7.5.2 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);
	}
}