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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 which performs 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 be used during the TLS certificate verification (see gnutls_certificate_verify_peers2). 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 creates an OCSP request object, stores some information about the certificate to check in the request, and then exports 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 4.8. 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 4.8: 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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