7.3.5 Client example with PSK authentication

The following client is a very simple PSK TLS client which connects to a server and authenticates using a username and a key.

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

/* A very basic TLS client, with PSK authentication.
 */

#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];
	const char *err;
	gnutls_psk_client_credentials_t pskcred;
	const gnutls_datum_t key = { (void *)"DEADBEEF", 8 };

	if (gnutls_check_version("3.6.3") == NULL) {
		fprintf(stderr,
			"GnuTLS 3.6.3 or later is required for this example\n");
		exit(1);
	}

	CHECK(gnutls_global_init());

	CHECK(gnutls_psk_allocate_client_credentials(&pskcred));
	CHECK(gnutls_psk_set_client_credentials(pskcred, "test", &key,
						GNUTLS_PSK_KEY_HEX));

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

	ret = gnutls_set_default_priority_append(
		session, "-KX-ALL:+ECDHE-PSK:+DHE-PSK:+PSK", &err, 0);

	/* Alternative for pre-3.6.3 versions:
	 * gnutls_priority_set_direct(session, "NORMAL:+ECDHE-PSK:+DHE-PSK:+PSK", &err)
	 */
	if (ret < 0) {
		if (ret == GNUTLS_E_INVALID_REQUEST) {
			fprintf(stderr, "Syntax error at: %s\n", err);
		}
		exit(1);
	}

	/* put the x509 credentials to the current session
	 */
	CHECK(gnutls_credentials_set(session, GNUTLS_CRD_PSK, pskcred));

	/* 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) {
		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);
	}

	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_psk_free_client_credentials(pskcred);

	gnutls_global_deinit();

	return 0;
}