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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 <assert.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 CHECK(x) assert((x)>=0)

#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;
        char buffer[MAX_BUF + 1];
        gnutls_certificate_credentials_t xcred;
        /* Allow connections to servers that have OpenPGP keys as well.
         */

        if (gnutls_check_version("3.1.4") == NULL) {
                fprintf(stderr, "GnuTLS 3.1.4 or later is required for this example\n");
                exit(1);
        }

        /* for backwards compatibility with gnutls < 3.3.0 */
        CHECK(gnutls_global_init());

        /* The PKCS11 private key operations may require PIN.
         * Register a callback. */
        gnutls_pkcs11_set_pin_function(pin_callback, NULL);

        /* X509 stuff */
        CHECK(gnutls_certificate_allocate_credentials(&xcred));

        /* sets the trusted cas file
         */
        CHECK(gnutls_certificate_set_x509_trust_file(xcred, CAFILE,
                                                     GNUTLS_X509_FMT_PEM));

        CHECK(gnutls_certificate_set_x509_key_file(xcred, CERT_URL, KEY_URL,
                                                   GNUTLS_X509_FMT_DER));

        /* Note that there is no server certificate verification in this example
         */


        /* Initialize TLS session
         */
        CHECK(gnutls_init(&session, GNUTLS_CLIENT));

        /* Use default priorities */
        CHECK(gnutls_set_default_priority(session));

        /* put the x509 credentials to the current session
         */
        CHECK(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);
        }

        CHECK(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);

        CHECK(gnutls_bye(session, GNUTLS_SHUT_RDWR));

      end:

        tcp_close(sd);

        gnutls_deinit(session);

        gnutls_certificate_free_credentials(xcred);

        gnutls_global_deinit();

        return 0;
}

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