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8.1.5 Using a callback to select the certificate to use
There are cases where a client holds several certificate and key pairs, and may not want to load all of them in the credentials structure. The following example demonstrates the use of the certificate selection callback.
/* 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 <gnutls/gnutls.h>
#include <gnutls/x509.h>
#include <gnutls/abstract.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
/* A TLS client that loads the certificate and key.
*/
#define MAX_BUF 1024
#define MSG "GET / HTTP/1.0\r\n\r\n"
#define CERT_FILE "cert.pem"
#define KEY_FILE "key.pem"
#define CAFILE "ca.pem"
extern int tcp_connect (void);
extern void tcp_close (int sd);
static int
cert_callback (gnutls_session_t session,
const gnutls_datum_t * req_ca_rdn, int nreqs,
const gnutls_pk_algorithm_t * sign_algos,
int sign_algos_length, gnutls_pcert_st ** pcert,
unsigned int *pcert_length, gnutls_privkey_t * pkey);
gnutls_pcert_st crt;
gnutls_privkey_t key;
/* Helper functions to load a certificate and key
* files into memory.
*/
static gnutls_datum_t
load_file (const char *file)
{
FILE *f;
gnutls_datum_t loaded_file = { NULL, 0 };
long filelen;
void *ptr;
if (!(f = fopen (file, "r"))
|| fseek (f, 0, SEEK_END) != 0
|| (filelen = ftell (f)) < 0
|| fseek (f, 0, SEEK_SET) != 0
|| !(ptr = malloc ((size_t) filelen))
|| fread (ptr, 1, (size_t) filelen, f) < (size_t) filelen)
{
return loaded_file;
}
loaded_file.data = ptr;
loaded_file.size = (unsigned int) filelen;
return loaded_file;
}
static void
unload_file (gnutls_datum_t data)
{
free (data.data);
}
/* Load the certificate and the private key.
*/
static void
load_keys (void)
{
int ret;
gnutls_datum_t data;
gnutls_x509_privkey_t x509_key;
data = load_file (CERT_FILE);
if (data.data == NULL)
{
fprintf (stderr, "*** Error loading certificate file.\n");
exit (1);
}
ret = gnutls_pcert_import_x509_raw (&crt, &data, GNUTLS_X509_FMT_PEM, 0);
if (ret < 0)
{
fprintf (stderr, "*** Error loading certificate file: %s\n",
gnutls_strerror (ret));
exit (1);
}
unload_file (data);
data = load_file (KEY_FILE);
if (data.data == NULL)
{
fprintf (stderr, "*** Error loading key file.\n");
exit (1);
}
gnutls_x509_privkey_init (&x509_key);
ret = gnutls_x509_privkey_import (x509_key, &data, GNUTLS_X509_FMT_PEM);
if (ret < 0)
{
fprintf (stderr, "*** Error loading key file: %s\n",
gnutls_strerror (ret));
exit (1);
}
gnutls_privkey_init (&key);
ret =
gnutls_privkey_import_x509 (key, x509_key,
GNUTLS_PRIVKEY_IMPORT_AUTO_RELEASE);
if (ret < 0)
{
fprintf (stderr, "*** Error importing key: %s\n",
gnutls_strerror (ret));
exit (1);
}
unload_file (data);
}
int
main (void)
{
int ret, sd, ii;
gnutls_session_t session;
gnutls_priority_t priorities_cache;
char buffer[MAX_BUF + 1];
gnutls_certificate_credentials_t xcred;
/* Allow connections to servers that have OpenPGP keys as well.
*/
gnutls_global_init ();
load_keys ();
/* X509 stuff */
gnutls_certificate_allocate_credentials (&xcred);
/* priorities */
gnutls_priority_init (&priorities_cache, "NORMAL", NULL);
/* sets the trusted cas file
*/
gnutls_certificate_set_x509_trust_file (xcred, CAFILE, GNUTLS_X509_FMT_PEM);
gnutls_certificate_set_retrieve_function2 (xcred, cert_callback);
/* Initialize TLS session
*/
gnutls_init (&session, GNUTLS_CLIENT);
/* Use default priorities */
gnutls_priority_set (session, priorities_cache);
/* put the x509 credentials to the current session
*/
gnutls_credentials_set (session, GNUTLS_CRD_CERTIFICATE, xcred);
/* connect to the peer
*/
sd = tcp_connect ();
gnutls_transport_set_ptr (session, (gnutls_transport_ptr_t) sd);
/* Perform the TLS handshake
*/
ret = gnutls_handshake (session);
if (ret < 0)
{
fprintf (stderr, "*** Handshake failed\n");
gnutls_perror (ret);
goto end;
}
else
{
printf ("- Handshake was completed\n");
}
gnutls_record_send (session, MSG, strlen (MSG));
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)
{
fprintf (stderr, "*** Error: %s\n", gnutls_strerror (ret));
goto end;
}
printf ("- Received %d bytes: ", ret);
for (ii = 0; ii < ret; ii++)
{
fputc (buffer[ii], stdout);
}
fputs ("\n", stdout);
gnutls_bye (session, GNUTLS_SHUT_RDWR);
end:
tcp_close (sd);
gnutls_deinit (session);
gnutls_certificate_free_credentials (xcred);
gnutls_priority_deinit (priorities_cache);
gnutls_global_deinit ();
return 0;
}
/* This callback should be associated with a session by calling
* gnutls_certificate_client_set_retrieve_function( session, cert_callback),
* before a handshake.
*/
static int
cert_callback (gnutls_session_t session,
const gnutls_datum_t * req_ca_rdn, int nreqs,
const gnutls_pk_algorithm_t * sign_algos,
int sign_algos_length, gnutls_pcert_st ** pcert,
unsigned int *pcert_length, gnutls_privkey_t * pkey)
{
char issuer_dn[256];
int i, ret;
size_t len;
gnutls_certificate_type_t type;
/* Print the server's trusted CAs
*/
if (nreqs > 0)
printf ("- Server's trusted authorities:\n");
else
printf ("- Server did not send us any trusted authorities names.\n");
/* print the names (if any) */
for (i = 0; i < nreqs; i++)
{
len = sizeof (issuer_dn);
ret = gnutls_x509_rdn_get (&req_ca_rdn[i], issuer_dn, &len);
if (ret >= 0)
{
printf (" [%d]: ", i);
printf ("%s\n", issuer_dn);
}
}
/* Select a certificate and return it.
* The certificate must be of any of the "sign algorithms"
* supported by the server.
*/
type = gnutls_certificate_type_get (session);
if (type == GNUTLS_CRT_X509)
{
*pcert_length = 1;
*pcert = &crt;
*pkey = key;
}
else
{
return -1;
}
return 0;
}
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