7.1.1 Client example with X.509 certificate support

Let’s assume now that we want to create a TCP client which communicates with servers that use X.509 certificate authentication. The following client is a very simple TLS client, which uses the high level verification functions for certificates, but does not support session resumption.

Note that this client utilizes functionality present in the latest GnuTLS version. For a reasonably portable version see Legacy client example with X.509 certificate support.

/* 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"

/* A very basic TLS client, with X.509 authentication and server certificate
 * verification. Note that error recovery is minimal for simplicity.
 */

#define CHECK(x) assert((x) >= 0)
#define LOOP_CHECK(rval, cmd)                                             \
	do {                                                              \
		rval = cmd;                                               \
	} while (rval == GNUTLS_E_AGAIN || rval == GNUTLS_E_INTERRUPTED); \
	assert(rval >= 0)

#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"

extern int tcp_connect(void);
extern void tcp_close(int sd);

int main(void)
{
	int ret, sd, ii;
	gnutls_session_t session;
	char buffer[MAX_BUF + 1], *desc;
	gnutls_datum_t out;
	int type;
	unsigned status;
	gnutls_certificate_credentials_t xcred;

	if (gnutls_check_version("3.4.6") == NULL) {
		fprintf(stderr,
			"GnuTLS 3.4.6 or later is required for this example\n");
		exit(1);
	}

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

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

	/* sets the system trusted CAs for Internet PKI */
	CHECK(gnutls_certificate_set_x509_system_trust(xcred));

	/* If client holds a certificate it can be set using the following:
	 *
	 gnutls_certificate_set_x509_key_file (xcred, "cert.pem", "key.pem", 
	 GNUTLS_X509_FMT_PEM); 
	 */

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

	CHECK(gnutls_server_name_set(session, GNUTLS_NAME_DNS,
				     "www.example.com",
				     strlen("www.example.com")));

	/* It is recommended to use the 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));
	gnutls_session_set_verify_cert(session, "www.example.com", 0);

	/* connect to the peer
	 */
	sd = tcp_connect();

	gnutls_transport_set_int(session, sd);
	gnutls_handshake_set_timeout(session, GNUTLS_DEFAULT_HANDSHAKE_TIMEOUT);

	/* Perform the TLS handshake
	 */
	do {
		ret = gnutls_handshake(session);
	} while (ret < 0 && gnutls_error_is_fatal(ret) == 0);
	if (ret < 0) {
		if (ret == GNUTLS_E_CERTIFICATE_VERIFICATION_ERROR) {
			/* check certificate verification status */
			type = gnutls_certificate_type_get(session);
			status = gnutls_session_get_verify_cert_status(session);
			CHECK(gnutls_certificate_verification_status_print(
				status, type, &out, 0));
			printf("cert verify output: %s\n", out.data);
			gnutls_free(out.data);
		}
		fprintf(stderr, "*** Handshake failed: %s\n",
			gnutls_strerror(ret));
		goto end;
	} else {
		desc = gnutls_session_get_desc(session);
		printf("- Session info: %s\n", desc);
		gnutls_free(desc);
	}

	/* send data */
	LOOP_CHECK(ret, gnutls_record_send(session, MSG, strlen(MSG)));

	LOOP_CHECK(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 && gnutls_error_is_fatal(ret) == 0) {
		fprintf(stderr, "*** Warning: %s\n", gnutls_strerror(ret));
	} else if (ret < 0) {
		fprintf(stderr, "*** Error: %s\n", gnutls_strerror(ret));
		goto end;
	}

	if (ret > 0) {
		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;
}