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 <assert.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>

#include "examples.h"

#define CHECK(x) assert((x)>=0)

/* 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;
        gnutls_datum_t txt;
        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.
         */
        CHECK(gnutls_x509_trust_list_init(&tlist, 0));

        CHECK(gnutls_x509_trust_list_add_cas(tlist, ca_list, ca_list_size, 0));
        CHECK(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++) {
                CHECK(gnutls_x509_crt_init(&cert[i]));
                CHECK(gnutls_x509_crt_import(cert[i], &cert_chain[i],
                                             GNUTLS_X509_FMT_DER));
        }

        CHECK(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) {
                CHECK(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\n");
                CHECK(gnutls_certificate_verification_status_print(
                                                     output,
                                                     GNUTLS_CRT_X509,
                                                     &txt, 0));

                fprintf(stderr, "Error: %s\n", txt.data);
                gnutls_free(txt.data);
        } 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;
}