7.1.5 Client example with SSH-style certificate verification

This is an alternative verification function that will use the X.509 certificate authorities for verification, but also assume an trust on first use (SSH-like) authentication system. That is the user is prompted on unknown public keys and known public keys are considered trusted.

/* 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 <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <assert.h>
#include "examples.h"

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

/* This function will verify the peer's certificate, check
 * if the hostname matches. In addition it will perform an
 * SSH-style authentication, where ultimately trusted keys
 * are only the keys that have been seen before.
 */
int _ssh_verify_certificate_callback(gnutls_session_t session)
{
	unsigned int status;
	const gnutls_datum_t *cert_list;
	unsigned int cert_list_size;
	int ret, type;
	gnutls_datum_t out;
	const char *hostname;

	/* read hostname */
	hostname = gnutls_session_get_ptr(session);

	/* This verification function uses the trusted CAs in the credentials
	 * structure. So you must have installed one or more CA certificates.
	 */
	CHECK(gnutls_certificate_verify_peers3(session, hostname, &status));

	type = gnutls_certificate_type_get(session);

	CHECK(gnutls_certificate_verification_status_print(status, type, &out,
							   0));
	printf("%s", out.data);

	gnutls_free(out.data);

	if (status != 0) /* Certificate is not trusted */
		return GNUTLS_E_CERTIFICATE_ERROR;

	/* Do SSH verification */
	cert_list = gnutls_certificate_get_peers(session, &cert_list_size);
	if (cert_list == NULL) {
		printf("No certificate was found!\n");
		return GNUTLS_E_CERTIFICATE_ERROR;
	}

	/* service may be obtained alternatively using getservbyport() */
	ret = gnutls_verify_stored_pubkey(NULL, NULL, hostname, "https", type,
					  &cert_list[0], 0);
	if (ret == GNUTLS_E_NO_CERTIFICATE_FOUND) {
		printf("Host %s is not known.", hostname);
		if (status == 0)
			printf("Its certificate is valid for %s.\n", hostname);

		/* the certificate must be printed and user must be asked on
		 * whether it is trustworthy. --see gnutls_x509_crt_print() */

		/* if not trusted */
		return GNUTLS_E_CERTIFICATE_ERROR;
	} else if (ret == GNUTLS_E_CERTIFICATE_KEY_MISMATCH) {
		printf("Warning: host %s is known but has another key associated.",
		       hostname);
		printf("It might be that the server has multiple keys, or you are under attack\n");
		if (status == 0)
			printf("Its certificate is valid for %s.\n", hostname);

		/* the certificate must be printed and user must be asked on
		 * whether it is trustworthy. --see gnutls_x509_crt_print() */

		/* if not trusted */
		return GNUTLS_E_CERTIFICATE_ERROR;
	} else if (ret < 0) {
		printf("gnutls_verify_stored_pubkey: %s\n",
		       gnutls_strerror(ret));
		return ret;
	}

	/* user trusts the key -> store it */
	if (ret != 0) {
		CHECK(gnutls_store_pubkey(NULL, NULL, hostname, "https", type,
					  &cert_list[0], 0, 0));
	}

	/* notify gnutls to continue handshake normally */
	return 0;
}