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7.3.4 Verifying Peer's Certificate
A TLS session is not secure just after the handshake procedure has finished. It must be considered secure, only after the peer's certificate and identity have been verified. That is, you have to verify the signature in peer's certificate, the hostname in the certificate, and expiration dates. Just after this step you should treat the connection as being a secure one.
/* This example code is placed in the public domain. */
#ifdef HAVE_CONFIG_H
# include <config.h>
#endif
#include <stdio.h>
#include <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include "examples.h"
/* This function will try to verify the peer's certificate, and
* also check if the hostname matches, and the activation, expiration dates.
*/
void
verify_certificate (gnutls_session_t session, const char *hostname)
{
unsigned int status;
const gnutls_datum_t *cert_list;
unsigned int cert_list_size;
int ret;
gnutls_x509_crt_t cert;
/* 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_peers2 (session, &status);
if (ret < 0)
{
printf ("Error\n");
return;
}
if (status & GNUTLS_CERT_INVALID)
printf ("The certificate is not trusted.\n");
if (status & GNUTLS_CERT_SIGNER_NOT_FOUND)
printf ("The certificate hasn't got a known issuer.\n");
if (status & GNUTLS_CERT_REVOKED)
printf ("The certificate has been revoked.\n");
/* Up to here the process is the same for X.509 certificates and
* OpenPGP keys. From now on X.509 certificates are assumed. This can
* be easily extended to work with openpgp keys as well.
*/
if (gnutls_certificate_type_get (session) != GNUTLS_CRT_X509)
return;
if (gnutls_x509_crt_init (&cert) < 0)
{
printf ("error in initialization\n");
return;
}
cert_list = gnutls_certificate_get_peers (session, &cert_list_size);
if (cert_list == NULL)
{
printf ("No certificate was found!\n");
return;
}
/* This is not a real world example, since we only check the first
* certificate in the given chain.
*/
if (gnutls_x509_crt_import (cert, &cert_list[0], GNUTLS_X509_FMT_DER) < 0)
{
printf ("error parsing certificate\n");
return;
}
/* Beware here we do not check for errors.
*/
if (gnutls_x509_crt_get_expiration_time (cert) < time (0))
{
printf ("The certificate has expired\n");
return;
}
if (gnutls_x509_crt_get_activation_time (cert) > time (0))
{
printf ("The certificate is not yet activated\n");
return;
}
if (!gnutls_x509_crt_check_hostname (cert, hostname))
{
printf ("The certificate's owner does not match hostname '%s'\n",
hostname);
return;
}
gnutls_x509_crt_deinit (cert);
return;
}
An other example is listed below which provides a more detailed verification output.
/* 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"
/* 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 void verify_cert2 (gnutls_x509_crt_t crt,
gnutls_x509_crt_t issuer,
gnutls_x509_crl_t * crl_list, int crl_list_size);
static void verify_last_cert (gnutls_x509_crt_t crt,
gnutls_x509_crt_t * ca_list, int ca_list_size,
gnutls_x509_crl_t * crl_list,
int crl_list_size);
/* This function will try to verify the peer's certificate chain, and
* also check if the hostname matches, and the activation, expiration dates.
*/
void
verify_certificate_chain (gnutls_session_t session,
const char *hostname,
const gnutls_datum_t * cert_chain,
int cert_chain_length)
{
int i;
gnutls_x509_crt_t *cert;
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);
}
/* If the last certificate in the chain is self signed ignore it.
* That is because we want to check against our trusted certificate
* list.
*/
if (gnutls_x509_crt_check_issuer (cert[cert_chain_length - 1],
cert[cert_chain_length - 1]) > 0
&& cert_chain_length > 0)
{
cert_chain_length--;
}
/* Now verify the certificates against their issuers
* in the chain.
*/
for (i = 1; i < cert_chain_length; i++)
{
verify_cert2 (cert[i - 1], cert[i], crl_list, crl_list_size);
}
/* Here we must verify the last certificate in the chain against
* our trusted CA list.
*/
verify_last_cert (cert[cert_chain_length - 1],
ca_list, ca_list_size, crl_list, crl_list_size);
/* 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);
}
for (i = 0; i < cert_chain_length; i++)
gnutls_x509_crt_deinit (cert[i]);
return;
}
/* Verifies a certificate against an other certificate
* which is supposed to be it's issuer. Also checks the
* crl_list if the certificate is revoked.
*/
static void
verify_cert2 (gnutls_x509_crt_t crt, gnutls_x509_crt_t issuer,
gnutls_x509_crl_t * crl_list, int crl_list_size)
{
unsigned int output;
int ret;
time_t now = time (0);
size_t name_size;
char name[64];
/* Print information about the certificates to
* be checked.
*/
name_size = sizeof (name);
gnutls_x509_crt_get_dn (crt, name, &name_size);
fprintf (stderr, "\nCertificate: %s\n", name);
name_size = sizeof (name);
gnutls_x509_crt_get_issuer_dn (crt, name, &name_size);
fprintf (stderr, "Issued by: %s\n", name);
/* Get the DN of the issuer cert.
*/
name_size = sizeof (name);
gnutls_x509_crt_get_dn (issuer, name, &name_size);
fprintf (stderr, "Checking against: %s\n", name);
/* Do the actual verification.
*/
gnutls_x509_crt_verify (crt, &issuer, 1, 0, &output);
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");
fprintf (stderr, "\n");
}
else
fprintf (stderr, "Trusted\n");
/* Now check the expiration dates.
*/
if (gnutls_x509_crt_get_activation_time (crt) > now)
fprintf (stderr, "Not yet activated\n");
if (gnutls_x509_crt_get_expiration_time (crt) < now)
fprintf (stderr, "Expired\n");
/* Check if the certificate is revoked.
*/
ret = gnutls_x509_crt_check_revocation (crt, crl_list, crl_list_size);
if (ret == 1)
{ /* revoked */
fprintf (stderr, "Revoked\n");
}
}
/* Verifies a certificate against our trusted CA list.
* Also checks the crl_list if the certificate is revoked.
*/
static void
verify_last_cert (gnutls_x509_crt_t crt,
gnutls_x509_crt_t * ca_list, int ca_list_size,
gnutls_x509_crl_t * crl_list, int crl_list_size)
{
unsigned int output;
int ret;
time_t now = time (0);
size_t name_size;
char name[64];
/* Print information about the certificates to
* be checked.
*/
name_size = sizeof (name);
gnutls_x509_crt_get_dn (crt, name, &name_size);
fprintf (stderr, "\nCertificate: %s\n", name);
name_size = sizeof (name);
gnutls_x509_crt_get_issuer_dn (crt, name, &name_size);
fprintf (stderr, "Issued by: %s\n", name);
/* Do the actual verification.
*/
gnutls_x509_crt_verify (crt, ca_list, ca_list_size,
GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT, &output);
if (output & GNUTLS_CERT_INVALID)
{
fprintf (stderr, "Not trusted");
if (output & GNUTLS_CERT_SIGNER_NOT_CA)
fprintf (stderr, ": Issuer is not a CA\n");
else
fprintf (stderr, "\n");
}
else
fprintf (stderr, "Trusted\n");
/* Now check the expiration dates.
*/
if (gnutls_x509_crt_get_activation_time (crt) > now)
fprintf (stderr, "Not yet activated\n");
if (gnutls_x509_crt_get_expiration_time (crt) < now)
fprintf (stderr, "Expired\n");
/* Check if the certificate is revoked.
*/
ret = gnutls_x509_crt_check_revocation (crt, crl_list, crl_list_size);
if (ret == 1)
{ /* revoked */
fprintf (stderr, "Revoked\n");
}
}
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