine, it might be desirable to override big parts of the cryptographic back-end, or even all of them. The following functions are provided for this reason.


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Appendix A Upgrading from previous versions

The GnuTLS library typically maintains binary and source code compatibility across versions. The releases that have the major version increased break binary compatibility but source compatibility is provided. This section lists exceptional cases where changes to existing code are required due to library changes.

Upgrading to 2.12.x from previous versions

GnuTLS 2.12.x is binary compatible with previous versions but changes the semantics of gnutls_transport_set_lowat, which might cause breakage in applications that relied on its default value be 1. Two fixes are proposed:

Upgrading to 3.0.x from 2.12.x

GnuTLS 3.0.x is source compatible with previous versions except for the functions listed below.

Old functionReplacement
gnutls_transport_set_lowatTo replace its functionality the function gnutls_record_check_pending has to be used, as described in Asynchronous operation
gnutls_session_get_server_random, gnutls_session_get_client_randomThey are replaced by the safer function gnutls_session_get_random
gnutls_session_get_master_secretReplaced by the keying material exporters discussed in Deriving keys for other applications/protocols
gnutls_transport_set_global_errnoReplaced by using the system’s errno fascility or gnutls_transport_set_errno.
gnutls_x509_privkey_verify_dataReplaced by gnutls_pubkey_verify_data.
gnutls_certificate_verify_peersReplaced by gnutls_certificate_verify_peers2.
gnutls_psk_netconf_derive_keyRemoved. The key derivation function was never standardized.
gnutls_session_set_finished_functionRemoved.
gnutls_ext_registerRemoved. Extension registration API is now internal to allow easier changes in the API.
gnutls_certificate_get_x509_crls, gnutls_certificate_get_x509_casRemoved to allow updating the internal structures. Replaced by gnutls_certificate_get_issuer.
gnutls_certificate_get_openpgp_keyringRemoved.
gnutls_ia_*Removed. The inner application extensions were completely removed (they failed to be standardized).

Upgrading to 3.1.x from 3.0.x

GnuTLS 3.1.x is source and binary compatible with GnuTLS 3.0.x releases. Few functions have been deprecated and are listed below.

Old functionReplacement
gnutls_pubkey_verify_hashThe function gnutls_pubkey_verify_hash2 is provided and is functionally equivalent and safer to use.
gnutls_pubkey_verify_dataThe function gnutls_pubkey_verify_data2 is provided and is functionally equivalent and safer to use.

Upgrading to 3.2.x from 3.1.x

GnuTLS 3.2.x is source and binary compatible with GnuTLS 3.1.x releases. Few functions have been deprecated and are listed below.

Old functionReplacement
gnutls_privkey_sign_raw_dataThe function gnutls_privkey_sign_hash is equivalent when the flag GNUTLS_PRIVKEY_SIGN_FLAG_TLS1_RSA is specified.

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Appendix B Support


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B.1 Getting Help

A mailing list where users may help each other exists, and you can reach it by sending e-mail to gnutls-help@gnutls.org. Archives of the mailing list discussions, and an interface to manage subscriptions, is available through the World Wide Web at http://lists.gnutls.org/pipermail/gnutls-help/.

A mailing list for developers are also available, see http://www.gnutls.org/lists.html. Bug reports should be sent to bugs@gnutls.org, see Bug Reports.


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B.2 Commercial Support

Commercial support is available for users of GnuTLS. The kind of support that can be purchased may include:

If you are interested, please write to:

Simon Josefsson Datakonsult
Hagagatan 24
113 47 Stockholm
Sweden

E-mail: simon@josefsson.org

If your company provides support related to GnuTLS and would like to be mentioned here, contact the authors.


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B.3 Bug Reports

If you think you have found a bug in GnuTLS, please investigate it and report it.

Please make an effort to produce a self-contained report, with something definite that can be tested or debugged. Vague queries or piecemeal messages are difficult to act on and don’t help the development effort.

If your bug report is good, we will do our best to help you to get a corrected version of the software; if the bug report is poor, we won’t do anything about it (apart from asking you to send better bug reports).

If you think something in this manual is unclear, or downright incorrect, or if the language needs to be improved, please also send a note.

Send your bug report to:

bugs@gnutls.org

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B.4 Contributing

If you want to submit a patch for inclusion – from solving a typo you discovered, up to adding support for a new feature – you should submit it as a bug report, using the process in Bug Reports. There are some things that you can do to increase the chances for it to be included in the official package.

Unless your patch is very small (say, under 10 lines) we require that you assign the copyright of your work to the Free Software Foundation. This is to protect the freedom of the project. If you have not already signed papers, we will send you the necessary information when you submit your contribution.

For contributions that doesn’t consist of actual programming code, the only guidelines are common sense. For code contributions, a number of style guides will help you:


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B.5 Certification

Many cryptographic libraries claim certifications from national or international bodies. These certifications are tied on a specific (and often restricted) version of the library or a specific product using the library, and typically in the case of software they assure that the algorithms implemented are correct. The major certifications known are:

Obtaining such a certification is an expensive and elaborate job that has no immediate value for a continuously developed free software library (as the certification is tied to the particular version tested), and in the case of algorithm verification of FIPS 140-2 it doesn’t make much sense as the library is freely available and anyone can verify the correctness of algorithm implementation. As such we are not actively pursuing this kind of certification. If you are, nevertheless, interested, see Commercial Support.


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Appendix C Error Codes and Descriptions

The error codes used throughout the library are described below. The return code GNUTLS_E_SUCCESS indicate successful operation, and is guaranteed to have the value 0, so you can use it in logical expressions.

0GNUTLS_E_SUCCESSSuccess.
-3GNUTLS_E_UNKNOWN_COMPRESSION_ALGORITHMCould not negotiate a supported compression method.
-6GNUTLS_E_UNKNOWN_CIPHER_TYPEThe cipher type is unsupported.
-7GNUTLS_E_LARGE_PACKETThe transmitted packet is too large (EMSGSIZE).
-8GNUTLS_E_UNSUPPORTED_VERSION_PACKETA record packet with illegal version was received.
-9GNUTLS_E_UNEXPECTED_PACKET_LENGTHA TLS packet with unexpected length was received.
-10GNUTLS_E_INVALID_SESSIONThe specified session has been invalidated for some reason.
-12GNUTLS_E_FATAL_ALERT_RECEIVEDA TLS fatal alert has been received.
-15GNUTLS_E_UNEXPECTED_PACKETAn unexpected TLS packet was received.
-16GNUTLS_E_WARNING_ALERT_RECEIVEDA TLS warning alert has been received.
-18GNUTLS_E_ERROR_IN_FINISHED_PACKETAn error was encountered at the TLS Finished packet calculation.
-19GNUTLS_E_UNEXPECTED_HANDSHAKE_PACKETAn unexpected TLS handshake packet was received.
-21GNUTLS_E_UNKNOWN_CIPHER_SUITECould not negotiate a supported cipher suite.
-22GNUTLS_E_UNWANTED_ALGORITHMAn algorithm that is not enabled was negotiated.
-23GNUTLS_E_MPI_SCAN_FAILEDThe scanning of a large integer has failed.
-24GNUTLS_E_DECRYPTION_FAILEDDecryption has failed.
-25GNUTLS_E_MEMORY_ERRORInternal error in memory allocation.
-26GNUTLS_E_DECOMPRESSION_FAILEDDecompression of the TLS record packet has failed.
-27GNUTLS_E_COMPRESSION_FAILEDCompression of the TLS record packet has failed.
-28GNUTLS_E_AGAINResource temporarily unavailable, try again.
-29GNUTLS_E_EXPIREDThe requested session has expired.
-30GNUTLS_E_DB_ERRORError in Database backend.
-31GNUTLS_E_SRP_PWD_ERRORError in password file.
-32GNUTLS_E_INSUFFICIENT_CREDENTIALSInsufficient credentials for that request.
-33GNUTLS_E_HASH_FAILEDHashing has failed.
-34GNUTLS_E_BASE64_DECODING_ERRORBase64 decoding error.
-35GNUTLS_E_MPI_PRINT_FAILEDCould not export a large integer.
-37GNUTLS_E_REHANDSHAKERehandshake was requested by the peer.
-38GNUTLS_E_GOT_APPLICATION_DATATLS Application data were received, while expecting handshake data.
-39GNUTLS_E_RECORD_LIMIT_REACHEDThe upper limit of record packet sequence numbers has been reached. Wow!
-40GNUTLS_E_ENCRYPTION_FAILEDEncryption has failed.
-43GNUTLS_E_CERTIFICATE_ERRORError in the certificate.
-44GNUTLS_E_PK_ENCRYPTION_FAILEDPublic key encryption has failed.
-45GNUTLS_E_PK_DECRYPTION_FAILEDPublic key decryption has failed.
-46GNUTLS_E_PK_SIGN_FAILEDPublic key signing has failed.
-47GNUTLS_E_X509_UNSUPPORTED_CRITICAL_EXTENSIONUnsupported critical extension in X.509 certificate.
-48GNUTLS_E_KEY_USAGE_VIOLATIONKey usage violation in certificate has been detected.
-49GNUTLS_E_NO_CERTIFICATE_FOUNDNo certificate was found.
-50GNUTLS_E_INVALID_REQUESTThe request is invalid.
-51GNUTLS_E_SHORT_MEMORY_BUFFERThe given memory buffer is too short to hold parameters.
-52GNUTLS_E_INTERRUPTEDFunction was interrupted.
-53GNUTLS_E_PUSH_ERRORError in the push function.
-54GNUTLS_E_PULL_ERRORError in the pull function.
-55GNUTLS_E_RECEIVED_ILLEGAL_PARAMETERAn illegal parameter has been received.
-56GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLEThe requested data were not available.
-57GNUTLS_E_PKCS1_WRONG_PADWrong padding in PKCS1 packet.
-58GNUTLS_E_RECEIVED_ILLEGAL_EXTENSIONAn illegal TLS extension was received.
-59GNUTLS_E_INTERNAL_ERRORGnuTLS internal error.
-60GNUTLS_E_CERTIFICATE_KEY_MISMATCHThe certificate and the given key do not match.
-61GNUTLS_E_UNSUPPORTED_CERTIFICATE_TYPEThe certificate type is not supported.
-62GNUTLS_E_X509_UNKNOWN_SANUnknown Subject Alternative name in X.509 certificate.
-63GNUTLS_E_DH_PRIME_UNACCEPTABLEThe Diffie-Hellman prime sent by the server is not acceptable (not long enough).
-64GNUTLS_E_FILE_ERRORError while reading file.
-67GNUTLS_E_ASN1_ELEMENT_NOT_FOUNDASN1 parser: Element was not found.
-68GNUTLS_E_ASN1_IDENTIFIER_NOT_FOUNDASN1 parser: Identifier was not found
-69GNUTLS_E_ASN1_DER_ERRORASN1 parser: Error in DER parsing.
-70GNUTLS_E_ASN1_VALUE_NOT_FOUNDASN1 parser: Value was not found.
-71GNUTLS_E_ASN1_GENERIC_ERRORASN1 parser: Generic parsing error.
-72GNUTLS_E_ASN1_VALUE_NOT_VALIDASN1 parser: Value is not valid.
-73GNUTLS_E_ASN1_TAG_ERRORASN1 parser: Error in TAG.
-74GNUTLS_E_ASN1_TAG_IMPLICITASN1 parser: error in implicit tag
-75GNUTLS_E_ASN1_TYPE_ANY_ERRORASN1 parser: Error in type ’ANY’.
-76GNUTLS_E_ASN1_SYNTAX_ERRORASN1 parser: Syntax error.
-77GNUTLS_E_ASN1_DER_OVERFLOWASN1 parser: Overflow in DER parsing.
-78GNUTLS_E_TOO_MANY_EMPTY_PACKETSToo many empty record packets have been received.
-79GNUTLS_E_OPENPGP_UID_REVOKEDThe OpenPGP User ID is revoked.
-80GNUTLS_E_UNKNOWN_PK_ALGORITHMAn unknown public key algorithm was encountered.
-81GNUTLS_E_TOO_MANY_HANDSHAKE_PACKETSToo many handshake packets have been received.
-84GNUTLS_E_NO_TEMPORARY_RSA_PARAMSNo temporary RSA parameters were found.
-86GNUTLS_E_NO_COMPRESSION_ALGORITHMSNo supported compression algorithms have been found.
-87GNUTLS_E_NO_CIPHER_SUITESNo supported cipher suites have been found.
-88GNUTLS_E_OPENPGP_GETKEY_FAILEDCould not get OpenPGP key.
-89GNUTLS_E_PK_SIG_VERIFY_FAILEDPublic key signature verification has failed.
-90GNUTLS_E_ILLEGAL_SRP_USERNAMEThe SRP username supplied is illegal.
-91GNUTLS_E_SRP_PWD_PARSING_ERRORParsing error in password file.
-93GNUTLS_E_NO_TEMPORARY_DH_PARAMSNo temporary DH parameters were found.
-94GNUTLS_E_OPENPGP_FINGERPRINT_UNSUPPORTEDThe OpenPGP fingerprint is not supported.
-95GNUTLS_E_X509_UNSUPPORTED_ATTRIBUTEThe certificate has unsupported attributes.
-96GNUTLS_E_UNKNOWN_HASH_ALGORITHMThe hash algorithm is unknown.
-97GNUTLS_E_UNKNOWN_PKCS_CONTENT_TYPEThe PKCS structure’s content type is unknown.
-98GNUTLS_E_UNKNOWN_PKCS_BAG_TYPEThe PKCS structure’s bag type is unknown.
-99GNUTLS_E_INVALID_PASSWORDThe given password contains invalid characters.
-100GNUTLS_E_MAC_VERIFY_FAILEDThe Message Authentication Code verification failed.
-101GNUTLS_E_CONSTRAINT_ERRORSome constraint limits were reached.
-102GNUTLS_E_WARNING_IA_IPHF_RECEIVEDReceived a TLS/IA Intermediate Phase Finished message
-103GNUTLS_E_WARNING_IA_FPHF_RECEIVEDReceived a TLS/IA Final Phase Finished message
-104GNUTLS_E_IA_VERIFY_FAILEDVerifying TLS/IA phase checksum failed
-105GNUTLS_E_UNKNOWN_ALGORITHMThe specified algorithm or protocol is unknown.
-106GNUTLS_E_UNSUPPORTED_SIGNATURE_ALGORITHMThe signature algorithm is not supported.
-107GNUTLS_E_SAFE_RENEGOTIATION_FAILEDSafe renegotiation failed.
-108GNUTLS_E_UNSAFE_RENEGOTIATION_DENIEDUnsafe renegotiation denied.
-109GNUTLS_E_UNKNOWN_SRP_USERNAMEThe SRP username supplied is unknown.
-110GNUTLS_E_PREMATURE_TERMINATIONThe TLS connection was non-properly terminated.
-201GNUTLS_E_BASE64_ENCODING_ERRORBase64 encoding error.
-202GNUTLS_E_INCOMPATIBLE_GCRYPT_LIBRARYThe crypto library version is too old.
-203GNUTLS_E_INCOMPATIBLE_LIBTASN1_LIBRARYThe tasn1 library version is too old.
-204GNUTLS_E_OPENPGP_KEYRING_ERRORError loading the keyring.
-205GNUTLS_E_X509_UNSUPPORTED_OIDThe OID is not supported.
-206GNUTLS_E_RANDOM_FAILEDFailed to acquire random data.
-207GNUTLS_E_BASE64_UNEXPECTED_HEADER_ERRORBase64 unexpected header error.
-208GNUTLS_E_OPENPGP_SUBKEY_ERRORCould not find OpenPGP subkey.
-209GNUTLS_E_CRYPTO_ALREADY_REGISTEREDThere is already a crypto algorithm with lower priority.
-210GNUTLS_E_HANDSHAKE_TOO_LARGEThe handshake data size is too large.
-211GNUTLS_E_CRYPTODEV_IOCTL_ERRORError interfacing with /dev/crypto
-212GNUTLS_E_CRYPTODEV_DEVICE_ERRORError opening /dev/crypto
-213GNUTLS_E_CHANNEL_BINDING_NOT_AVAILABLEChannel binding data not available
-214GNUTLS_E_BAD_COOKIEThe cookie was bad.
-215GNUTLS_E_OPENPGP_PREFERRED_KEY_ERRORThe OpenPGP key has not a preferred key set.
-216GNUTLS_E_INCOMPAT_DSA_KEY_WITH_TLS_PROTOCOLThe given DSA key is incompatible with the selected TLS protocol.
-292GNUTLS_E_HEARTBEAT_PONG_RECEIVEDA heartbeat pong message was received.
-293GNUTLS_E_HEARTBEAT_PING_RECEIVEDA heartbeat ping message was received.
-300GNUTLS_E_PKCS11_ERRORPKCS #11 error.
-301GNUTLS_E_PKCS11_LOAD_ERRORPKCS #11 initialization error.
-302GNUTLS_E_PARSING_ERRORError in parsing.
-303GNUTLS_E_PKCS11_PIN_ERRORError in provided PIN.
-305GNUTLS_E_PKCS11_SLOT_ERRORPKCS #11 error in slot
-306GNUTLS_E_LOCKING_ERRORThread locking error
-307GNUTLS_E_PKCS11_ATTRIBUTE_ERRORPKCS #11 error in attribute
-308GNUTLS_E_PKCS11_DEVICE_ERRORPKCS #11 error in device
-309GNUTLS_E_PKCS11_DATA_ERRORPKCS #11 error in data
-310GNUTLS_E_PKCS11_UNSUPPORTED_FEATURE_ERRORPKCS #11 unsupported feature
-311GNUTLS_E_PKCS11_KEY_ERRORPKCS #11 error in key
-312GNUTLS_E_PKCS11_PIN_EXPIREDPKCS #11 PIN expired
-313GNUTLS_E_PKCS11_PIN_LOCKEDPKCS #11 PIN locked
-314GNUTLS_E_PKCS11_SESSION_ERRORPKCS #11 error in session
-315GNUTLS_E_PKCS11_SIGNATURE_ERRORPKCS #11 error in signature
-316GNUTLS_E_PKCS11_TOKEN_ERRORPKCS #11 error in token
-317GNUTLS_E_PKCS11_USER_ERRORPKCS #11 user error
-318GNUTLS_E_CRYPTO_INIT_FAILEDThe initialization of crypto backend has failed.
-319GNUTLS_E_TIMEDOUTThe operation timed out
-320GNUTLS_E_USER_ERRORThe operation was cancelled due to user error
-321GNUTLS_E_ECC_NO_SUPPORTED_CURVESNo supported ECC curves were found
-322GNUTLS_E_ECC_UNSUPPORTED_CURVEThe curve is unsupported
-323GNUTLS_E_PKCS11_REQUESTED_OBJECT_NOT_AVAILBLEThe requested PKCS #11 object is not available
-324GNUTLS_E_CERTIFICATE_LIST_UNSORTEDThe provided X.509 certificate list is not sorted (in subject to issuer order)
-325GNUTLS_E_ILLEGAL_PARAMETERAn illegal parameter was found.
-326GNUTLS_E_NO_PRIORITIES_WERE_SETNo or insufficient priorities were set.
-327GNUTLS_E_X509_UNSUPPORTED_EXTENSIONUnsupported extension in X.509 certificate.
-328GNUTLS_E_SESSION_EOFPeer has terminated the connection
-329GNUTLS_E_TPM_ERRORTPM error.
-330GNUTLS_E_TPM_KEY_PASSWORD_ERRORError in provided password for key to be loaded in TPM.
-331GNUTLS_E_TPM_SRK_PASSWORD_ERRORError in provided SRK password for TPM.
-332GNUTLS_E_TPM_SESSION_ERRORCannot initialize a session with the TPM.
-333GNUTLS_E_TPM_KEY_NOT_FOUNDTPM key was not found in persistent storage.
-334GNUTLS_E_TPM_UNINITIALIZEDTPM is not initialized.
-340GNUTLS_E_NO_CERTIFICATE_STATUSThere is no certificate status (OCSP).
-341GNUTLS_E_OCSP_RESPONSE_ERRORThe OCSP response is invalid
-342GNUTLS_E_RANDOM_DEVICE_ERRORError in the system’s randomness device.
-343GNUTLS_E_AUTH_ERRORCould not authenticate peer.
-344GNUTLS_E_NO_APPLICATION_PROTOCOLNo common application protocol could be negotiated.

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Appendix D Supported Ciphersuites

Ciphersuites

Ciphersuite nameTLS IDSince
TLS_RSA_NULL_MD50x00 0x01SSL3.0
TLS_RSA_NULL_SHA10x00 0x02SSL3.0
TLS_RSA_NULL_SHA2560x00 0x3BTLS1.2
TLS_RSA_ARCFOUR_128_SHA10x00 0x05SSL3.0
TLS_RSA_ARCFOUR_128_MD50x00 0x04SSL3.0
TLS_RSA_3DES_EDE_CBC_SHA10x00 0x0ASSL3.0
TLS_RSA_AES_128_CBC_SHA10x00 0x2FSSL3.0
TLS_RSA_AES_256_CBC_SHA10x00 0x35SSL3.0
TLS_RSA_CAMELLIA_128_CBC_SHA2560x00 0xBATLS1.0
TLS_RSA_CAMELLIA_256_CBC_SHA2560x00 0xC0TLS1.0
TLS_RSA_CAMELLIA_128_CBC_SHA10x00 0x41TLS1.0
TLS_RSA_CAMELLIA_256_CBC_SHA10x00 0x84TLS1.0
TLS_RSA_AES_128_CBC_SHA2560x00 0x3CTLS1.2
TLS_RSA_AES_256_CBC_SHA2560x00 0x3DTLS1.2
TLS_RSA_AES_128_GCM_SHA2560x00 0x9CTLS1.2
TLS_RSA_AES_256_GCM_SHA3840x00 0x9DTLS1.2
TLS_RSA_CAMELLIA_128_GCM_SHA2560xC0 0x7ATLS1.2
TLS_RSA_CAMELLIA_256_GCM_SHA3840xC0 0x7BTLS1.2
TLS_RSA_SALSA20_256_SHA10xE4 0x11SSL3.0
TLS_RSA_SALSA20_256_UMAC960xE4 0x31SSL3.0
TLS_RSA_ESTREAM_SALSA20_256_SHA10xE4 0x10SSL3.0
TLS_RSA_ESTREAM_SALSA20_256_UMAC960xE4 0x30SSL3.0
TLS_DHE_DSS_ARCFOUR_128_SHA10x00 0x66TLS1.0
TLS_DHE_DSS_3DES_EDE_CBC_SHA10x00 0x13SSL3.0
TLS_DHE_DSS_AES_128_CBC_SHA10x00 0x32SSL3.0
TLS_DHE_DSS_AES_256_CBC_SHA10x00 0x38SSL3.0
TLS_DHE_DSS_CAMELLIA_128_CBC_SHA2560x00 0xBDTLS1.0
TLS_DHE_DSS_CAMELLIA_256_CBC_SHA2560x00 0xC3TLS1.0
TLS_DHE_DSS_CAMELLIA_128_CBC_SHA10x00 0x44TLS1.0
TLS_DHE_DSS_CAMELLIA_256_CBC_SHA10x00 0x87TLS1.0
TLS_DHE_DSS_AES_128_CBC_SHA2560x00 0x40TLS1.2
TLS_DHE_DSS_AES_256_CBC_SHA2560x00 0x6ATLS1.2
TLS_DHE_DSS_AES_128_GCM_SHA2560x00 0xA2TLS1.2
TLS_DHE_DSS_AES_256_GCM_SHA3840x00 0xA3TLS1.2
TLS_DHE_DSS_CAMELLIA_128_GCM_SHA2560xC0 0x80TLS1.2
TLS_DHE_DSS_CAMELLIA_256_GCM_SHA3840xC0 0x81TLS1.2
TLS_DHE_RSA_3DES_EDE_CBC_SHA10x00 0x16SSL3.0
TLS_DHE_RSA_AES_128_CBC_SHA10x00 0x33SSL3.0
TLS_DHE_RSA_AES_256_CBC_SHA10x00 0x39SSL3.0
TLS_DHE_RSA_CAMELLIA_128_CBC_SHA2560x00 0xBETLS1.0
TLS_DHE_RSA_CAMELLIA_256_CBC_SHA2560x00 0xC4TLS1.0
TLS_DHE_RSA_CAMELLIA_128_CBC_SHA10x00 0x45TLS1.0
TLS_DHE_RSA_CAMELLIA_256_CBC_SHA10x00 0x88TLS1.0
TLS_DHE_RSA_AES_128_CBC_SHA2560x00 0x67TLS1.2
TLS_DHE_RSA_AES_256_CBC_SHA2560x00 0x6BTLS1.2
TLS_DHE_RSA_AES_128_GCM_SHA2560x00 0x9ETLS1.2
TLS_DHE_RSA_AES_256_GCM_SHA3840x00 0x9FTLS1.2
TLS_DHE_RSA_CAMELLIA_128_GCM_SHA2560xC0 0x7CTLS1.2
TLS_DHE_RSA_CAMELLIA_256_GCM_SHA3840xC0 0x7DTLS1.2
TLS_ECDHE_RSA_NULL_SHA10xC0 0x10SSL3.0
TLS_ECDHE_RSA_3DES_EDE_CBC_SHA10xC0 0x12SSL3.0
TLS_ECDHE_RSA_AES_128_CBC_SHA10xC0 0x13SSL3.0
TLS_ECDHE_RSA_AES_256_CBC_SHA10xC0 0x14SSL3.0
TLS_ECDHE_RSA_AES_256_CBC_SHA3840xC0 0x28TLS1.0
TLS_ECDHE_RSA_ARCFOUR_128_SHA10xC0 0x11SSL3.0
TLS_ECDHE_RSA_CAMELLIA_128_CBC_SHA2560xC0 0x76TLS1.0
TLS_ECDHE_RSA_CAMELLIA_256_CBC_SHA3840xC0 0x77TLS1.0
TLS_ECDHE_ECDSA_NULL_SHA10xC0 0x06SSL3.0
TLS_ECDHE_ECDSA_3DES_EDE_CBC_SHA10xC0 0x08SSL3.0
TLS_ECDHE_ECDSA_AES_128_CBC_SHA10xC0 0x09SSL3.0
TLS_ECDHE_ECDSA_AES_256_CBC_SHA10xC0 0x0ASSL3.0
TLS_ECDHE_ECDSA_ARCFOUR_128_SHA10xC0 0x07SSL3.0
TLS_ECDHE_ECDSA_CAMELLIA_128_CBC_SHA2560xC0 0x72TLS1.0
TLS_ECDHE_ECDSA_CAMELLIA_256_CBC_SHA3840xC0 0x73TLS1.0
TLS_ECDHE_ECDSA_AES_128_CBC_SHA2560xC0 0x23TLS1.2
TLS_ECDHE_RSA_AES_128_CBC_SHA2560xC0 0x27TLS1.2
TLS_ECDHE_ECDSA_CAMELLIA_128_GCM_SHA2560xC0 0x86TLS1.2
TLS_ECDHE_ECDSA_CAMELLIA_256_GCM_SHA3840xC0 0x87TLS1.2
TLS_ECDHE_ECDSA_AES_128_GCM_SHA2560xC0 0x2BTLS1.2
TLS_ECDHE_ECDSA_AES_256_GCM_SHA3840xC0 0x2CTLS1.2
TLS_ECDHE_RSA_AES_128_GCM_SHA2560xC0 0x2FTLS1.2
TLS_ECDHE_RSA_AES_256_GCM_SHA3840xC0 0x30TLS1.2
TLS_ECDHE_ECDSA_AES_256_CBC_SHA3840xC0 0x24TLS1.2
TLS_ECDHE_RSA_CAMELLIA_128_GCM_SHA2560xC0 0x8ATLS1.2
TLS_ECDHE_RSA_CAMELLIA_256_GCM_SHA3840xC0 0x8BTLS1.2
TLS_ECDHE_RSA_SALSA20_256_SHA10xE4 0x13SSL3.0
TLS_ECDHE_RSA_SALSA20_256_UMAC960xE4 0x33SSL3.0
TLS_ECDHE_ECDSA_SALSA20_256_SHA10xE4 0x15SSL3.0
TLS_ECDHE_ECDSA_SALSA20_256_UMAC960xE4 0x35SSL3.0
TLS_ECDHE_RSA_ESTREAM_SALSA20_256_SHA10xE4 0x12SSL3.0
TLS_ECDHE_RSA_ESTREAM_SALSA20_256_UMAC960xE4 0x32SSL3.0
TLS_ECDHE_ECDSA_ESTREAM_SALSA20_256_SHA10xE4 0x14SSL3.0
TLS_ECDHE_ECDSA_ESTREAM_SALSA20_256_UMAC960xE4 0x34SSL3.0
TLS_ECDHE_PSK_3DES_EDE_CBC_SHA10xC0 0x34SSL3.0
TLS_ECDHE_PSK_AES_128_CBC_SHA10xC0 0x35SSL3.0
TLS_ECDHE_PSK_AES_256_CBC_SHA10xC0 0x36SSL3.0
TLS_ECDHE_PSK_AES_128_CBC_SHA2560xC0 0x37TLS1.2
TLS_ECDHE_PSK_AES_256_CBC_SHA3840xC0 0x38TLS1.2
TLS_ECDHE_PSK_ARCFOUR_128_SHA10xC0 0x33SSL3.0
TLS_ECDHE_PSK_NULL_SHA2560xC0 0x3ATLS1.2
TLS_ECDHE_PSK_NULL_SHA3840xC0 0x3BTLS1.2
TLS_ECDHE_PSK_CAMELLIA_128_CBC_SHA2560xC0 0x9ATLS1.2
TLS_ECDHE_PSK_CAMELLIA_256_CBC_SHA3840xC0 0x9BTLS1.2
TLS_ECDHE_PSK_SALSA20_256_SHA10xE4 0x19SSL3.0
TLS_ECDHE_PSK_SALSA20_256_UMAC960xE4 0x39SSL3.0
TLS_ECDHE_PSK_ESTREAM_SALSA20_256_SHA10xE4 0x18SSL3.0
TLS_ECDHE_PSK_ESTREAM_SALSA20_256_UMAC960xE4 0x38SSL3.0
TLS_PSK_ARCFOUR_128_SHA10x00 0x8ATLS1.0
TLS_PSK_3DES_EDE_CBC_SHA10x00 0x8BTLS1.0
TLS_PSK_AES_128_CBC_SHA10x00 0x8CTLS1.0
TLS_PSK_AES_256_CBC_SHA10x00 0x8DTLS1.0
TLS_PSK_AES_128_CBC_SHA2560x00 0xAETLS1.2
TLS_PSK_AES_256_GCM_SHA3840x00 0xA9TLS1.2
TLS_PSK_CAMELLIA_128_GCM_SHA2560xC0 0x8ETLS1.2
TLS_PSK_CAMELLIA_256_GCM_SHA3840xC0 0x8FTLS1.2
TLS_PSK_AES_128_GCM_SHA2560x00 0xA8TLS1.2
TLS_PSK_NULL_SHA2560x00 0xB0TLS1.2
TLS_PSK_CAMELLIA_128_CBC_SHA2560xC0 0x94TLS1.0
TLS_PSK_CAMELLIA_256_CBC_SHA3840xC0 0x95TLS1.0
TLS_PSK_SALSA20_256_SHA10xE4 0x17SSL3.0
TLS_PSK_SALSA20_256_UMAC960xE4 0x37SSL3.0
TLS_PSK_ESTREAM_SALSA20_256_SHA10xE4 0x16SSL3.0
TLS_PSK_ESTREAM_SALSA20_256_UMAC960xE4 0x36SSL3.0
TLS_PSK_AES_256_CBC_SHA3840x00 0xAFTLS1.0
TLS_PSK_NULL_SHA3840x00 0xB1TLS1.0
TLS_RSA_PSK_ARCFOUR_128_SHA10x00 0x92TLS1.0
TLS_RSA_PSK_3DES_EDE_CBC_SHA10x00 0x93TLS1.0
TLS_RSA_PSK_AES_128_CBC_SHA10x00 0x94TLS1.0
TLS_RSA_PSK_AES_256_CBC_SHA10x00 0x95TLS1.0
TLS_RSA_PSK_CAMELLIA_128_GCM_SHA2560xC0 0x92TLS1.2
TLS_RSA_PSK_CAMELLIA_256_GCM_SHA3840xC0 0x93TLS1.2
TLS_RSA_PSK_AES_128_GCM_SHA2560x00 0xACTLS1.2
TLS_RSA_PSK_AES_128_CBC_SHA2560x00 0xB6TLS1.0
TLS_RSA_PSK_NULL_SHA2560x00 0xB8TLS1.0
TLS_RSA_PSK_AES_256_GCM_SHA3840x00 0xADTLS1.2
TLS_RSA_PSK_AES_256_CBC_SHA3840x00 0xB7TLS1.0
TLS_RSA_PSK_NULL_SHA3840x00 0xB9TLS1.0
TLS_RSA_PSK_CAMELLIA_128_CBC_SHA2560xC0 0x98TLS1.0
TLS_RSA_PSK_CAMELLIA_256_CBC_SHA3840xC0 0x99TLS1.0
TLS_DHE_PSK_ARCFOUR_128_SHA10x00 0x8ETLS1.0
TLS_DHE_PSK_3DES_EDE_CBC_SHA10x00 0x8FTLS1.0
TLS_DHE_PSK_AES_128_CBC_SHA10x00 0x90TLS1.0
TLS_DHE_PSK_AES_256_CBC_SHA10x00 0x91TLS1.0
TLS_DHE_PSK_AES_128_CBC_SHA2560x00 0xB2TLS1.2
TLS_DHE_PSK_AES_128_GCM_SHA2560x00 0xAATLS1.2
TLS_DHE_PSK_NULL_SHA2560x00 0xB4TLS1.2
TLS_DHE_PSK_NULL_SHA3840x00 0xB5TLS1.0
TLS_DHE_PSK_AES_256_CBC_SHA3840x00 0xB3TLS1.0
TLS_DHE_PSK_AES_256_GCM_SHA3840x00 0xABTLS1.2
TLS_DHE_PSK_CAMELLIA_128_CBC_SHA2560xC0 0x96TLS1.0
TLS_DHE_PSK_CAMELLIA_256_CBC_SHA3840xC0 0x97TLS1.0
TLS_DHE_PSK_CAMELLIA_128_GCM_SHA2560xC0 0x90TLS1.2
TLS_DHE_PSK_CAMELLIA_256_GCM_SHA3840xC0 0x91TLS1.2
TLS_DH_ANON_ARCFOUR_128_MD50x00 0x18SSL3.0
TLS_DH_ANON_3DES_EDE_CBC_SHA10x00 0x1BSSL3.0
TLS_DH_ANON_AES_128_CBC_SHA10x00 0x34SSL3.0
TLS_DH_ANON_AES_256_CBC_SHA10x00 0x3ASSL3.0
TLS_DH_ANON_CAMELLIA_128_CBC_SHA2560x00 0xBFTLS1.0
TLS_DH_ANON_CAMELLIA_256_CBC_SHA2560x00 0xC5TLS1.0
TLS_DH_ANON_CAMELLIA_128_CBC_SHA10x00 0x46TLS1.0
TLS_DH_ANON_CAMELLIA_256_CBC_SHA10x00 0x89TLS1.0
TLS_DH_ANON_AES_128_CBC_SHA2560x00 0x6CTLS1.2
TLS_DH_ANON_AES_256_CBC_SHA2560x00 0x6DTLS1.2
TLS_DH_ANON_AES_128_GCM_SHA2560x00 0xA6TLS1.2
TLS_DH_ANON_AES_256_GCM_SHA3840x00 0xA7TLS1.2
TLS_DH_ANON_CAMELLIA_128_GCM_SHA2560xC0 0x84TLS1.2
TLS_DH_ANON_CAMELLIA_256_GCM_SHA3840xC0 0x85TLS1.2
TLS_ECDH_ANON_NULL_SHA10xC0 0x15SSL3.0
TLS_ECDH_ANON_3DES_EDE_CBC_SHA10xC0 0x17SSL3.0
TLS_ECDH_ANON_AES_128_CBC_SHA10xC0 0x18SSL3.0
TLS_ECDH_ANON_AES_256_CBC_SHA10xC0 0x19SSL3.0
TLS_ECDH_ANON_ARCFOUR_128_SHA10xC0 0x16SSL3.0
TLS_SRP_SHA_3DES_EDE_CBC_SHA10xC0 0x1ATLS1.0
TLS_SRP_SHA_AES_128_CBC_SHA10xC0 0x1DTLS1.0
TLS_SRP_SHA_AES_256_CBC_SHA10xC0 0x20TLS1.0
TLS_SRP_SHA_DSS_3DES_EDE_CBC_SHA10xC0 0x1CTLS1.0
TLS_SRP_SHA_RSA_3DES_EDE_CBC_SHA10xC0 0x1BTLS1.0
TLS_SRP_SHA_DSS_AES_128_CBC_SHA10xC0 0x1FTLS1.0
TLS_SRP_SHA_RSA_AES_128_CBC_SHA10xC0 0x1ETLS1.0
TLS_SRP_SHA_DSS_AES_256_CBC_SHA10xC0 0x22TLS1.0
TLS_SRP_SHA_RSA_AES_256_CBC_SHA10xC0 0x21TLS1.0

Certificate types

X.509
OPENPGP

Protocols

SSL3.0
TLS1.0
TLS1.1
TLS1.2
DTLS0.9
DTLS1.0
DTLS1.2

Ciphers

AES-256-CBC
AES-192-CBC
AES-128-CBC
AES-128-GCM
AES-256-GCM
ARCFOUR-128
ESTREAM-SALSA20-256
SALSA20-256
CAMELLIA-256-CBC
CAMELLIA-192-CBC
CAMELLIA-128-CBC
CAMELLIA-128-GCM
CAMELLIA-256-GCM
3DES-CBC
DES-CBC
ARCFOUR-40
RC2-40

MAC algorithms

SHA1
MD5
SHA256
SHA384
SHA512
SHA224
UMAC-96
UMAC-128
AEAD

Key exchange methods

ANON-DH
ANON-ECDH
RSA
DHE-RSA
DHE-DSS
ECDHE-RSA
ECDHE-ECDSA
SRP-DSS
SRP-RSA
SRP
PSK
RSA-PSK
DHE-PSK
ECDHE-PSK

Public key algorithms

RSA
DSA
EC

Public key signature algorithms

RSA-SHA1
RSA-SHA1
RSA-SHA224
RSA-SHA256
RSA-SHA384
RSA-SHA512
RSA-RMD160
DSA-SHA1
DSA-SHA1
DSA-SHA224
DSA-SHA256
RSA-MD5
RSA-MD5
RSA-MD2
ECDSA-SHA1
ECDSA-SHA224
ECDSA-SHA256
ECDSA-SHA384
ECDSA-SHA512

Elliptic curves

SECP192R1
SECP224R1
SECP256R1
SECP384R1
SECP521R1

Compression methods

DEFLATE
NULL

Next: , Previous: , Up: Top   [Contents][Index]

Appendix E API reference


Next: , Up: API reference   [Contents][Index]

E.1 Core TLS API

The prototypes for the following functions lie in gnutls/gnutls.h.

gnutls_alert_get

Function: gnutls_alert_description_t gnutls_alert_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function will return the last alert number received. This function should be called when GNUTLS_E_WARNING_ALERT_RECEIVED or GNUTLS_E_FATAL_ALERT_RECEIVED errors are returned by a gnutls function. The peer may send alerts if he encounters an error. If no alert has been received the returned value is undefined.

Returns: the last alert received, a gnutls_alert_description_t value.

gnutls_alert_get_name

Function: const char * gnutls_alert_get_name (gnutls_alert_description_t alert)

alert: is an alert number.

This function will return a string that describes the given alert number, or NULL . See gnutls_alert_get() .

Returns: string corresponding to gnutls_alert_description_t value.

gnutls_alert_get_strname

Function: const char * gnutls_alert_get_strname (gnutls_alert_description_t alert)

alert: is an alert number.

This function will return a string of the name of the alert.

Returns: string corresponding to gnutls_alert_description_t value.

Since: 3.0

gnutls_alert_send

Function: int gnutls_alert_send (gnutls_session_t session, gnutls_alert_level_t level, gnutls_alert_description_t desc)

session: is a gnutls_session_t structure.

level: is the level of the alert

desc: is the alert description

This function will send an alert to the peer in order to inform him of something important (eg. his Certificate could not be verified). If the alert level is Fatal then the peer is expected to close the connection, otherwise he may ignore the alert and continue.

The error code of the underlying record send function will be returned, so you may also receive GNUTLS_E_INTERRUPTED or GNUTLS_E_AGAIN as well.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_alert_send_appropriate

Function: int gnutls_alert_send_appropriate (gnutls_session_t session, int err)

session: is a gnutls_session_t structure.

err: is an integer

Sends an alert to the peer depending on the error code returned by a gnutls function. This function will call gnutls_error_to_alert() to determine the appropriate alert to send.

This function may also return GNUTLS_E_AGAIN , or GNUTLS_E_INTERRUPTED .

If the return value is GNUTLS_E_INVALID_REQUEST , then no alert has been sent to the peer.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_alpn_get_selected_protocol

Function: int gnutls_alpn_get_selected_protocol (gnutls_session_t session, gnutls_datum_t * protocol)

session: is a gnutls_session_t structure.

protocol: will hold the protocol name

This function allows you to get the negotiated protocol name. The returned protocol should be treated as opaque, constant value and only valid during the session life.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since 3.1.11

gnutls_alpn_set_protocols

Function: int gnutls_alpn_set_protocols (gnutls_session_t session, const gnutls_datum_t * protocols, unsigned protocols_size, unsigned int flags)

session: is a gnutls_session_t structure.

protocols: is the protocol names to add.

protocols_size: the number of protocols to add.

flags: zero or GNUTLS_ALPN_ *

This function is to be used by both clients and servers, to declare the supported ALPN protocols, which are used during peer negotiation.

If GNUTLS_ALPN_MAND is specified the connection will be aborted if no matching ALPN protocol is found.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since 3.1.11

gnutls_anon_allocate_client_credentials

Function: int gnutls_anon_allocate_client_credentials (gnutls_anon_client_credentials_t * sc)

sc: is a pointer to a gnutls_anon_client_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_anon_allocate_server_credentials

Function: int gnutls_anon_allocate_server_credentials (gnutls_anon_server_credentials_t * sc)

sc: is a pointer to a gnutls_anon_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_anon_free_client_credentials

Function: void gnutls_anon_free_client_credentials (gnutls_anon_client_credentials_t sc)

sc: is a gnutls_anon_client_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.

gnutls_anon_free_server_credentials

Function: void gnutls_anon_free_server_credentials (gnutls_anon_server_credentials_t sc)

sc: is a gnutls_anon_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.

gnutls_anon_set_params_function

Function: void gnutls_anon_set_params_function (gnutls_anon_server_credentials_t res, gnutls_params_function * func)

res: is a gnutls_anon_server_credentials_t structure

func: is the function to be called

This function will set a callback in order for the server to get the Diffie-Hellman or RSA parameters for anonymous authentication. The callback should return GNUTLS_E_SUCCESS (0) on success.

gnutls_anon_set_server_dh_params

Function: void gnutls_anon_set_server_dh_params (gnutls_anon_server_credentials_t res, gnutls_dh_params_t dh_params)

res: is a gnutls_anon_server_credentials_t structure

dh_params: is a structure that holds Diffie-Hellman parameters.

This function will set the Diffie-Hellman parameters for an anonymous server to use. These parameters will be used in Anonymous Diffie-Hellman cipher suites.

gnutls_anon_set_server_params_function

Function: void gnutls_anon_set_server_params_function (gnutls_anon_server_credentials_t res, gnutls_params_function * func)

res: is a gnutls_certificate_credentials_t structure

func: is the function to be called

This function will set a callback in order for the server to get the Diffie-Hellman parameters for anonymous authentication. The callback should return GNUTLS_E_SUCCESS (0) on success.

gnutls_auth_client_get_type

Function: gnutls_credentials_type_t gnutls_auth_client_get_type (gnutls_session_t session)

session: is a gnutls_session_t structure.

Returns the type of credentials that were used for client authentication. The returned information is to be used to distinguish the function used to access authentication data.

Returns: The type of credentials for the client authentication schema, a gnutls_credentials_type_t type.

gnutls_auth_get_type

Function: gnutls_credentials_type_t gnutls_auth_get_type (gnutls_session_t session)

session: is a gnutls_session_t structure.

Returns type of credentials for the current authentication schema. The returned information is to be used to distinguish the function used to access authentication data.

Eg. for CERTIFICATE ciphersuites (key exchange algorithms: GNUTLS_KX_RSA , GNUTLS_KX_DHE_RSA ), the same function are to be used to access the authentication data.

Returns: The type of credentials for the current authentication schema, a gnutls_credentials_type_t type.

gnutls_auth_server_get_type

Function: gnutls_credentials_type_t gnutls_auth_server_get_type (gnutls_session_t session)

session: is a gnutls_session_t structure.

Returns the type of credentials that were used for server authentication. The returned information is to be used to distinguish the function used to access authentication data.

Returns: The type of credentials for the server authentication schema, a gnutls_credentials_type_t type.

gnutls_bye

Function: int gnutls_bye (gnutls_session_t session, gnutls_close_request_t how)

session: is a gnutls_session_t structure.

how: is an integer

Terminates the current TLS/SSL connection. The connection should have been initiated using gnutls_handshake() . how should be one of GNUTLS_SHUT_RDWR , GNUTLS_SHUT_WR .

In case of GNUTLS_SHUT_RDWR the TLS session gets terminated and further receives and sends will be disallowed. If the return value is zero you may continue using the underlying transport layer. GNUTLS_SHUT_RDWR sends an alert containing a close request and waits for the peer to reply with the same message.

In case of GNUTLS_SHUT_WR the TLS session gets terminated and further sends will be disallowed. In order to reuse the connection you should wait for an EOF from the peer. GNUTLS_SHUT_WR sends an alert containing a close request.

Note that not all implementations will properly terminate a TLS connection. Some of them, usually for performance reasons, will terminate only the underlying transport layer, and thus not distinguishing between a malicious party prematurely terminating the connection and normal termination.

This function may also return GNUTLS_E_AGAIN or GNUTLS_E_INTERRUPTED ; cf. gnutls_record_get_direction() .

Returns: GNUTLS_E_SUCCESS on success, or an error code, see function documentation for entire semantics.

gnutls_certificate_activation_time_peers

Function: time_t gnutls_certificate_activation_time_peers (gnutls_session_t session)

session: is a gnutls session

This function will return the peer’s certificate activation time. This is the creation time for openpgp keys.

Returns: (time_t)-1 on error.

Deprecated: gnutls_certificate_verify_peers2() now verifies activation times.

gnutls_certificate_allocate_credentials

Function: int gnutls_certificate_allocate_credentials (gnutls_certificate_credentials_t * res)

res: is a pointer to a gnutls_certificate_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_certificate_client_get_request_status

Function: int gnutls_certificate_client_get_request_status (gnutls_session_t session)

session: is a gnutls session

Get whether client certificate is requested or not.

Returns: 0 if the peer (server) did not request client authentication or 1 otherwise, or a negative error code in case of error.

gnutls_certificate_expiration_time_peers

Function: time_t gnutls_certificate_expiration_time_peers (gnutls_session_t session)

session: is a gnutls session

This function will return the peer’s certificate expiration time.

Returns: (time_t)-1 on error.

Deprecated: gnutls_certificate_verify_peers2() now verifies expiration times.

gnutls_certificate_free_ca_names

Function: void gnutls_certificate_free_ca_names (gnutls_certificate_credentials_t sc)

sc: is a gnutls_certificate_credentials_t structure.

This function will delete all the CA name in the given credentials. Clients may call this to save some memory since in client side the CA names are not used. Servers might want to use this function if a large list of trusted CAs is present and sending the names of it would just consume bandwidth without providing information to client.

CA names are used by servers to advertise the CAs they support to clients.

gnutls_certificate_free_cas

Function: void gnutls_certificate_free_cas (gnutls_certificate_credentials_t sc)

sc: is a gnutls_certificate_credentials_t structure.

This function will delete all the CAs associated with the given credentials. Servers that do not use gnutls_certificate_verify_peers2() may call this to save some memory.

gnutls_certificate_free_credentials

Function: void gnutls_certificate_free_credentials (gnutls_certificate_credentials_t sc)

sc: is a gnutls_certificate_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to free (deallocate) it.

This function does not free any temporary parameters associated with this structure (ie RSA and DH parameters are not freed by this function).

gnutls_certificate_free_crls

Function: void gnutls_certificate_free_crls (gnutls_certificate_credentials_t sc)

sc: is a gnutls_certificate_credentials_t structure.

This function will delete all the CRLs associated with the given credentials.

gnutls_certificate_free_keys

Function: void gnutls_certificate_free_keys (gnutls_certificate_credentials_t sc)

sc: is a gnutls_certificate_credentials_t structure.

This function will delete all the keys and the certificates associated with the given credentials. This function must not be called when a TLS negotiation that uses the credentials is in progress.

gnutls_certificate_get_crt_raw

Function: int gnutls_certificate_get_crt_raw (gnutls_certificate_credentials_t sc, unsigned idx1, unsigned idx2, gnutls_datum_t * cert)

sc: is a gnutls_certificate_credentials_t structure.

idx1: the index of the certificate if multiple are present

idx2: the index in the certificate list. Zero gives the server’s certificate.

cert: Will hold the DER encoded certificate.

This function will return the DER encoded certificate of the server or any other certificate on its certificate chain (based on idx2 ). The returned data should be treated as constant and only accessible during the lifetime of sc .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value. In case the indexes are out of bounds GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE is returned.

Since: 3.2.5

gnutls_certificate_get_issuer

Function: int gnutls_certificate_get_issuer (gnutls_certificate_credentials_t sc, gnutls_x509_crt_t cert, gnutls_x509_crt_t * issuer, unsigned int flags)

sc: is a gnutls_certificate_credentials_t structure.

cert: is the certificate to find issuer for

issuer: Will hold the issuer if any. Should be treated as constant.

flags: Use zero.

This function will return the issuer of a given certificate.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.0

gnutls_certificate_get_ours

Function: const gnutls_datum_t * gnutls_certificate_get_ours (gnutls_session_t session)

session: is a gnutls session

Gets the certificate as sent to the peer in the last handshake. The certificate is in raw (DER) format. No certificate list is being returned. Only the first certificate.

Returns: a pointer to a gnutls_datum_t containing our certificate, or NULL in case of an error or if no certificate was used.

gnutls_certificate_get_peers

Function: const gnutls_datum_t * gnutls_certificate_get_peers (gnutls_session_t session, unsigned int * list_size)

session: is a gnutls session

list_size: is the length of the certificate list

Get the peer’s raw certificate (chain) as sent by the peer. These certificates are in raw format (DER encoded for X.509). In case of a X.509 then a certificate list may be present. The first certificate in the list is the peer’s certificate, following the issuer’s certificate, then the issuer’s issuer etc.

In case of OpenPGP keys a single key will be returned in raw format.

Returns: a pointer to a gnutls_datum_t containing our certificates, or NULL in case of an error or if no certificate was used.

gnutls_certificate_get_peers_subkey_id

Function: int gnutls_certificate_get_peers_subkey_id (gnutls_session_t session, gnutls_datum_t * id)

session: is a gnutls session

id: will contain the ID

Get the peer’s subkey ID when OpenPGP certificates are used. The returned id should be treated as constant.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

Since: 3.1.3

gnutls_certificate_send_x509_rdn_sequence

Function: void gnutls_certificate_send_x509_rdn_sequence (gnutls_session_t session, int status)

session: is a pointer to a gnutls_session_t structure.

status: is 0 or 1

If status is non zero, this function will order gnutls not to send the rdnSequence in the certificate request message. That is the server will not advertise its trusted CAs to the peer. If status is zero then the default behaviour will take effect, which is to advertise the server’s trusted CAs.

This function has no effect in clients, and in authentication methods other than certificate with X.509 certificates.

gnutls_certificate_server_set_request

Function: void gnutls_certificate_server_set_request (gnutls_session_t session, gnutls_certificate_request_t req)

session: is a gnutls_session_t structure.

req: is one of GNUTLS_CERT_REQUEST, GNUTLS_CERT_REQUIRE

This function specifies if we (in case of a server) are going to send a certificate request message to the client. If req is GNUTLS_CERT_REQUIRE then the server will return an error if the peer does not provide a certificate. If you do not call this function then the client will not be asked to send a certificate.

gnutls_certificate_set_dh_params

Function: void gnutls_certificate_set_dh_params (gnutls_certificate_credentials_t res, gnutls_dh_params_t dh_params)

res: is a gnutls_certificate_credentials_t structure

dh_params: is a structure that holds Diffie-Hellman parameters.

This function will set the Diffie-Hellman parameters for a certificate server to use. These parameters will be used in Ephemeral Diffie-Hellman cipher suites. Note that only a pointer to the parameters are stored in the certificate handle, so you must not deallocate the parameters before the certificate is deallocated.

gnutls_certificate_set_ocsp_status_request_file

Function: int gnutls_certificate_set_ocsp_status_request_file (gnutls_certificate_credentials_t sc, const char * response_file, unsigned int flags)

sc: – undescribed –

response_file: a filename of the OCSP response

flags: should be zero

This function sets the filename of an OCSP response, that will be sent to the client if requests an OCSP certificate status. This is a convenience function which is inefficient on busy servers since the file is opened on every access. Use gnutls_certificate_set_ocsp_status_request_function() to fine-tune file accesses.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since: 3.1.3

gnutls_certificate_set_ocsp_status_request_function

Function: void gnutls_certificate_set_ocsp_status_request_function (gnutls_certificate_credentials_t sc, gnutls_status_request_ocsp_func ocsp_func, void * ptr)

sc: is a gnutls_certificate_credentials_t structure.

ocsp_func: function pointer to OCSP status request callback.

ptr: opaque pointer passed to callback function

This function is to be used by server to register a callback to handle OCSP status requests from the client. The callback will be invoked if the client supplied a status-request OCSP extension. The callback function prototype is:

typedef int (*gnutls_status_request_ocsp_func) (gnutls_session_t session, void *ptr, gnutls_datum_t *ocsp_response);

The callback will be invoked if the client requests an OCSP certificate status. The callback may return GNUTLS_E_NO_CERTIFICATE_STATUS , if there is no recent OCSP response. If the callback returns GNUTLS_E_SUCCESS , the server will provide the client with the ocsp_response.

The response must be a value allocated using gnutls_malloc() , and will be deinitialized when needed.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since: 3.1.3

gnutls_certificate_set_params_function

Function: void gnutls_certificate_set_params_function (gnutls_certificate_credentials_t res, gnutls_params_function * func)

res: is a gnutls_certificate_credentials_t structure

func: is the function to be called

This function will set a callback in order for the server to get the Diffie-Hellman or RSA parameters for certificate authentication. The callback should return GNUTLS_E_SUCCESS (0) on success.

gnutls_certificate_set_pin_function

Function: void gnutls_certificate_set_pin_function (gnutls_certificate_credentials_t cred, gnutls_pin_callback_t fn, void * userdata)

cred: is a gnutls_certificate_credentials_t structure.

fn: A PIN callback

userdata: Data to be passed in the callback

This function will set a callback function to be used when required to access a protected object. This function overrides any other global PIN functions.

Note that this function must be called right after initialization to have effect.

Since: 3.1.0

gnutls_certificate_set_retrieve_function

Function: void gnutls_certificate_set_retrieve_function (gnutls_certificate_credentials_t cred, gnutls_certificate_retrieve_function * func)

cred: is a gnutls_certificate_credentials_t structure.

func: is the callback function

This function sets a callback to be called in order to retrieve the certificate to be used in the handshake. You are advised to use gnutls_certificate_set_retrieve_function2() because it is much more efficient in the processing it requires from gnutls.

The callback’s function prototype is: int (*callback)(gnutls_session_t, const gnutls_datum_t* req_ca_dn, int nreqs, const gnutls_pk_algorithm_t* pk_algos, int pk_algos_length, gnutls_retr2_st* st);

req_ca_cert is only used in X.509 certificates. Contains a list with the CA names that the server considers trusted. Normally we should send a certificate that is signed by one of these CAs. These names are DER encoded. To get a more meaningful value use the function gnutls_x509_rdn_get() .

pk_algos contains a list with server’s acceptable signature algorithms. The certificate returned should support the server’s given algorithms.

st should contain the certificates and private keys.

If the callback function is provided then gnutls will call it, in the handshake, after the certificate request message has been received.

In server side pk_algos and req_ca_dn are NULL.

The callback function should set the certificate list to be sent, and return 0 on success. If no certificate was selected then the number of certificates should be set to zero. The value (-1) indicates error and the handshake will be terminated.

Since: 3.0

gnutls_certificate_set_verify_flags

Function: void gnutls_certificate_set_verify_flags (gnutls_certificate_credentials_t res, unsigned int flags)

res: is a gnutls_certificate_credentials_t structure

flags: are the flags

This function will set the flags to be used for verification of certificates and override any defaults. The provided flags must be an OR of the gnutls_certificate_verify_flags enumerations.

gnutls_certificate_set_verify_function

Function: void gnutls_certificate_set_verify_function (gnutls_certificate_credentials_t cred, gnutls_certificate_verify_function * func)

cred: is a gnutls_certificate_credentials_t structure.

func: is the callback function

This function sets a callback to be called when peer’s certificate has been received in order to verify it on receipt rather than doing after the handshake is completed.

The callback’s function prototype is: int (*callback)(gnutls_session_t);

If the callback function is provided then gnutls will call it, in the handshake, just after the certificate message has been received. To verify or obtain the certificate the gnutls_certificate_verify_peers2() , gnutls_certificate_type_get() , gnutls_certificate_get_peers() functions can be used.

The callback function should return 0 for the handshake to continue or non-zero to terminate.

Since: 2.10.0

gnutls_certificate_set_verify_limits

Function: void gnutls_certificate_set_verify_limits (gnutls_certificate_credentials_t res, unsigned int max_bits, unsigned int max_depth)

res: is a gnutls_certificate_credentials structure

max_bits: is the number of bits of an acceptable certificate (default 8200)

max_depth: is maximum depth of the verification of a certificate chain (default 5)

This function will set some upper limits for the default verification function, gnutls_certificate_verify_peers2() , to avoid denial of service attacks. You can set them to zero to disable limits.

gnutls_certificate_set_x509_crl

Function: int gnutls_certificate_set_x509_crl (gnutls_certificate_credentials_t res, gnutls_x509_crl_t * crl_list, int crl_list_size)

res: is a gnutls_certificate_credentials_t structure.

crl_list: is a list of trusted CRLs. They should have been verified before.

crl_list_size: holds the size of the crl_list

This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

Returns: number of CRLs processed, or a negative error code on error.

Since: 2.4.0

gnutls_certificate_set_x509_crl_file

Function: int gnutls_certificate_set_x509_crl_file (gnutls_certificate_credentials_t res, const char * crlfile, gnutls_x509_crt_fmt_t type)

res: is a gnutls_certificate_credentials_t structure.

crlfile: is a file containing the list of verified CRLs (DER or PEM list)

type: is PEM or DER

This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

Returns: number of CRLs processed or a negative error code on error.

gnutls_certificate_set_x509_crl_mem

Function: int gnutls_certificate_set_x509_crl_mem (gnutls_certificate_credentials_t res, const gnutls_datum_t * CRL, gnutls_x509_crt_fmt_t type)

res: is a gnutls_certificate_credentials_t structure.

CRL: is a list of trusted CRLs. They should have been verified before.

type: is DER or PEM

This function adds the trusted CRLs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

Returns: number of CRLs processed, or a negative error code on error.

gnutls_certificate_set_x509_key

Function: int gnutls_certificate_set_x509_key (gnutls_certificate_credentials_t res, gnutls_x509_crt_t * cert_list, int cert_list_size, gnutls_x509_privkey_t key)

res: is a gnutls_certificate_credentials_t structure.

cert_list: contains a certificate list (path) for the specified private key

cert_list_size: holds the size of the certificate list

key: is a gnutls_x509_privkey_t key

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server. For clients that wants to send more than their own end entity certificate (e.g., also an intermediate CA cert) then put the certificate chain in cert_list .

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

Since: 2.4.0

gnutls_certificate_set_x509_key_file

Function: int gnutls_certificate_set_x509_key_file (gnutls_certificate_credentials_t res, const char * certfile, const char * keyfile, gnutls_x509_crt_fmt_t type)

res: is a gnutls_certificate_credentials_t structure.

certfile: is a file that containing the certificate list (path) for the specified private key, in PKCS7 format, or a list of certificates

keyfile: is a file that contains the private key

type: is PEM or DER

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server. For clients that need to send more than its own end entity certificate, e.g., also an intermediate CA cert, then the certfile must contain the ordered certificate chain.

Note that the names in the certificate provided will be considered when selecting the appropriate certificate to use (in case of multiple certificate/key pairs).

This function can also accept URLs at keyfile and certfile . In that case it will import the private key and certificate indicated by the URLs. Note that the supported URLs are the ones indicated by gnutls_url_is_supported() .

In case the certfile is provided as a PKCS 11 URL, then the certificate, and its present issuers in the token are are imported (i.e., the required trust chain).

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

gnutls_certificate_set_x509_key_file2

Function: int gnutls_certificate_set_x509_key_file2 (gnutls_certificate_credentials_t res, const char * certfile, const char * keyfile, gnutls_x509_crt_fmt_t type, const char * pass, unsigned int flags)

res: is a gnutls_certificate_credentials_t structure.

certfile: is a file that containing the certificate list (path) for the specified private key, in PKCS7 format, or a list of certificates

keyfile: is a file that contains the private key

type: is PEM or DER

pass: is the password of the key

flags: an ORed sequence of gnutls_pkcs_encrypt_flags_t

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server. For clients that need to send more than its own end entity certificate, e.g., also an intermediate CA cert, then the certfile must contain the ordered certificate chain.

Note that the names in the certificate provided will be considered when selecting the appropriate certificate to use (in case of multiple certificate/key pairs).

This function can also accept URLs at keyfile and certfile . In that case it will import the private key and certificate indicated by the URLs. Note that the supported URLs are the ones indicated by gnutls_url_is_supported() .

In case the certfile is provided as a PKCS 11 URL, then the certificate, and its present issuers in the token are are imported (i.e., the required trust chain).

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

gnutls_certificate_set_x509_key_mem

Function: int gnutls_certificate_set_x509_key_mem (gnutls_certificate_credentials_t res, const gnutls_datum_t * cert, const gnutls_datum_t * key, gnutls_x509_crt_fmt_t type)

res: is a gnutls_certificate_credentials_t structure.

cert: contains a certificate list (path) for the specified private key

key: is the private key, or NULL

type: is PEM or DER

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server.

Note that the keyUsage (2.5.29.15) PKIX extension in X.509 certificates is supported. This means that certificates intended for signing cannot be used for ciphersuites that require encryption.

If the certificate and the private key are given in PEM encoding then the strings that hold their values must be null terminated.

The key may be NULL if you are using a sign callback, see gnutls_sign_callback_set() .

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

gnutls_certificate_set_x509_key_mem2

Function: int gnutls_certificate_set_x509_key_mem2 (gnutls_certificate_credentials_t res, const gnutls_datum_t * cert, const gnutls_datum_t * key, gnutls_x509_crt_fmt_t type, const char * pass, unsigned int flags)

res: is a gnutls_certificate_credentials_t structure.

cert: contains a certificate list (path) for the specified private key

key: is the private key, or NULL

type: is PEM or DER

pass: is the key’s password

flags: an ORed sequence of gnutls_pkcs_encrypt_flags_t

This function sets a certificate/private key pair in the gnutls_certificate_credentials_t structure. This function may be called more than once, in case multiple keys/certificates exist for the server.

Note that the keyUsage (2.5.29.15) PKIX extension in X.509 certificates is supported. This means that certificates intended for signing cannot be used for ciphersuites that require encryption.

If the certificate and the private key are given in PEM encoding then the strings that hold their values must be null terminated.

The key may be NULL if you are using a sign callback, see gnutls_sign_callback_set() .

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

gnutls_certificate_set_x509_simple_pkcs12_file

Function: int gnutls_certificate_set_x509_simple_pkcs12_file (gnutls_certificate_credentials_t res, const char * pkcs12file, gnutls_x509_crt_fmt_t type, const char * password)

res: is a gnutls_certificate_credentials_t structure.

pkcs12file: filename of file containing PKCS12 blob.

type: is PEM or DER of the pkcs12file .

password: optional password used to decrypt PKCS12 file, bags and keys.

This function sets a certificate/private key pair and/or a CRL in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).

PKCS12 files with a MAC, encrypted bags and PKCS 8 private keys are supported. However, only password based security, and the same password for all operations, are supported.

PKCS12 file may contain many keys and/or certificates, and this function will try to auto-detect based on the key ID the certificate and key pair to use. If the PKCS12 file contain the issuer of the selected certificate, it will be appended to the certificate to form a chain.

If more than one private keys are stored in the PKCS12 file, then only one key will be read (and it is undefined which one).

It is believed that the limitations of this function is acceptable for most usage, and that any more flexibility would introduce complexity that would make it harder to use this functionality at all.

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

gnutls_certificate_set_x509_simple_pkcs12_mem

Function: int gnutls_certificate_set_x509_simple_pkcs12_mem (gnutls_certificate_credentials_t res, const gnutls_datum_t * p12blob, gnutls_x509_crt_fmt_t type, const char * password)

res: is a gnutls_certificate_credentials_t structure.

p12blob: the PKCS12 blob.

type: is PEM or DER of the pkcs12file .

password: optional password used to decrypt PKCS12 file, bags and keys.

This function sets a certificate/private key pair and/or a CRL in the gnutls_certificate_credentials_t structure. This function may be called more than once (in case multiple keys/certificates exist for the server).

Encrypted PKCS12 bags and PKCS8 private keys are supported. However, only password based security, and the same password for all operations, are supported.

PKCS12 file may contain many keys and/or certificates, and this function will try to auto-detect based on the key ID the certificate and key pair to use. If the PKCS12 file contain the issuer of the selected certificate, it will be appended to the certificate to form a chain.

If more than one private keys are stored in the PKCS12 file, then only one key will be read (and it is undefined which one).

It is believed that the limitations of this function is acceptable for most usage, and that any more flexibility would introduce complexity that would make it harder to use this functionality at all.

Returns: GNUTLS_E_SUCCESS (0) on success, or a negative error code.

Since: 2.8.0

gnutls_certificate_set_x509_system_trust

Function: int gnutls_certificate_set_x509_system_trust (gnutls_certificate_credentials_t cred)

cred: is a gnutls_certificate_credentials_t structure.

This function adds the system’s default trusted CAs in order to verify client or server certificates.

In the case the system is currently unsupported GNUTLS_E_UNIMPLEMENTED_FEATURE is returned.

Returns: the number of certificates processed or a negative error code on error.

Since: 3.0

gnutls_certificate_set_x509_trust

Function: int gnutls_certificate_set_x509_trust (gnutls_certificate_credentials_t res, gnutls_x509_crt_t * ca_list, int ca_list_size)

res: is a gnutls_certificate_credentials_t structure.

ca_list: is a list of trusted CAs

ca_list_size: holds the size of the CA list

This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

In case of a server the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using gnutls_certificate_send_x509_rdn_sequence() .

Returns: the number of certificates processed or a negative error code on error.

Since: 2.4.0

gnutls_certificate_set_x509_trust_file

Function: int gnutls_certificate_set_x509_trust_file (gnutls_certificate_credentials_t cred, const char * cafile, gnutls_x509_crt_fmt_t type)

cred: is a gnutls_certificate_credentials_t structure.

cafile: is a file containing the list of trusted CAs (DER or PEM list)

type: is PEM or DER

This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

In case of a server the names of the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using gnutls_certificate_send_x509_rdn_sequence() .

This function can also accept URLs. In that case it will import all certificates that are marked as trusted. Note that the supported URLs are the ones indicated by gnutls_url_is_supported() .

Returns: number of certificates processed, or a negative error code on error.

gnutls_certificate_set_x509_trust_mem

Function: int gnutls_certificate_set_x509_trust_mem (gnutls_certificate_credentials_t res, const gnutls_datum_t * ca, gnutls_x509_crt_fmt_t type)

res: is a gnutls_certificate_credentials_t structure.

ca: is a list of trusted CAs or a DER certificate

type: is DER or PEM

This function adds the trusted CAs in order to verify client or server certificates. In case of a client this is not required to be called if the certificates are not verified using gnutls_certificate_verify_peers2() . This function may be called multiple times.

In case of a server the CAs set here will be sent to the client if a certificate request is sent. This can be disabled using gnutls_certificate_send_x509_rdn_sequence() .

Returns: the number of certificates processed or a negative error code on error.

gnutls_certificate_type_get

Function: gnutls_certificate_type_t gnutls_certificate_type_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

The certificate type is by default X.509, unless it is negotiated as a TLS extension.

Returns: the currently used gnutls_certificate_type_t certificate type.

gnutls_certificate_type_get_id

Function: gnutls_certificate_type_t gnutls_certificate_type_get_id (const char * name)

name: is a certificate type name

The names are compared in a case insensitive way.

Returns: a gnutls_certificate_type_t for the specified in a string certificate type, or GNUTLS_CRT_UNKNOWN on error.

gnutls_certificate_type_get_name

Function: const char * gnutls_certificate_type_get_name (gnutls_certificate_type_t type)

type: is a certificate type

Convert a gnutls_certificate_type_t type to a string.

Returns: a string that contains the name of the specified certificate type, or NULL in case of unknown types.

gnutls_certificate_type_list

Function: const gnutls_certificate_type_t * gnutls_certificate_type_list ( void)

Get a list of certificate types.

Returns: a (0)-terminated list of gnutls_certificate_type_t integers indicating the available certificate types.

gnutls_certificate_verification_status_print

Function: int gnutls_certificate_verification_status_print (unsigned int status, gnutls_certificate_type_t type, gnutls_datum_t * out, unsigned int flags)

status: The status flags to be printed

type: The certificate type

out: Newly allocated datum with (0) terminated string.

flags: should be zero

This function will pretty print the status of a verification process – eg. the one obtained by gnutls_certificate_verify_peers3() .

The output out needs to be deallocated using gnutls_free() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error value.

Since: 3.1.4

gnutls_certificate_verify_peers2

Function: int gnutls_certificate_verify_peers2 (gnutls_session_t session, unsigned int * status)

session: is a gnutls session

status: is the output of the verification

This function will verify the peer’s certificate and store the status in the status variable as a bitwise or’d gnutls_certificate_status_t values or zero if the certificate is trusted. Note that value in status is set only when the return value of this function is success (i.e, failure to trust a certificate does not imply a negative return value).

If available the OCSP Certificate Status extension will be utilized by this function.

To avoid denial of service attacks some default upper limits regarding the certificate key size and chain size are set. To override them use gnutls_certificate_set_verify_limits() .

Note that you must also check the peer’s name in order to check if the verified certificate belongs to the actual peer, see gnutls_x509_crt_check_hostname() , or use gnutls_certificate_verify_peers3() .

Returns: a negative error code on error and GNUTLS_E_SUCCESS (0) on success.

gnutls_certificate_verify_peers3

Function: int gnutls_certificate_verify_peers3 (gnutls_session_t session, const char * hostname, unsigned int * status)

session: is a gnutls session

hostname: is the expected name of the peer; may be NULL

status: is the output of the verification

This function will verify the peer’s certificate and store the status in the status variable as a bitwise or’d gnutls_certificate_status_t values or zero if the certificate is trusted. Note that value in status is set only when the return value of this function is success (i.e, failure to trust a certificate does not imply a negative return value).

If the hostname provided is non-NULL then this function will compare the hostname in the certificate against the given. If they do not match the GNUTLS_CERT_UNEXPECTED_OWNER status flag will be set.

If available the OCSP Certificate Status extension will be utilized by this function.

To avoid denial of service attacks some default upper limits regarding the certificate key size and chain size are set. To override them use gnutls_certificate_set_verify_limits() .

Returns: a negative error code on error and GNUTLS_E_SUCCESS (0) on success.

Since: 3.1.4

gnutls_check_version

Function: const char * gnutls_check_version (const char * req_version)

req_version: version string to compare with, or NULL .

Check GnuTLS Library version.

See GNUTLS_VERSION for a suitable req_version string.

Returns: Check that the version of the library is at minimum the one given as a string in req_version and return the actual version string of the library; return NULL if the condition is not met. If NULL is passed to this function no check is done and only the version string is returned.

gnutls_cipher_get

Function: gnutls_cipher_algorithm_t gnutls_cipher_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get currently used cipher.

Returns: the currently used cipher, a gnutls_cipher_algorithm_t type.

gnutls_cipher_get_id

Function: gnutls_cipher_algorithm_t gnutls_cipher_get_id (const char * name)

name: is a cipher algorithm name

The names are compared in a case insensitive way.

Returns: return a gnutls_cipher_algorithm_t value corresponding to the specified cipher, or GNUTLS_CIPHER_UNKNOWN on error.

gnutls_cipher_get_key_size

Function: size_t gnutls_cipher_get_key_size (gnutls_cipher_algorithm_t algorithm)

algorithm: is an encryption algorithm

Get key size for cipher.

Returns: length (in bytes) of the given cipher’s key size, or 0 if the given cipher is invalid.

gnutls_cipher_get_name

Function: const char * gnutls_cipher_get_name (gnutls_cipher_algorithm_t algorithm)

algorithm: is an encryption algorithm

Convert a gnutls_cipher_algorithm_t type to a string.

Returns: a pointer to a string that contains the name of the specified cipher, or NULL .

gnutls_cipher_list

Function: const gnutls_cipher_algorithm_t * gnutls_cipher_list ( void)

Get a list of supported cipher algorithms. Note that not necessarily all ciphers are supported as TLS cipher suites. For example, DES is not supported as a cipher suite, but is supported for other purposes (e.g., PKCS8 or similar).

This function is not thread safe.

Returns: a (0)-terminated list of gnutls_cipher_algorithm_t integers indicating the available ciphers.

gnutls_cipher_suite_get_name

Function: const char * gnutls_cipher_suite_get_name (gnutls_kx_algorithm_t kx_algorithm, gnutls_cipher_algorithm_t cipher_algorithm, gnutls_mac_algorithm_t mac_algorithm)

kx_algorithm: is a Key exchange algorithm

cipher_algorithm: is a cipher algorithm

mac_algorithm: is a MAC algorithm

Note that the full cipher suite name must be prepended by TLS or SSL depending of the protocol in use.

Returns: a string that contains the name of a TLS cipher suite, specified by the given algorithms, or NULL .

gnutls_cipher_suite_info

Function: const char * gnutls_cipher_suite_info (size_t idx, unsigned char * cs_id, gnutls_kx_algorithm_t * kx, gnutls_cipher_algorithm_t * cipher, gnutls_mac_algorithm_t * mac, gnutls_protocol_t * min_version)

idx: index of cipher suite to get information about, starts on 0.

cs_id: output buffer with room for 2 bytes, indicating cipher suite value

kx: output variable indicating key exchange algorithm, or NULL .

cipher: output variable indicating cipher, or NULL .

mac: output variable indicating MAC algorithm, or NULL .

min_version: output variable indicating TLS protocol version, or NULL .

Get information about supported cipher suites. Use the function iteratively to get information about all supported cipher suites. Call with idx=0 to get information about first cipher suite, then idx=1 and so on until the function returns NULL.

Returns: the name of idx cipher suite, and set the information about the cipher suite in the output variables. If idx is out of bounds, NULL is returned.

gnutls_compression_get

Function: gnutls_compression_method_t gnutls_compression_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get currently used compression algorithm.

Returns: the currently used compression method, a gnutls_compression_method_t value.

gnutls_compression_get_id

Function: gnutls_compression_method_t gnutls_compression_get_id (const char * name)

name: is a compression method name

The names are compared in a case insensitive way.

Returns: an id of the specified in a string compression method, or GNUTLS_COMP_UNKNOWN on error.

gnutls_compression_get_name

Function: const char * gnutls_compression_get_name (gnutls_compression_method_t algorithm)

algorithm: is a Compression algorithm

Convert a gnutls_compression_method_t value to a string.

Returns: a pointer to a string that contains the name of the specified compression algorithm, or NULL .

gnutls_compression_list

Function: const gnutls_compression_method_t * gnutls_compression_list ( void)

Get a list of compression methods.

Returns: a zero-terminated list of gnutls_compression_method_t integers indicating the available compression methods.

gnutls_credentials_clear

Function: void gnutls_credentials_clear (gnutls_session_t session)

session: is a gnutls_session_t structure.

Clears all the credentials previously set in this session.

gnutls_credentials_set

Function: int gnutls_credentials_set (gnutls_session_t session, gnutls_credentials_type_t type, void * cred)

session: is a gnutls_session_t structure.

type: is the type of the credentials

cred: is a pointer to a structure.

Sets the needed credentials for the specified type. Eg username, password - or public and private keys etc. The cred parameter is a structure that depends on the specified type and on the current session (client or server).

In order to minimize memory usage, and share credentials between several threads gnutls keeps a pointer to cred, and not the whole cred structure. Thus you will have to keep the structure allocated until you call gnutls_deinit() .

For GNUTLS_CRD_ANON , cred should be gnutls_anon_client_credentials_t in case of a client. In case of a server it should be gnutls_anon_server_credentials_t .

For GNUTLS_CRD_SRP , cred should be gnutls_srp_client_credentials_t in case of a client, and gnutls_srp_server_credentials_t , in case of a server.

For GNUTLS_CRD_CERTIFICATE , cred should be gnutls_certificate_credentials_t .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_db_check_entry

Function: int gnutls_db_check_entry (gnutls_session_t session, gnutls_datum_t session_entry)

session: is a gnutls_session_t structure.

session_entry: is the session data (not key)

This function has no effect.

Returns: Returns GNUTLS_E_EXPIRED , if the database entry has expired or 0 otherwise.

gnutls_db_check_entry_time

Function: time_t gnutls_db_check_entry_time (gnutls_datum_t * entry)

entry: is a pointer to a gnutls_datum_t structure.

This function returns the time that this entry was active. It can be used for database entry expiration.

Returns: The time this entry was created, or zero on error.

gnutls_db_get_default_cache_expiration

Function: unsigned gnutls_db_get_default_cache_expiration ( void)

Returns: the expiration time (in seconds) of stored sessions for resumption.

Since 3.2.10

gnutls_db_get_ptr

Function: void * gnutls_db_get_ptr (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get db function pointer.

Returns: the pointer that will be sent to db store, retrieve and delete functions, as the first argument.

gnutls_db_remove_session

Function: void gnutls_db_remove_session (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function will remove the current session data from the session database. This will prevent future handshakes reusing these session data. This function should be called if a session was terminated abnormally, and before gnutls_deinit() is called.

Normally gnutls_deinit() will remove abnormally terminated sessions.

gnutls_db_set_cache_expiration

Function: void gnutls_db_set_cache_expiration (gnutls_session_t session, int seconds)

session: is a gnutls_session_t structure.

seconds: is the number of seconds.

Set the expiration time for resumed sessions. The default is 3600 (one hour) at the time of this writing.

gnutls_db_set_ptr

Function: void gnutls_db_set_ptr (gnutls_session_t session, void * ptr)

session: is a gnutls_session_t structure.

ptr: is the pointer

Sets the pointer that will be provided to db store, retrieve and delete functions, as the first argument.

gnutls_db_set_remove_function

Function: void gnutls_db_set_remove_function (gnutls_session_t session, gnutls_db_remove_func rem_func)

session: is a gnutls_session_t structure.

rem_func: is the function.

Sets the function that will be used to remove data from the resumed sessions database. This function must return 0 on success.

The first argument to rem_func will be null unless gnutls_db_set_ptr() has been called.

gnutls_db_set_retrieve_function

Function: void gnutls_db_set_retrieve_function (gnutls_session_t session, gnutls_db_retr_func retr_func)

session: is a gnutls_session_t structure.

retr_func: is the function.

Sets the function that will be used to retrieve data from the resumed sessions database. This function must return a gnutls_datum_t containing the data on success, or a gnutls_datum_t containing null and 0 on failure.

The datum’s data must be allocated using the function gnutls_malloc() .

The first argument to retr_func will be null unless gnutls_db_set_ptr() has been called.

gnutls_db_set_store_function

Function: void gnutls_db_set_store_function (gnutls_session_t session, gnutls_db_store_func store_func)

session: is a gnutls_session_t structure.

store_func: is the function

Sets the function that will be used to store data in the resumed sessions database. This function must return 0 on success.

The first argument to store_func will be null unless gnutls_db_set_ptr() has been called.

gnutls_deinit

Function: void gnutls_deinit (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function clears all buffers associated with the session . This function will also remove session data from the session database if the session was terminated abnormally.

gnutls_dh_get_group

Function: int gnutls_dh_get_group (gnutls_session_t session, gnutls_datum_t * raw_gen, gnutls_datum_t * raw_prime)

session: is a gnutls session

raw_gen: will hold the generator.

raw_prime: will hold the prime.

This function will return the group parameters used in the last Diffie-Hellman key exchange with the peer. These are the prime and the generator used. This function should be used for both anonymous and ephemeral Diffie-Hellman. The output parameters must be freed with gnutls_free() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_dh_get_peers_public_bits

Function: int gnutls_dh_get_peers_public_bits (gnutls_session_t session)

session: is a gnutls session

Get the Diffie-Hellman public key bit size. Can be used for both anonymous and ephemeral Diffie-Hellman.

Returns: The public key bit size used in the last Diffie-Hellman key exchange with the peer, or a negative error code in case of error.

gnutls_dh_get_prime_bits

Function: int gnutls_dh_get_prime_bits (gnutls_session_t session)

session: is a gnutls session

This function will return the bits of the prime used in the last Diffie-Hellman key exchange with the peer. Should be used for both anonymous and ephemeral Diffie-Hellman. Note that some ciphers, like RSA and DSA without DHE, do not use a Diffie-Hellman key exchange, and then this function will return 0.

Returns: The Diffie-Hellman bit strength is returned, or 0 if no Diffie-Hellman key exchange was done, or a negative error code on failure.

gnutls_dh_get_pubkey

Function: int gnutls_dh_get_pubkey (gnutls_session_t session, gnutls_datum_t * raw_key)

session: is a gnutls session

raw_key: will hold the public key.

This function will return the peer’s public key used in the last Diffie-Hellman key exchange. This function should be used for both anonymous and ephemeral Diffie-Hellman. The output parameters must be freed with gnutls_free() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_dh_get_secret_bits

Function: int gnutls_dh_get_secret_bits (gnutls_session_t session)

session: is a gnutls session

This function will return the bits used in the last Diffie-Hellman key exchange with the peer. Should be used for both anonymous and ephemeral Diffie-Hellman.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_dh_params_cpy

Function: int gnutls_dh_params_cpy (gnutls_dh_params_t dst, gnutls_dh_params_t src)

dst: Is the destination structure, which should be initialized.

src: Is the source structure

This function will copy the DH parameters structure from source to destination.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_deinit

Function: void gnutls_dh_params_deinit (gnutls_dh_params_t dh_params)

dh_params: Is a structure that holds the prime numbers

This function will deinitialize the DH parameters structure.

gnutls_dh_params_export2_pkcs3

Function: int gnutls_dh_params_export2_pkcs3 (gnutls_dh_params_t params, gnutls_x509_crt_fmt_t format, gnutls_datum_t * out)

params: Holds the DH parameters

format: the format of output params. One of PEM or DER.

out: will contain a PKCS3 DHParams structure PEM or DER encoded

This function will export the given dh parameters to a PKCS3 DHParams structure. This is the format generated by "openssl dhparam" tool. The data in out will be allocated using gnutls_malloc() .

If the structure is PEM encoded, it will have a header of "BEGIN DH PARAMETERS".

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since: 3.1.3

gnutls_dh_params_export_pkcs3

Function: int gnutls_dh_params_export_pkcs3 (gnutls_dh_params_t params, gnutls_x509_crt_fmt_t format, unsigned char * params_data, size_t * params_data_size)

params: Holds the DH parameters

format: the format of output params. One of PEM or DER.

params_data: will contain a PKCS3 DHParams structure PEM or DER encoded

params_data_size: holds the size of params_data (and will be replaced by the actual size of parameters)

This function will export the given dh parameters to a PKCS3 DHParams structure. This is the format generated by "openssl dhparam" tool. If the buffer provided is not long enough to hold the output, then GNUTLS_E_SHORT_MEMORY_BUFFER will be returned.

If the structure is PEM encoded, it will have a header of "BEGIN DH PARAMETERS".

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_export_raw

Function: int gnutls_dh_params_export_raw (gnutls_dh_params_t params, gnutls_datum_t * prime, gnutls_datum_t * generator, unsigned int * bits)

params: Holds the DH parameters

prime: will hold the new prime

generator: will hold the new generator

bits: if non null will hold the secret key’s number of bits

This function will export the pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters will be allocated using gnutls_malloc() and will be stored in the appropriate datum.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_generate2

Function: int gnutls_dh_params_generate2 (gnutls_dh_params_t params, unsigned int bits)

params: Is the structure that the DH parameters will be stored

bits: is the prime’s number of bits

This function will generate a new pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters will be allocated using gnutls_malloc() and will be stored in the appropriate datum. This function is normally slow.

Do not set the number of bits directly, use gnutls_sec_param_to_pk_bits() to get bits for GNUTLS_PK_DSA . Also note that the DH parameters are only useful to servers. Since clients use the parameters sent by the server, it’s of no use to call this in client side.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_import_pkcs3

Function: int gnutls_dh_params_import_pkcs3 (gnutls_dh_params_t params, const gnutls_datum_t * pkcs3_params, gnutls_x509_crt_fmt_t format)

params: A structure where the parameters will be copied to

pkcs3_params: should contain a PKCS3 DHParams structure PEM or DER encoded

format: the format of params. PEM or DER.

This function will extract the DHParams found in a PKCS3 formatted structure. This is the format generated by "openssl dhparam" tool.

If the structure is PEM encoded, it should have a header of "BEGIN DH PARAMETERS".

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_import_raw

Function: int gnutls_dh_params_import_raw (gnutls_dh_params_t dh_params, const gnutls_datum_t * prime, const gnutls_datum_t * generator)

dh_params: Is a structure that will hold the prime numbers

prime: holds the new prime

generator: holds the new generator

This function will replace the pair of prime and generator for use in the Diffie-Hellman key exchange. The new parameters should be stored in the appropriate gnutls_datum.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_params_init

Function: int gnutls_dh_params_init (gnutls_dh_params_t * dh_params)

dh_params: Is a structure that will hold the prime numbers

This function will initialize the DH parameters structure.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_dh_set_prime_bits

Function: void gnutls_dh_set_prime_bits (gnutls_session_t session, unsigned int bits)

session: is a gnutls_session_t structure.

bits: is the number of bits

This function sets the number of bits, for use in a Diffie-Hellman key exchange. This is used both in DH ephemeral and DH anonymous cipher suites. This will set the minimum size of the prime that will be used for the handshake.

In the client side it sets the minimum accepted number of bits. If a server sends a prime with less bits than that GNUTLS_E_DH_PRIME_UNACCEPTABLE will be returned by the handshake.

Note that values lower than 512 bits may allow decryption of the exchanged data.

The function has no effect in server side.

Note that since 3.1.7 this function is deprecated. The minimum number of bits is set by the priority string level. Also this function must be called after gnutls_priority_set_direct() or the set value may be overridden by the selected priority options.

gnutls_digest_get_id

Function: gnutls_digest_algorithm_t gnutls_digest_get_id (const char * name)

name: is a digest algorithm name

Convert a string to a gnutls_digest_algorithm_t value. The names are compared in a case insensitive way.

Returns: a gnutls_digest_algorithm_t id of the specified MAC algorithm string, or GNUTLS_DIG_UNKNOWN on failures.

gnutls_digest_get_name

Function: const char * gnutls_digest_get_name (gnutls_digest_algorithm_t algorithm)

algorithm: is a digest algorithm

Convert a gnutls_digest_algorithm_t value to a string.

Returns: a string that contains the name of the specified digest algorithm, or NULL .

gnutls_digest_list

Function: const gnutls_digest_algorithm_t * gnutls_digest_list ( void)

Get a list of hash (digest) algorithms supported by GnuTLS.

This function is not thread safe.

Returns: Return a (0)-terminated list of gnutls_digest_algorithm_t integers indicating the available digests.

gnutls_ecc_curve_get

Function: gnutls_ecc_curve_t gnutls_ecc_curve_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

Returns the currently used elliptic curve. Only valid when using an elliptic curve ciphersuite.

Returns: the currently used curve, a gnutls_ecc_curve_t type.

Since: 3.0

gnutls_ecc_curve_get_name

Function: const char * gnutls_ecc_curve_get_name (gnutls_ecc_curve_t curve)

curve: is an ECC curve

Convert a gnutls_ecc_curve_t value to a string.

Returns: a string that contains the name of the specified curve or NULL .

Since: 3.0

gnutls_ecc_curve_get_size

Function: int gnutls_ecc_curve_get_size (gnutls_ecc_curve_t curve)

curve: is an ECC curve

Returns the size in bytes of the curve.

Returns: a the size or (0).

Since: 3.0

gnutls_ecc_curve_list

Function: const gnutls_ecc_curve_t * gnutls_ecc_curve_list ( void)

Get the list of supported elliptic curves.

This function is not thread safe.

Returns: Return a (0)-terminated list of gnutls_ecc_curve_t integers indicating the available curves.

gnutls_error_is_fatal

Function: int gnutls_error_is_fatal (int error)

error: is a GnuTLS error code, a negative error code

If a GnuTLS function returns a negative error code you may feed that value to this function to see if the error condition is fatal to a TLS session (i.e., must be terminated).

Note that you may also want to check the error code manually, since some non-fatal errors to the protocol (such as a warning alert or a rehandshake request) may be fatal for your program.

This function is only useful if you are dealing with errors from functions that relate to a TLS session (e.g., record layer or handshake layer handling functions).

Returns: zero on non fatal errors or positive error values. Non-zero on fatal error codes.

gnutls_error_to_alert

Function: int gnutls_error_to_alert (int err, int * level)

err: is a negative integer

level: the alert level will be stored there

Get an alert depending on the error code returned by a gnutls function. All alerts sent by this function should be considered fatal. The only exception is when err is GNUTLS_E_REHANDSHAKE , where a warning alert should be sent to the peer indicating that no renegotiation will be performed.

If there is no mapping to a valid alert the alert to indicate internal error is returned.

Returns: the alert code to use for a particular error code.

gnutls_est_record_overhead_size

Function: size_t gnutls_est_record_overhead_size (gnutls_protocol_t version, gnutls_cipher_algorithm_t cipher, gnutls_mac_algorithm_t mac, gnutls_compression_method_t comp, unsigned int flags)

version: is a gnutls_protocol_t value

cipher: is a gnutls_cipher_algorithm_t value

mac: is a gnutls_mac_algorithm_t value

comp: is a gnutls_compression_method_t value

flags: must be zero

This function will return the set size in bytes of the overhead due to TLS (or DTLS) per record.

Note that this function may provide inacurate values when TLS extensions that modify the record format are negotiated. In these cases a more accurate value can be obtained using gnutls_record_overhead_size() after a completed handshake.

Since: 3.2.2

gnutls_fingerprint

Function: int gnutls_fingerprint (gnutls_digest_algorithm_t algo, const gnutls_datum_t * data, void * result, size_t * result_size)

algo: is a digest algorithm

data: is the data

result: is the place where the result will be copied (may be null).

result_size: should hold the size of the result. The actual size of the returned result will also be copied there.

This function will calculate a fingerprint (actually a hash), of the given data. The result is not printable data. You should convert it to hex, or to something else printable.

This is the usual way to calculate a fingerprint of an X.509 DER encoded certificate. Note however that the fingerprint of an OpenPGP certificate is not just a hash and cannot be calculated with this function.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_global_deinit

Function: void gnutls_global_deinit ( void)

This function deinitializes the global data, that were initialized using gnutls_global_init() .

Note! This function is not thread safe. See the discussion for gnutls_global_init() for more information.

gnutls_global_init

Function: int gnutls_global_init ( void)

This function performs any required precalculations, detects the supported CPU capabilities and initializes the underlying cryptographic backend. In order to free any resources taken by this call you should gnutls_global_deinit() when gnutls usage is no longer needed.

This function increments a global counter, so that gnutls_global_deinit() only releases resources when it has been called as many times as gnutls_global_init() . This is useful when GnuTLS is used by more than one library in an application. This function can be called many times, but will only do something the first time.

Note! This function is not thread safe. If two threads call this function simultaneously, they can cause a race between checking the global counter and incrementing it, causing both threads to execute the library initialization code. That could lead to a memory leak or even a crash. To handle this, your application should invoke this function after aquiring a thread mutex.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

gnutls_global_set_audit_log_function

Function: void gnutls_global_set_audit_log_function (gnutls_audit_log_func log_func)

log_func: it is the audit log function

This is the function to set the audit logging function. This is a function to report important issues, such as possible attacks in the protocol. This is different from gnutls_global_set_log_function() because it will report also session-specific events. The session parameter will be null if there is no corresponding TLS session.

gnutls_audit_log_func is of the form, void (*gnutls_audit_log_func)( gnutls_session_t, const char*);

Since: 3.0

gnutls_global_set_log_function

Function: void gnutls_global_set_log_function (gnutls_log_func log_func)

log_func: it’s a log function

This is the function where you set the logging function gnutls is going to use. This function only accepts a character array. Normally you may not use this function since it is only used for debugging purposes.

gnutls_log_func is of the form, void (*gnutls_log_func)( int level, const char*);

gnutls_global_set_log_level

Function: void gnutls_global_set_log_level (int level)

level: it’s an integer from 0 to 9.

This is the function that allows you to set the log level. The level is an integer between 0 and 9. Higher values mean more verbosity. The default value is 0. Larger values should only be used with care, since they may reveal sensitive information.

Use a log level over 10 to enable all debugging options.

gnutls_global_set_mem_functions

Function: void gnutls_global_set_mem_functions (gnutls_alloc_function alloc_func, gnutls_alloc_function secure_alloc_func, gnutls_is_secure_function is_secure_func, gnutls_realloc_function realloc_func, gnutls_free_function free_func)

alloc_func: it’s the default memory allocation function. Like malloc() .

secure_alloc_func: This is the memory allocation function that will be used for sensitive data.

is_secure_func: a function that returns 0 if the memory given is not secure. May be NULL.

realloc_func: A realloc function

free_func: The function that frees allocated data. Must accept a NULL pointer.

This is the function where you set the memory allocation functions gnutls is going to use. By default the libc’s allocation functions (malloc() , free() ), are used by gnutls, to allocate both sensitive and not sensitive data. This function is provided to set the memory allocation functions to something other than the defaults

This function must be called before gnutls_global_init() is called. This function is not thread safe.

gnutls_global_set_mutex

Function: void gnutls_global_set_mutex (mutex_init_func init, mutex_deinit_func deinit, mutex_lock_func lock, mutex_unlock_func unlock)

init: mutex initialization function

deinit: mutex deinitialization function

lock: mutex locking function

unlock: mutex unlocking function

With this function you are allowed to override the default mutex locks used in some parts of gnutls and dependent libraries. This function should be used if you have complete control of your program and libraries. Do not call this function from a library. Instead only initialize gnutls and the default OS mutex locks will be used.

This function must be called before gnutls_global_init() .

Since: 2.12.0

gnutls_global_set_time_function

Function: void gnutls_global_set_time_function (gnutls_time_func time_func)

time_func: it’s the system time function, a gnutls_time_func() callback.

This is the function where you can override the default system time function. The application provided function should behave the same as the standard function.

Since: 2.12.0

gnutls_handshake

Function: int gnutls_handshake (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function does the handshake of the TLS/SSL protocol, and initializes the TLS connection.

This function will fail if any problem is encountered, and will return a negative error code. In case of a client, if the client has asked to resume a session, but the server couldn’t, then a full handshake will be performed.

The non-fatal errors such as GNUTLS_E_AGAIN and GNUTLS_E_INTERRUPTED interrupt the handshake procedure, which should be resumed later. Call this function again, until it returns 0; cf. gnutls_record_get_direction() and gnutls_error_is_fatal() .

If this function is called by a server after a rehandshake request then GNUTLS_E_GOT_APPLICATION_DATA or GNUTLS_E_WARNING_ALERT_RECEIVED may be returned. Note that these are non fatal errors, only in the specific case of a rehandshake. Their meaning is that the client rejected the rehandshake request or in the case of GNUTLS_E_GOT_APPLICATION_DATA it might also mean that some data were pending.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code.

gnutls_handshake_description_get_name

Function: const char * gnutls_handshake_description_get_name (gnutls_handshake_description_t type)

type: is a handshake message description

Convert a gnutls_handshake_description_t value to a string.

Returns: a string that contains the name of the specified handshake message or NULL .

gnutls_handshake_get_last_in

Function: gnutls_handshake_description_t gnutls_handshake_get_last_in (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function is only useful to check where the last performed handshake failed. If the previous handshake succeed or was not performed at all then no meaningful value will be returned.

Check gnutls_handshake_description_t in gnutls.h for the available handshake descriptions.

Returns: the last handshake message type received, a gnutls_handshake_description_t .

gnutls_handshake_get_last_out

Function: gnutls_handshake_description_t gnutls_handshake_get_last_out (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function is only useful to check where the last performed handshake failed. If the previous handshake succeed or was not performed at all then no meaningful value will be returned.

Check gnutls_handshake_description_t in gnutls.h for the available handshake descriptions.

Returns: the last handshake message type sent, a gnutls_handshake_description_t .

gnutls_handshake_set_hook_function

Function: void gnutls_handshake_set_hook_function (gnutls_session_t session, unsigned int htype, int post, gnutls_handshake_hook_func func)

session: is a gnutls_session_t structure

htype: the gnutls_handshake_description_t of the message to hook at

post: GNUTLS_HOOK_ * depending on when the hook function should be called

func: is the function to be called

This function will set a callback to be called after or before the specified handshake message has been received or generated. This is a generalization of gnutls_handshake_set_post_client_hello_function() .

To call the hook function prior to the message being sent/generation use GNUTLS_HOOK_PRE as post parameter, GNUTLS_HOOK_POST to call after, and GNUTLS_HOOK_BOTH for both cases.

This callback must return 0 on success or a gnutls error code to terminate the handshake.

Note to hook at all handshake messages use an htype of GNUTLS_HANDSHAKE_ANY .

Warning: You should not use this function to terminate the handshake based on client input unless you know what you are doing. Before the handshake is finished there is no way to know if there is a man-in-the-middle attack being performed.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_handshake_set_max_packet_length

Function: void gnutls_handshake_set_max_packet_length (gnutls_session_t session, size_t max)

session: is a gnutls_session_t structure.

max: is the maximum number.

This function will set the maximum size of all handshake messages. Handshakes over this size are rejected with GNUTLS_E_HANDSHAKE_TOO_LARGE error code. The default value is 48kb which is typically large enough. Set this to 0 if you do not want to set an upper limit.

The reason for restricting the handshake message sizes are to limit Denial of Service attacks.

gnutls_handshake_set_post_client_hello_function

Function: void gnutls_handshake_set_post_client_hello_function (gnutls_session_t session, gnutls_handshake_post_client_hello_func func)

session: is a gnutls_session_t structure.

func: is the function to be called

This function will set a callback to be called after the client hello has been received (callback valid in server side only). This allows the server to adjust settings based on received extensions.

Those settings could be ciphersuites, requesting certificate, or anything else except for version negotiation (this is done before the hello message is parsed).

This callback must return 0 on success or a gnutls error code to terminate the handshake.

Warning: You should not use this function to terminate the handshake based on client input unless you know what you are doing. Before the handshake is finished there is no way to know if there is a man-in-the-middle attack being performed.

gnutls_handshake_set_private_extensions

Function: void gnutls_handshake_set_private_extensions (gnutls_session_t session, int allow)

session: is a gnutls_session_t structure.

allow: is an integer (0 or 1)

This function will enable or disable the use of private cipher suites (the ones that start with 0xFF). By default or if allow is 0 then these cipher suites will not be advertised nor used.

Currently GnuTLS does not include such cipher-suites or compression algorithms.

Enabling the private ciphersuites when talking to other than gnutls servers and clients may cause interoperability problems.

gnutls_handshake_set_random

Function: int gnutls_handshake_set_random (gnutls_session_t session, const gnutls_datum_t * random)

session: is a gnutls_session_t structure.

random: a random value of 32-bytes

This function will explicitly set the server or client hello random value in the subsequent TLS handshake. The random value should be a 32-byte value.

Note that this function should not normally be used as gnutls will select automatically a random value for the handshake.

This function should not be used when resuming a session.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

Since 3.1.9

gnutls_handshake_set_timeout

Function: void gnutls_handshake_set_timeout (gnutls_session_t session, unsigned int ms)

session: is a gnutls_session_t structure.

ms: is a timeout value in milliseconds

This function sets the timeout for the handshake process to the provided value. Use an ms value of zero to disable timeout.

Note that in order for the timeout to be enforced gnutls_transport_set_pull_timeout_function() must be set (it is set by default in most systems).

gnutls_heartbeat_allowed

Function: int gnutls_heartbeat_allowed (gnutls_session_t session, unsigned int type)

session: is a gnutls_session_t structure.

type: one of GNUTLS_HB_LOCAL_ALLOWED_TO_SEND and GNUTLS_HB_PEER_ALLOWED_TO_SEND

This function will check whether heartbeats are allowed to be sent or received in this session.

Returns: Non zero if heartbeats are allowed.

Since: 3.1.2

gnutls_heartbeat_enable

Function: void gnutls_heartbeat_enable (gnutls_session_t session, unsigned int type)

session: is a gnutls_session_t structure.

type: one of the GNUTLS_HB_* flags

If this function is called with the GNUTLS_HB_PEER_ALLOWED_TO_SEND GnuTLS will allow heartbeat messages to be received. Moreover it also request the peer to accept heartbeat messages.

The function gnutls_heartbeat_allowed() can be used to test Whether locally generated heartbeat messages can be accepted by the peer.

Since: 3.1.2

gnutls_heartbeat_get_timeout

Function: unsigned int gnutls_heartbeat_get_timeout (gnutls_session_t session)

session: is a gnutls_session_t structure.

This function will return the milliseconds remaining for a retransmission of the previously sent ping message. This function is useful when ping is used in non-blocking mode, to estimate when to call gnutls_heartbeat_ping() if no packets have been received.

Returns: the remaining time in milliseconds.

Since: 3.1.2

gnutls_heartbeat_ping

Function: int gnutls_heartbeat_ping (gnutls_session_t session, size_t data_size, unsigned int max_tries, unsigned int flags)

session: is a gnutls_session_t structure.

data_size: is the length of the ping payload.

max_tries: if flags is GNUTLS_HEARTBEAT_WAIT then this sets the number of retransmissions. Use zero for indefinite (until timeout).

flags: if GNUTLS_HEARTBEAT_WAIT then wait for pong or timeout instead of returning immediately.

This function sends a ping to the peer. If the flags is set to GNUTLS_HEARTBEAT_WAIT then it waits for a reply from the peer.

Note that it is highly recommended to use this function with the flag GNUTLS_HEARTBEAT_WAIT , or you need to handle retransmissions and timeouts manually.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code.

Since: 3.1.2

gnutls_heartbeat_pong

Function: int gnutls_heartbeat_pong (gnutls_session_t session, unsigned int flags)

session: is a gnutls_session_t structure.

flags: should be zero

This function replies to a ping by sending a pong to the peer.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code.

Since: 3.1.2

gnutls_heartbeat_set_timeouts

Function: void gnutls_heartbeat_set_timeouts (gnutls_session_t session, unsigned int retrans_timeout, unsigned int total_timeout)

session: is a gnutls_session_t structure.

retrans_timeout: The time at which a retransmission will occur in milliseconds

total_timeout: The time at which the connection will be aborted, in milliseconds.

This function will override the timeouts for the DTLS heartbeat protocol. The retransmission timeout is the time after which a message from the peer is not received, the previous request will be retransmitted. The total timeout is the time after which the handshake will be aborted with GNUTLS_E_TIMEDOUT .

If the retransmission timeout is zero then the handshake will operate in a non-blocking way, i.e., return GNUTLS_E_AGAIN .

Since: 3.1.2

gnutls_hex2bin

Function: int gnutls_hex2bin (const char * hex_data, size_t hex_size, void * bin_data, size_t * bin_size)

hex_data: string with data in hex format

hex_size: size of hex data

bin_data: output array with binary data

bin_size: when calling should hold maximum size of bin_data , on return will hold actual length of bin_data .

Convert a buffer with hex data to binary data.

Returns: GNUTLS_E_SUCCESS on success, otherwise a negative error code.

Since: 2.4.0

gnutls_hex_decode

Function: int gnutls_hex_decode (const gnutls_datum_t * hex_data, void * result, size_t * result_size)

hex_data: contain the encoded data

result: the place where decoded data will be copied

result_size: holds the size of the result

This function will decode the given encoded data, using the hex encoding used by PSK password files.

Note that hex_data should be null terminated.

Returns: GNUTLS_E_SHORT_MEMORY_BUFFER if the buffer given is not long enough, or 0 on success.

gnutls_hex_encode

Function: int gnutls_hex_encode (const gnutls_datum_t * data, char * result, size_t * result_size)

data: contain the raw data

result: the place where hex data will be copied

result_size: holds the size of the result

This function will convert the given data to printable data, using the hex encoding, as used in the PSK password files.

Note that the size of the result includes the null terminator.

Returns: GNUTLS_E_SHORT_MEMORY_BUFFER if the buffer given is not long enough, or 0 on success.

gnutls_init

Function: int gnutls_init (gnutls_session_t * session, unsigned int flags)

session: is a pointer to a gnutls_session_t structure.

flags: indicate if this session is to be used for server or client.

This function initializes the current session to null. Every session must be initialized before use, so internal structures can be allocated. This function allocates structures which can only be free’d by calling gnutls_deinit() . Returns GNUTLS_E_SUCCESS (0) on success.

flags can be one of GNUTLS_CLIENT and GNUTLS_SERVER . For a DTLS entity, the flags GNUTLS_DATAGRAM and GNUTLS_NONBLOCK are also available. The latter flag will enable a non-blocking operation of the DTLS timers.

The flag GNUTLS_NO_REPLAY_PROTECTION will disable any replay protection in DTLS mode. That must only used when replay protection is achieved using other means.

Note that since version 3.1.2 this function enables some common TLS extensions such as session tickets and OCSP certificate status request in client side by default. To prevent that use the GNUTLS_NO_EXTENSIONS flag.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_key_generate

Function: int gnutls_key_generate (gnutls_datum_t * key, unsigned int key_size)

key: is a pointer to a gnutls_datum_t which will contain a newly created key.

key_size: The number of bytes of the key.

Generates a random key of key_size bytes.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, or an error code.

Since: 3.0

gnutls_kx_get

Function: gnutls_kx_algorithm_t gnutls_kx_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get currently used key exchange algorithm.

Returns: the key exchange algorithm used in the last handshake, a gnutls_kx_algorithm_t value.

gnutls_kx_get_id

Function: gnutls_kx_algorithm_t gnutls_kx_get_id (const char * name)

name: is a KX name

Convert a string to a gnutls_kx_algorithm_t value. The names are compared in a case insensitive way.

Returns: an id of the specified KX algorithm, or GNUTLS_KX_UNKNOWN on error.

gnutls_kx_get_name

Function: const char * gnutls_kx_get_name (gnutls_kx_algorithm_t algorithm)

algorithm: is a key exchange algorithm

Convert a gnutls_kx_algorithm_t value to a string.

Returns: a pointer to a string that contains the name of the specified key exchange algorithm, or NULL .

gnutls_kx_list

Function: const gnutls_kx_algorithm_t * gnutls_kx_list ( void)

Get a list of supported key exchange algorithms.

This function is not thread safe.

Returns: a (0)-terminated list of gnutls_kx_algorithm_t integers indicating the available key exchange algorithms.

gnutls_load_file

Function: int gnutls_load_file (const char * filename, gnutls_datum_t * data)

filename: the name of the file to load

data: Where the file will be stored

This function will load a file into a datum. The data are zero terminated but the terminating null is not included in length. The returned data are allocated using gnutls_malloc() .

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

Since 3.1.0

gnutls_mac_get

Function: gnutls_mac_algorithm_t gnutls_mac_get (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get currently used MAC algorithm.

Returns: the currently used mac algorithm, a gnutls_mac_algorithm_t value.

gnutls_mac_get_id

Function: gnutls_mac_algorithm_t gnutls_mac_get_id (const char * name)

name: is a MAC algorithm name

Convert a string to a gnutls_mac_algorithm_t value. The names are compared in a case insensitive way.

Returns: a gnutls_mac_algorithm_t id of the specified MAC algorithm string, or GNUTLS_MAC_UNKNOWN on failures.

gnutls_mac_get_key_size

Function: size_t gnutls_mac_get_key_size (gnutls_mac_algorithm_t algorithm)

algorithm: is an encryption algorithm

Returns the size of the MAC key used in TLS.

Returns: length (in bytes) of the given MAC key size, or 0 if the given MAC algorithm is invalid.

gnutls_mac_get_name

Function: const char * gnutls_mac_get_name (gnutls_mac_algorithm_t algorithm)

algorithm: is a MAC algorithm

Convert a gnutls_mac_algorithm_t value to a string.

Returns: a string that contains the name of the specified MAC algorithm, or NULL .

gnutls_mac_list

Function: const gnutls_mac_algorithm_t * gnutls_mac_list ( void)

Get a list of hash algorithms for use as MACs. Note that not necessarily all MACs are supported in TLS cipher suites. This function is not thread safe.

Returns: Return a (0)-terminated list of gnutls_mac_algorithm_t integers indicating the available MACs.

gnutls_ocsp_status_request_enable_client

Function: int gnutls_ocsp_status_request_enable_client (gnutls_session_t session, gnutls_datum_t * responder_id, size_t responder_id_size, gnutls_datum_t * extensions)

session: is a gnutls_session_t structure.

responder_id: array with gnutls_datum_t with DER data of responder id

responder_id_size: number of members in responder_id array

extensions: a gnutls_datum_t with DER encoded OCSP extensions

This function is to be used by clients to request OCSP response from the server, using the "status_request" TLS extension. Only OCSP status type is supported. A typical server has a single OCSP response cached, so responder_id and extensions should be null.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since: 3.1.3

gnutls_ocsp_status_request_get

Function: int gnutls_ocsp_status_request_get (gnutls_session_t session, gnutls_datum_t * response)

session: is a gnutls_session_t structure.

response: a gnutls_datum_t with DER encoded OCSP response

This function returns the OCSP status response received from the TLS server. The response should be treated as constant. If no OCSP response is available then GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE is returned.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise a negative error code is returned.

Since: 3.1.3

gnutls_ocsp_status_request_is_checked

Function: int gnutls_ocsp_status_request_is_checked (gnutls_session_t session, unsigned int flags)

session: is a gnutls session

flags: should be zero

Check whether an OCSP status response was included in the handshake and whether it was checked and valid (not too old or superseded). This is a helper function when needing to decide whether to perform an OCSP validity check on the peer’s certificate. Must be called after gnutls_certificate_verify_peers3() is called.

Returns: non zero it was valid, or a zero if it wasn’t sent, or sent and was invalid.

gnutls_openpgp_send_cert

Function: void gnutls_openpgp_send_cert (gnutls_session_t session, gnutls_openpgp_crt_status_t status)

session: is a pointer to a gnutls_session_t structure.

status: is one of GNUTLS_OPENPGP_CERT, or GNUTLS_OPENPGP_CERT_FINGERPRINT

This function will order gnutls to send the key fingerprint instead of the key in the initial handshake procedure. This should be used with care and only when there is indication or knowledge that the server can obtain the client’s key.

gnutls_pem_base64_decode

Function: int gnutls_pem_base64_decode (const char * header, const gnutls_datum_t * b64_data, unsigned char * result, size_t * result_size)

header: A null terminated string with the PEM header (eg. CERTIFICATE)

b64_data: contain the encoded data

result: the place where decoded data will be copied

result_size: holds the size of the result

This function will decode the given encoded data. If the header given is non null this function will search for "—–BEGIN header" and decode only this part. Otherwise it will decode the first PEM packet found.

Returns: On success GNUTLS_E_SUCCESS (0) is returned, GNUTLS_E_SHORT_MEMORY_BUFFER is returned if the buffer given is not long enough, or 0 on success.

gnutls_pem_base64_decode_alloc

Function: int gnutls_pem_base64_decode_alloc (const char * header, const gnutls_datum_t * b64_data, gnutls_datum_t * result)

header: The PEM header (eg. CERTIFICATE)

b64_data: contains the encoded data

result: the place where decoded data lie

This function will decode the given encoded data. The decoded data will be allocated, and stored into result. If the header given is non null this function will search for "—–BEGIN header" and decode only this part. Otherwise it will decode the first PEM packet found.

You should use gnutls_free() to free the returned data.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_pem_base64_encode

Function: int gnutls_pem_base64_encode (const char * msg, const gnutls_datum_t * data, char * result, size_t * result_size)

msg: is a message to be put in the header

data: contain the raw data

result: the place where base64 data will be copied

result_size: holds the size of the result

This function will convert the given data to printable data, using the base64 encoding. This is the encoding used in PEM messages.

The output string will be null terminated, although the size will not include the terminating null.

Returns: On success GNUTLS_E_SUCCESS (0) is returned, GNUTLS_E_SHORT_MEMORY_BUFFER is returned if the buffer given is not long enough, or 0 on success.

gnutls_pem_base64_encode_alloc

Function: int gnutls_pem_base64_encode_alloc (const char * msg, const gnutls_datum_t * data, gnutls_datum_t * result)

msg: is a message to be put in the encoded header

data: contains the raw data

result: will hold the newly allocated encoded data

This function will convert the given data to printable data, using the base64 encoding. This is the encoding used in PEM messages. This function will allocate the required memory to hold the encoded data.

You should use gnutls_free() to free the returned data.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_perror

Function: void gnutls_perror (int error)

error: is a GnuTLS error code, a negative error code

This function is like perror() . The only difference is that it accepts an error number returned by a gnutls function.

gnutls_pk_algorithm_get_name

Function: const char * gnutls_pk_algorithm_get_name (gnutls_pk_algorithm_t algorithm)

algorithm: is a pk algorithm

Convert a gnutls_pk_algorithm_t value to a string.

Returns: a string that contains the name of the specified public key algorithm, or NULL .

gnutls_pk_bits_to_sec_param

Function: gnutls_sec_param_t gnutls_pk_bits_to_sec_param (gnutls_pk_algorithm_t algo, unsigned int bits)

algo: is a public key algorithm

bits: is the number of bits

This is the inverse of gnutls_sec_param_to_pk_bits() . Given an algorithm and the number of bits, it will return the security parameter. This is a rough indication.

Returns: The security parameter.

Since: 2.12.0

gnutls_pk_get_id

Function: gnutls_pk_algorithm_t gnutls_pk_get_id (const char * name)

name: is a string containing a public key algorithm name.

Convert a string to a gnutls_pk_algorithm_t value. The names are compared in a case insensitive way. For example, gnutls_pk_get_id("RSA") will return GNUTLS_PK_RSA .

Returns: a gnutls_pk_algorithm_t id of the specified public key algorithm string, or GNUTLS_PK_UNKNOWN on failures.

Since: 2.6.0

gnutls_pk_get_name

Function: const char * gnutls_pk_get_name (gnutls_pk_algorithm_t algorithm)

algorithm: is a public key algorithm

Convert a gnutls_pk_algorithm_t value to a string.

Returns: a pointer to a string that contains the name of the specified public key algorithm, or NULL .

Since: 2.6.0

gnutls_pk_list

Function: const gnutls_pk_algorithm_t * gnutls_pk_list ( void)

Get a list of supported public key algorithms.

This function is not thread safe.

Returns: a (0)-terminated list of gnutls_pk_algorithm_t integers indicating the available ciphers.

Since: 2.6.0

gnutls_pk_to_sign

Function: gnutls_sign_algorithm_t gnutls_pk_to_sign (gnutls_pk_algorithm_t pk, gnutls_digest_algorithm_t hash)

pk: is a public key algorithm

hash: a hash algorithm

This function maps public key and hash algorithms combinations to signature algorithms.

Returns: return a gnutls_sign_algorithm_t value, or GNUTLS_SIGN_UNKNOWN on error.

gnutls_prf

Function: int gnutls_prf (gnutls_session_t session, size_t label_size, const char * label, int server_random_first, size_t extra_size, const char * extra, size_t outsize, char * out)

session: is a gnutls_session_t structure.

label_size: length of the label variable.

label: label used in PRF computation, typically a short string.

server_random_first: non-0 if server random field should be first in seed

extra_size: length of the extra variable.

extra: optional extra data to seed the PRF with.

outsize: size of pre-allocated output buffer to hold the output.

out: pre-allocated buffer to hold the generated data.

Applies the TLS Pseudo-Random-Function (PRF) on the master secret and the provided data, seeded with the client and server random fields, as specified in RFC5705.

The label variable usually contains a string denoting the purpose for the generated data. The server_random_first indicates whether the client random field or the server random field should be first in the seed. Non-0 indicates that the server random field is first, 0 that the client random field is first.

The extra variable can be used to add more data to the seed, after the random variables. It can be used to make sure the generated output is strongly connected to some additional data (e.g., a string used in user authentication).

The output is placed in out , which must be pre-allocated.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_prf_raw

Function: int gnutls_prf_raw (gnutls_session_t session, size_t label_size, const char * label, size_t seed_size, const char * seed, size_t outsize, char * out)

session: is a gnutls_session_t structure.

label_size: length of the label variable.

label: label used in PRF computation, typically a short string.

seed_size: length of the seed variable.

seed: optional extra data to seed the PRF with.

outsize: size of pre-allocated output buffer to hold the output.

out: pre-allocated buffer to hold the generated data.

Apply the TLS Pseudo-Random-Function (PRF) on the master secret and the provided data.

The label variable usually contains a string denoting the purpose for the generated data. The seed usually contains data such as the client and server random, perhaps together with some additional data that is added to guarantee uniqueness of the output for a particular purpose.

Because the output is not guaranteed to be unique for a particular session unless seed includes the client random and server random fields (the PRF would output the same data on another connection resumed from the first one), it is not recommended to use this function directly. The gnutls_prf() function seeds the PRF with the client and server random fields directly, and is recommended if you want to generate pseudo random data unique for each session.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_priority_certificate_type_list

Function: int gnutls_priority_certificate_type_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available certificate types in the priority structure.

Returns: the number of certificate types, or an error code.

Since: 3.0

gnutls_priority_cipher_list

Function: int gnutls_priority_cipher_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available ciphers in the priority structure.

Returns: the number of curves, or an error code.

Since: 3.2.3

gnutls_priority_compression_list

Function: int gnutls_priority_compression_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available compression method in the priority structure.

Returns: the number of methods, or an error code.

Since: 3.0

gnutls_priority_deinit

Function: void gnutls_priority_deinit (gnutls_priority_t priority_cache)

priority_cache: is a gnutls_prioritity_t structure.

Deinitializes the priority cache.

gnutls_priority_ecc_curve_list

Function: int gnutls_priority_ecc_curve_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available elliptic curves in the priority structure.

Returns: the number of curves, or an error code.

Since: 3.0

gnutls_priority_get_cipher_suite_index

Function: int gnutls_priority_get_cipher_suite_index (gnutls_priority_t pcache, unsigned int idx, unsigned int * sidx)

pcache: is a gnutls_prioritity_t structure.

idx: is an index number.

sidx: internal index of cipher suite to get information about.

Provides the internal ciphersuite index to be used with gnutls_cipher_suite_info() . The index idx provided is an index kept at the priorities structure. It might be that a valid priorities index does not correspond to a ciphersuite and in that case GNUTLS_E_UNKNOWN_CIPHER_SUITE will be returned. Once the last available index is crossed then GNUTLS_E_REQUESTED_DATA_NOT_AVAILABLE will be returned.

Returns: On success it returns GNUTLS_E_SUCCESS (0), or a negative error value otherwise.

gnutls_priority_init

Function: int gnutls_priority_init (gnutls_priority_t * priority_cache, const char * priorities, const char ** err_pos)

priority_cache: is a gnutls_prioritity_t structure.

priorities: is a string describing priorities

err_pos: In case of an error this will have the position in the string the error occured

Sets priorities for the ciphers, key exchange methods, macs and compression methods.

The priorities option allows you to specify a colon separated list of the cipher priorities to enable. Some keywords are defined to provide quick access to common preferences.

Unless there is a special need, using "NORMAL" or "NORMAL:COMPAT " for compatibility is recommended.

"PERFORMANCE" means all the "secure" ciphersuites are enabled, limited to 128 bit ciphers and sorted by terms of speed performance.

"NORMAL" means all "secure" ciphersuites. The 256-bit ciphers are included as a fallback only. The ciphers are sorted by security margin.

"PFS" means all "secure" ciphersuites that support perfect forward secrecy. The 256-bit ciphers are included as a fallback only. The ciphers are sorted by security margin.

"SECURE128" means all "secure" ciphersuites of security level 128-bit or more.

"SECURE192" means all "secure" ciphersuites of security level 192-bit or more.

"SUITEB128" means all the NSA SuiteB ciphersuites with security level of 128.

"SUITEB192" means all the NSA SuiteB ciphersuites with security level of 192.

"EXPORT" means all ciphersuites are enabled, including the low-security 40 bit ciphers.

"NONE" means nothing is enabled. This disables even protocols and compression methods.

Special keywords are "!", "-" and "+". "!" or "-" appended with an algorithm will remove this algorithm. "+" appended with an algorithm will add this algorithm.

Check the GnuTLS manual section "Priority strings" for detailed information.

Examples: "NONE:+VERS-TLS-ALL:+MAC-ALL:+RSA:+AES-128-CBC:+SIGN-ALL:+COMP-NULL"

"NORMAL:-ARCFOUR-128" means normal ciphers except for ARCFOUR-128.

"SECURE128:-VERS-SSL3.0:+COMP-DEFLATE" means that only secure ciphers are enabled, SSL3.0 is disabled, and libz compression enabled.

"NONE:+VERS-TLS-ALL:+AES-128-CBC:+RSA:+SHA1:+COMP-NULL:+SIGN-RSA-SHA1",

"NONE:+VERS-TLS-ALL:+AES-128-CBC:+ECDHE-RSA:+SHA1:+COMP-NULL:+SIGN-RSA-SHA1:+CURVE-SECP256R1",

"SECURE256:+SECURE128",

Note that "NORMAL:COMPAT " is the most compatible mode.

Returns: On syntax error GNUTLS_E_INVALID_REQUEST is returned, GNUTLS_E_SUCCESS on success, or an error code.

gnutls_priority_kx_list

Function: int gnutls_priority_kx_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available key exchange methods in the priority structure.

Returns: the number of curves, or an error code.

Since: 3.2.3

gnutls_priority_mac_list

Function: int gnutls_priority_mac_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available MAC algorithms in the priority structure.

Returns: the number of curves, or an error code.

Since: 3.2.3

gnutls_priority_protocol_list

Function: int gnutls_priority_protocol_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available TLS version numbers in the priority structure.

Returns: the number of protocols, or an error code.

Since: 3.0

gnutls_priority_set

Function: int gnutls_priority_set (gnutls_session_t session, gnutls_priority_t priority)

session: is a gnutls_session_t structure.

priority: is a gnutls_priority_t structure.

Sets the priorities to use on the ciphers, key exchange methods, macs and compression methods.

Returns: GNUTLS_E_SUCCESS on success, or an error code.

gnutls_priority_set_direct

Function: int gnutls_priority_set_direct (gnutls_session_t session, const char * priorities, const char ** err_pos)

session: is a gnutls_session_t structure.

priorities: is a string describing priorities

err_pos: In case of an error this will have the position in the string the error occured

Sets the priorities to use on the ciphers, key exchange methods, macs and compression methods. This function avoids keeping a priority cache and is used to directly set string priorities to a TLS session. For documentation check the gnutls_priority_init() .

Returns: On syntax error GNUTLS_E_INVALID_REQUEST is returned, GNUTLS_E_SUCCESS on success, or an error code.

gnutls_priority_sign_list

Function: int gnutls_priority_sign_list (gnutls_priority_t pcache, const unsigned int ** list)

pcache: is a gnutls_prioritity_t structure.

list: will point to an integer list

Get a list of available signature algorithms in the priority structure.

Returns: the number of algorithms, or an error code.

Since: 3.0

gnutls_protocol_get_id

Function: gnutls_protocol_t gnutls_protocol_get_id (const char * name)

name: is a protocol name

The names are compared in a case insensitive way.

Returns: an id of the specified protocol, or GNUTLS_VERSION_UNKNOWN on error.

gnutls_protocol_get_name

Function: const char * gnutls_protocol_get_name (gnutls_protocol_t version)

version: is a (gnutls) version number

Convert a gnutls_protocol_t value to a string.

Returns: a string that contains the name of the specified TLS version (e.g., "TLS1.0"), or NULL .

gnutls_protocol_get_version

Function: gnutls_protocol_t gnutls_protocol_get_version (gnutls_session_t session)

session: is a gnutls_session_t structure.

Get TLS version, a gnutls_protocol_t value.

Returns: The version of the currently used protocol.

gnutls_protocol_list

Function: const gnutls_protocol_t * gnutls_protocol_list ( void)

Get a list of supported protocols, e.g. SSL 3.0, TLS 1.0 etc.

This function is not thread safe.

Returns: a (0)-terminated list of gnutls_protocol_t integers indicating the available protocols.

gnutls_psk_allocate_client_credentials

Function: int gnutls_psk_allocate_client_credentials (gnutls_psk_client_credentials_t * sc)

sc: is a pointer to a gnutls_psk_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_psk_allocate_server_credentials

Function: int gnutls_psk_allocate_server_credentials (gnutls_psk_server_credentials_t * sc)

sc: is a pointer to a gnutls_psk_server_credentials_t structure.

This structure is complex enough to manipulate directly thus this helper function is provided in order to allocate it.

Returns: On success, GNUTLS_E_SUCCESS (0) is returned, otherwise an error code is returned.

gnutls_psk_client_get_hint

Function: const char * gnutls_psk_client_get_hint (gnutls_session_t session)

session: is a gnutls session

The PSK identity hint may give the client help in deciding which username to use. This should only be called in case of PSK authentication and in case of a client.

Returns: the identity hint of the peer, or NULL in case of an error.

Since: 2.4.0

gnutls_psk_free_client_credentials