How to generate PSK keys for GnuTLS

Question

I'm using these examples from GnuTLS to run server and a client which use PSK for authentication:

/* Server */

#ifdef HAVE_CONFIG_H
#include <config.h>
#endif

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#include <string.h>
#include <unistd.h>
#include <gnutls/gnutls.h>

#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define CRLFILE "crl.pem"

/* This is a sample TLS echo server, supporting X.509 and PSK
   authentication.
 */

#define SOCKET_ERR(err,s) if(err==-1) {perror(s);return(1);}
#define MAX_BUF 1024
#define PORT 5556               /* listen to 5556 port */
#define DH_BITS 1024

/* These are global */
gnutls_certificate_credentials_t x509_cred;
gnutls_psk_server_credentials_t psk_cred;
gnutls_priority_t priority_cache;

static gnutls_session_t
initialize_tls_session (void)
{
  gnutls_session_t session;

  gnutls_init (&session, GNUTLS_SERVER);

  gnutls_priority_set (session, priority_cache);

  gnutls_credentials_set (session, GNUTLS_CRD_CERTIFICATE, x509_cred);
  gnutls_credentials_set (session, GNUTLS_CRD_PSK, psk_cred);

  /* request client certificate if any.
   */
  gnutls_certificate_server_set_request (session, GNUTLS_CERT_REQUEST);

  return session;
}

static gnutls_dh_params_t dh_params;

static int
generate_dh_params (void)
{

  /* Generate Diffie-Hellman parameters - for use with DHE
   * kx algorithms. When short bit length is used, it might
   * be wise to regenerate parameters.
   *
   * Check the ex-serv-export.c example for using static
   * parameters.
   */
  gnutls_dh_params_init (&dh_params);
  gnutls_dh_params_generate2 (dh_params, DH_BITS);

  return 0;
}

static int
pskfunc (gnutls_session_t session, const char *username, gnutls_datum_t * key)
{
  printf ("psk: username %s\n", username);
  key->data = gnutls_malloc (4);
  key->data[0] = 0xDE;
  key->data[1] = 0xAD;
  key->data[2] = 0xBE;
  key->data[3] = 0xEF;
  key->size = 4;
  return 0;
}

int
main (void)
{
  int err, listen_sd;
  int sd, ret;
  struct sockaddr_in sa_serv;
  struct sockaddr_in sa_cli;
  socklen_t client_len;
  char topbuf[512];
  gnutls_session_t session;
  char buffer[MAX_BUF + 1];
  int optval = 1;
  int kx;

  /* this must be called once in the program
   */
  gnutls_global_init ();

  gnutls_certificate_allocate_credentials (&x509_cred);
  gnutls_certificate_set_x509_trust_file (x509_cred, CAFILE,
                                          GNUTLS_X509_FMT_PEM);

  gnutls_certificate_set_x509_crl_file (x509_cred, CRLFILE,
                                        GNUTLS_X509_FMT_PEM);

  gnutls_certificate_set_x509_key_file (x509_cred, CERTFILE, KEYFILE,
                                        GNUTLS_X509_FMT_PEM);

  gnutls_psk_allocate_server_credentials (&psk_cred);
  gnutls_psk_set_server_credentials_function (psk_cred, pskfunc);

  generate_dh_params ();

  gnutls_priority_init (&priority_cache, "NORMAL:+PSK:+ECDHE-PSK:+DHE-PSK", NULL);

  gnutls_certificate_set_dh_params (x509_cred, dh_params);

  /* Socket operations
   */
  listen_sd = socket (AF_INET, SOCK_STREAM, 0);
  SOCKET_ERR (listen_sd, "socket");

  memset (&sa_serv, '\0', sizeof (sa_serv));
  sa_serv.sin_family = AF_INET;
  sa_serv.sin_addr.s_addr = INADDR_ANY;
  sa_serv.sin_port = htons (PORT);      /* Server Port number */

  setsockopt (listen_sd, SOL_SOCKET, SO_REUSEADDR, (void *) &optval,
              sizeof (int));

  err = bind (listen_sd, (struct sockaddr *) & sa_serv, sizeof (sa_serv));
  SOCKET_ERR (err, "bind");
  err = listen (listen_sd, 1024);
  SOCKET_ERR (err, "listen");

  printf ("Server ready. Listening to port '%d'.\n\n", PORT);

  client_len = sizeof (sa_cli);
  for (;;)
    {
      session = initialize_tls_session ();

      sd = accept (listen_sd, (struct sockaddr *) & sa_cli, &client_len);

      printf ("- connection from %s, port %d\n",
              inet_ntop (AF_INET, &sa_cli.sin_addr, topbuf,
                         sizeof (topbuf)), ntohs (sa_cli.sin_port));

      gnutls_transport_set_ptr (session, (gnutls_transport_ptr_t) sd);
      ret = gnutls_handshake (session);
      if (ret < 0)
        {
          close (sd);
          gnutls_deinit (session);
          fprintf (stderr, "*** Handshake has failed (%s)\n\n",
                   gnutls_strerror (ret));
          continue;
        }
      printf ("- Handshake was completed\n");

      kx = gnutls_kx_get(session);
      if (kx == GNUTLS_KX_PSK || kx == GNUTLS_KX_DHE_PSK || 
          kx == GNUTLS_KX_ECDHE_PSK)
        {
          printf("- User %s was connected\n", gnutls_psk_server_get_username(session));
        }

      /* see the Getting peer's information example */
      /* print_info(session); */

      for (;;)
        {
          memset (buffer, 0, MAX_BUF + 1);
          ret = gnutls_record_recv (session, buffer, MAX_BUF);

          if (ret == 0)
            {
              printf ("\n- Peer has closed the GnuTLS connection\n");
              break;
            }
          else if (ret < 0)
            {
              fprintf (stderr, "\n*** Received corrupted "
                       "data(%d). Closing the connection.\n\n", ret);
              break;
            }
          else if (ret > 0)
            {
              /* echo data back to the client
               */
              gnutls_record_send (session, buffer, strlen (buffer));
            }
        }
      printf ("\n");
      /* do not wait for the peer to close the connection.
       */
      gnutls_bye (session, GNUTLS_SHUT_WR);

      close (sd);
      gnutls_deinit (session);

    }
  close (listen_sd);

  gnutls_certificate_free_credentials (x509_cred);
  gnutls_psk_free_server_credentials (psk_cred);

  gnutls_priority_deinit (priority_cache);

  gnutls_global_deinit ();

  return 0;

}





/* Client */

#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>

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

#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 };

  gnutls_global_init ();

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

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

  /* Use default priorities */
  ret = gnutls_priority_set_direct (session, "PERFORMANCE", &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
   */
  gnutls_credentials_set (session, GNUTLS_CRD_PSK, pskcred);

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

  gnutls_transport_set_ptr (session, (gnutls_transport_ptr_t) sd);

  /* 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
    {
      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_psk_free_client_credentials (pskcred);

  gnutls_global_deinit ();

  return 0;
}

I generated self signed certificate using these commands:

certtool --generate-privkey --outfile key.pem
certtool --generate-self-signed --load-privkey key.pem --outfile cert.pem

I compiled the examples ex-serv-psk.c and ex-client-psk.c. When I try to run them I get this error:

[root@localhost test]# ./server
Server ready. Listening to port '5556'.

- connection from 127.0.0.1, port 38184
*** Handshake has failed (The TLS connection was non-properly terminated.)

[root@localhost test]# ./client
*** Handshake failed
GnuTLS error: Insufficient credentials for that request.
[root@localhost test]#

Do I need something else to run these examples? I modified some code

#define KEYFILE "key.pem"
#define CERTFILE "cert.pem"
#define CAFILE "/etc/ssl/certs/ca-certificates.crt"
#define CRLFILE "crl.pem"

How must I create CAFILE and CRLFILE? I have created self-signed certificate. Do I need these files in this case?

Answer 1

you generate the crl file with this command. certtool --generate-crl --load-ca-privkey key.pem --load-ca-certificate cert.pem

You don't need to generate the CA file, and from what I can see, you don't even need that, just remove the call to "gnutls_certificate_set_x509_trust_file"