Write computed Diffie-Hellman-Merkel parameters to VLC cache.

These are are quite slow to compute, so it could be used to speed up start up
once reading them from the cache is also implemented.

Hmm, there is a security problem in this commit (cache file is world-readable).

modules/misc/gnutls.c

@@ -1032,7 +1032,7 @@ gnutls_ServerSessionPrepare( tls_server_t *p_server )
     gnutls_dh_set_prime_bits (session, i_val);
 
     /* Session resumption support */
-    i_val = config_GetInt (p_server, "gnutls-cache-expiration");
+    i_val = config_GetInt (p_server, "gnutls-cache-timeout");
     gnutls_db_set_cache_expiration (session, i_val);
     gnutls_db_set_retrieve_function( session, cb_fetch );
     gnutls_db_set_remove_function( session, cb_delete );
@@ -1189,13 +1189,38 @@ static int OpenServer (vlc_object_t *obj)
     val = gnutls_dh_params_init( &p_sys->dh_params );
     if( val >= 0 )
     {
-        msg_Dbg( p_server, "computing Diffie Hellman ciphers parameters" );
+        msg_Dbg( p_server, "computing DHE ciphers parameters" );
         val = gnutls_dh_params_generate2 (p_sys->dh_params,
                                           config_GetInt (obj, "gnutls-dh-bits"));
+
+        /* Write the DH parameter to cache */
+        const char *cachedir = p_server->p_libvlc->psz_cachedir;
+        char cachefile[strlen (cachedir) + sizeof ("/dh_params.pem")];
+        sprintf (cachefile, "%s/dh_params.pem", cachedir);
+
+        FILE *cache = utf8_fopen (cachefile, "wb");
+        if (cache != NULL)
+        {
+            size_t len = 0;
+            gnutls_dh_params_export_pkcs3 (p_sys->dh_params,
+                                           GNUTLS_X509_FMT_PEM, NULL, &len);
+            msg_Dbg (p_server, "caching DHE parameters (%u bytes) to %s",
+                     (unsigned)len, cachefile);
+
+            unsigned char buf[len];
+            gnutls_dh_params_export_pkcs3 (p_sys->dh_params,
+                                           GNUTLS_X509_FMT_PEM, buf, &len);
+            if (fwrite (buf, 1, len, cache) != len)
+                msg_Warn (p_server, "cannot write to %s: %m", cachefile);
+            fclose (cache);
+        }
+        else
+            msg_Warn (p_server, "cannot open to %s: %m", cachefile);
+
     }
     if( val < 0 )
     {
-        msg_Err( p_server, "cannot initialize DH cipher suites: %s",
+        msg_Err( p_server, "cannot initialize DHE cipher suites: %s",
                  gnutls_strerror( val ) );
         gnutls_certificate_free_credentials( p_sys->x509_cred );
         goto error;