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<title>GnuTLS 3.5.2: Simple client example with SSH-style certificate verification</title>
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<a name="Simple-client-example-with-SSH_002dstyle-certificate-verification"></a>
<div class="header">
<p>
Next: <a href="Simple-client-example-with-anonymous-authentication.html#Simple-client-example-with-anonymous-authentication" accesskey="n" rel="next">Simple client example with anonymous authentication</a>, Previous: <a href="Simple-client-example-with-X_002e509-certificate-support.html#Simple-client-example-with-X_002e509-certificate-support" accesskey="p" rel="prev">Simple client example with X.509 certificate support</a>, Up: <a href="Client-examples.html#Client-examples" accesskey="u" rel="up">Client examples</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Simple-client-example-with-SSH_002dstyle-certificate-verification-1"></a>
<h4 class="subsection">7.1.2 Simple client example with SSH-style certificate verification</h4>
<p>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.
</p>
<pre class="verbatim">/* 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;
}
</pre>
<hr>
<div class="header">
<p>
Next: <a href="Simple-client-example-with-anonymous-authentication.html#Simple-client-example-with-anonymous-authentication" accesskey="n" rel="next">Simple client example with anonymous authentication</a>, Previous: <a href="Simple-client-example-with-X_002e509-certificate-support.html#Simple-client-example-with-X_002e509-certificate-support" accesskey="p" rel="prev">Simple client example with X.509 certificate support</a>, Up: <a href="Client-examples.html#Client-examples" accesskey="u" rel="up">Client examples</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Function-and-Data-Index.html#Function-and-Data-Index" title="Index" rel="index">Index</a>]</p>
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