Remove ciphersuite from SSL session if single suite hardcoded

If MBEDTLS_SSL_SINGLE_CIPHERSUITE is enabled, the type

  mbedtls_ssl_ciphersuite_handle_t

is logically a boolean (concretely realized as `unsigned char`),
containing the invalid handle and the unique valid handle, which
represents the single enabled ciphersuite.

The SSL session structure mbedtls_ssl_session contains an instance
of mbedtls_ssl_ciphersuite_handle_t which is guaranteed to be valid,
and which is hence redundant in any two-valued implementation of
mbedtls_ssl_ciphersuite_handle_t.

This commit replaces read-uses of

  mbedtls_ssl_session::ciphersuite_info

by a getter functions which, and defines this getter function
either by just reading the field from the session structure
(in case MBEDTLS_SSL_SINGLE_CIPHERSUITE is disabled), or by
returning the single valid ciphersuite handle (in case
MBEDTLS_SSL_SINGLE_CIPHERSUITE is enabled) and removing the
field from mbedtls_ssl_session in this case.

include/mbedtls/ssl.h

@@ -923,7 +923,9 @@ struct mbedtls_ssl_session
 #if defined(MBEDTLS_HAVE_TIME)
     mbedtls_time_t start;       /*!< starting time      */
 #endif
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     int ciphersuite;            /*!< chosen ciphersuite */
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
     int compression;            /*!< chosen compression */
     size_t id_len;              /*!< session id length  */
     unsigned char id[32];       /*!< session identifier */

include/mbedtls/ssl_ciphersuites.h

@@ -681,6 +681,21 @@ static inline int mbedtls_ssl_ciphersuite_uses_server_signature(
     }
 }
 
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
+static inline int mbedtls_ssl_session_get_ciphersuite(
+    mbedtls_ssl_session const * session )
+{
+    return( session->ciphersuite );
+}
+#else /* !MBEDTLS_SSL_SINGLE_CIPHERSUITE */
+static inline int mbedtls_ssl_session_get_ciphersuite(
+    mbedtls_ssl_session const * session )
+{
+    ((void) session);
+    return( MBEDTLS_SSL_SUITE_ID( MBEDTLS_SSL_SINGLE_CIPHERSUITE ) );
+}
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
+
 const int *mbedtls_ssl_list_ciphersuites( void );
 
 mbedtls_ssl_ciphersuite_handle_t mbedtls_ssl_ciphersuite_from_string( const char *ciphersuite_name );

library/ssl_cache.c

@@ -84,10 +84,13 @@ int mbedtls_ssl_cache_get( void *data, mbedtls_ssl_session *session )
             continue;
 #endif
 
-        if( session->ciphersuite != entry->session.ciphersuite ||
+        if( mbedtls_ssl_session_get_ciphersuite( session ) !=
+            mbedtls_ssl_session_get_ciphersuite( &entry->session ) ||
             session->compression != entry->session.compression ||
             session->id_len != entry->session.id_len )
+        {
             continue;
+        }
 
         if( memcmp( session->id, entry->session.id,
                     entry->session.id_len ) != 0 )

library/ssl_cli.c

@@ -1845,7 +1845,7 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
 #if !defined(MBEDTLS_SSL_NO_SESSION_RESUMPTION)
     if( n == 0 ||
         mbedtls_ssl_get_renego_status( ssl ) != MBEDTLS_SSL_INITIAL_HANDSHAKE ||
-        ssl->session_negotiate->ciphersuite != i ||
+        mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate ) != i ||
         ssl->session_negotiate->compression != comp ||
         ssl->session_negotiate->id_len != n ||
         memcmp( ssl->session_negotiate->id, buf + 35, n ) != 0 )
@@ -1874,7 +1874,9 @@ static int ssl_parse_server_hello( mbedtls_ssl_context *ssl )
 #if defined(MBEDTLS_HAVE_TIME)
         ssl->session_negotiate->start = mbedtls_time( NULL );
 #endif
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
         ssl->session_negotiate->ciphersuite = i;
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
         ssl->session_negotiate->compression = comp;
         ssl->session_negotiate->id_len = n;
         memcpy( ssl->session_negotiate->id, buf + 35, n );

library/ssl_srv.c

@@ -1037,7 +1037,9 @@ static int ssl_parse_client_hello_v2( mbedtls_ssl_context *ssl )
     size_t n;
     unsigned int ciph_len, sess_len, chal_len;
     unsigned char *buf, *p;
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     mbedtls_ssl_ciphersuite_handle_t ciphersuite_info;
+#endif
 
     MBEDTLS_SSL_DEBUG_MSG( 2, ( "=> parse client hello v2" ) );
 
@@ -1256,7 +1258,9 @@ static int ssl_parse_client_hello_v2( mbedtls_ssl_context *ssl )
 
             if( ssl_ciphersuite_is_match( ssl, cur_info, NULL ) )
             {
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
                 ciphersuite_info = cur_info;
+#endif
                 goto have_ciphersuite_v2;
             }
 
@@ -1289,9 +1293,9 @@ static int ssl_parse_client_hello_v2( mbedtls_ssl_context *ssl )
 
 have_ciphersuite_v2:
 
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     ssl->session_negotiate->ciphersuite =
         mbedtls_ssl_suite_get_id( ciphersuite_info );
-#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     ssl->handshake->ciphersuite_info = ciphersuite_info;
 #endif
 
@@ -1341,7 +1345,10 @@ static int ssl_parse_client_hello( mbedtls_ssl_context *ssl )
     int extended_ms_seen = 0;
 #endif
     int handshake_failure = 0;
+
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     mbedtls_ssl_ciphersuite_handle_t ciphersuite_info;
+#endif
     int major, minor;
 
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
@@ -2175,7 +2182,9 @@ read_record_header:
             if( ssl_ciphersuite_is_match( ssl, cur_info,
                                           acceptable_ec_grp_ids) )
             {
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
                 ciphersuite_info = cur_info;
+#endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
                 goto have_ciphersuite;
             }
 
@@ -2212,9 +2221,9 @@ read_record_header:
 
 have_ciphersuite:
 
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     ssl->session_negotiate->ciphersuite =
         mbedtls_ssl_suite_get_id( ciphersuite_info );
-#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
     ssl->handshake->ciphersuite_info = ciphersuite_info;
 #endif /* MBEDTLS_SSL_SINGLE_CIPHERSUITE */
 
@@ -2354,7 +2363,7 @@ static void ssl_write_encrypt_then_mac_ext( mbedtls_ssl_context *ssl,
      * encrypt-then-MAC response extension back to the client."
      */
     suite = mbedtls_ssl_ciphersuite_from_id(
-        ssl->session_negotiate->ciphersuite );
+        mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate ) );
     if( suite == MBEDTLS_SSL_CIPHERSUITE_INVALID_HANDLE )
     {
         *olen = 0;
@@ -2695,6 +2704,7 @@ static int ssl_write_server_hello( mbedtls_ssl_context *ssl )
     mbedtls_time_t t;
 #endif
     int ret;
+    int ciphersuite;
     size_t olen, ext_len = 0, n;
     unsigned char *buf, *p;
 
@@ -2844,12 +2854,13 @@ static int ssl_write_server_hello( mbedtls_ssl_context *ssl )
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "%s session has been resumed",
             mbedtls_ssl_handshake_get_resume( ssl->handshake ) ? "a" : "no" ) );
 
-    *p++ = (unsigned char)( ssl->session_negotiate->ciphersuite >> 8 );
+    ciphersuite = mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate );
-    *p++ = (unsigned char)( ssl->session_negotiate->ciphersuite      );
+    *p++ = (unsigned char)( ciphersuite >> 8 );
+    *p++ = (unsigned char)( ciphersuite      );
     *p++ = (unsigned char)( ssl->session_negotiate->compression      );
 
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, chosen ciphersuite: %s",
-           mbedtls_ssl_get_ciphersuite_name( ssl->session_negotiate->ciphersuite ) ) );
+           mbedtls_ssl_get_ciphersuite_name( ciphersuite ) ) );
     MBEDTLS_SSL_DEBUG_MSG( 3, ( "server hello, compress alg.: 0x%02X",
                    ssl->session_negotiate->compression ) );
 
@@ -2898,7 +2909,8 @@ static int ssl_write_server_hello( mbedtls_ssl_context *ssl )
 #if defined(MBEDTLS_ECDH_C) || defined(MBEDTLS_ECDSA_C) || \
     defined(MBEDTLS_KEY_EXCHANGE_ECJPAKE_ENABLED)
     if ( mbedtls_ssl_ciphersuite_uses_ec(
-         mbedtls_ssl_ciphersuite_from_id( ssl->session_negotiate->ciphersuite ) ) )
+           mbedtls_ssl_ciphersuite_from_id(
+             mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate ) ) ) )
     {
         ssl_write_supported_point_formats_ext( ssl, p + 2 + ext_len, &olen );
         ext_len += olen;

library/ssl_tls.c

@@ -1378,7 +1378,7 @@ int mbedtls_ssl_derive_keys( mbedtls_ssl_context *ssl )
 
     /* Populate transform structure */
     ret = ssl_populate_transform( ssl->transform_negotiate,
-                                  ssl->session_negotiate->ciphersuite,
+                  mbedtls_ssl_session_get_ciphersuite( ssl->session_negotiate ),
                   ssl->session_negotiate->master,
 #if defined(MBEDTLS_SSL_SOME_MODES_USE_MAC)
 #if defined(MBEDTLS_SSL_ENCRYPT_THEN_MAC)
@@ -8968,10 +8968,13 @@ uint32_t mbedtls_ssl_get_verify_result( const mbedtls_ssl_context *ssl )
 
 const char *mbedtls_ssl_get_ciphersuite( const mbedtls_ssl_context *ssl )
 {
+    int suite;
+
     if( ssl == NULL || ssl->session == NULL )
         return( NULL );
 
-    return mbedtls_ssl_get_ciphersuite_name( ssl->session->ciphersuite );
+    suite = mbedtls_ssl_session_get_ciphersuite( ssl->session );
+    return( mbedtls_ssl_get_ciphersuite_name( suite ) );
 }
 
 const char *mbedtls_ssl_get_version( const mbedtls_ssl_context *ssl )
@@ -9393,8 +9396,10 @@ int mbedtls_ssl_session_save( const mbedtls_ssl_session *session,
 
     if( used <= buf_len )
     {
-        *p++ = (unsigned char)( ( session->ciphersuite >> 8 ) & 0xFF );
+        const int ciphersuite =
-        *p++ = (unsigned char)( ( session->ciphersuite      ) & 0xFF );
+            mbedtls_ssl_session_get_ciphersuite( session );
+        *p++ = (unsigned char)( ( ciphersuite >> 8 ) & 0xFF );
+        *p++ = (unsigned char)( ( ciphersuite      ) & 0xFF );
 
         *p++ = (unsigned char)( session->compression & 0xFF );
 
@@ -9532,6 +9537,7 @@ static int ssl_session_load( mbedtls_ssl_session *session,
 {
     const unsigned char *p = buf;
     const unsigned char * const end = buf + len;
+    int ciphersuite;
 #if defined(MBEDTLS_HAVE_TIME)
     uint64_t start;
 #endif
@@ -9578,12 +9584,23 @@ static int ssl_session_load( mbedtls_ssl_session *session,
     /*
      * Basic mandatory fields
      */
+
     if( 2 + 1 + 1 + 32 + 48 + 4 > (size_t)( end - p ) )
         return( MBEDTLS_ERR_SSL_BAD_INPUT_DATA );
 
-    session->ciphersuite = ( p[0] << 8 ) | p[1];
+    ciphersuite = ( p[0] << 8 ) | p[1];
     p += 2;
 
+#if !defined(MBEDTLS_SSL_SINGLE_CIPHERSUITE)
+    session->ciphersuite = ciphersuite;
+#else
+    if( ciphersuite !=
+        MBEDTLS_SSL_SUITE_ID( MBEDTLS_SSL_SINGLE_CIPHERSUITE ) )
+    {
+        return( MBEDTLS_ERR_SSL_VERSION_MISMATCH );
+    }
+#endif
+
     session->compression = *p++;
 
     session->id_len = *p++;

tests/suites/test_suite_ssl.function

@@ -279,7 +279,9 @@ static int ssl_populate_session( mbedtls_ssl_session *session,
 #if defined(MBEDTLS_HAVE_TIME)
     session->start = mbedtls_time( NULL ) - 42;
 #endif
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE)
     session->ciphersuite = 0xabcd;
+#endif
     session->compression = 1;
     session->id_len = sizeof( session->id );
     memset( session->id, 66, session->id_len );
@@ -698,7 +700,9 @@ void ssl_serialize_session_save_load( int ticket_len, char *crt_file )
 #if defined(MBEDTLS_HAVE_TIME)
     TEST_ASSERT( original.start == restored.start );
 #endif
+#if !defined(MBEDTLS_SSL_CONF_SINGLE_CIPHERSUITE)
     TEST_ASSERT( original.ciphersuite == restored.ciphersuite );
+#endif
     TEST_ASSERT( original.compression == restored.compression );
     TEST_ASSERT( original.id_len == restored.id_len );
     TEST_ASSERT( memcmp( original.id,