Return the message index when decrypting group messages.

Applications can use the index to detect replays of the same message.

include/olm/inbound_group_session.h

@@ -140,7 +140,8 @@ size_t olm_group_decrypt(
     uint8_t * message, size_t message_length,
 
     /* output */
-    uint8_t * plaintext, size_t max_plaintext_length
+    uint8_t * plaintext, size_t max_plaintext_length,
+    uint32_t * message_index
 );
 
 

javascript/demo/group_demo.js

@@ -403,8 +403,8 @@ DemoUser.prototype.decryptGroup = function(jsonpacket, callback) {
             throw new Error("Unknown session id " + session_id);
         }
 
-        var plaintext = session.decrypt(packet.body);
+        var result = session.decrypt(packet.body);
-        done(plaintext);
+        done(result.plaintext);
     }, callback);
 };
 

javascript/olm_inbound_group_session.js

@@ -73,10 +73,12 @@ InboundGroupSession.prototype['decrypt'] = restore_stack(function(
     // So we copy the array to a new buffer
     var message_buffer = stack(message_array);
     var plaintext_buffer = stack(max_plaintext_length + NULL_BYTE_PADDING_LENGTH);
+    var message_index = stack(4);
     var plaintext_length = inbound_group_session_method(Module["_olm_group_decrypt"])(
         this.ptr,
         message_buffer, message_array.length,
-        plaintext_buffer, max_plaintext_length
+        plaintext_buffer, max_plaintext_length,
+        message_index
     );
 
     // Pointer_stringify requires a null-terminated argument (the optional
@@ -86,7 +88,10 @@ InboundGroupSession.prototype['decrypt'] = restore_stack(function(
         0, "i8"
     );
 
-    return Pointer_stringify(plaintext_buffer);
+    return {
+        "plaintext": Pointer_stringify(plaintext_buffer),
+        "message_index": Module['getValue'](message_index, "i32")
+    }
 });
 
 InboundGroupSession.prototype['session_id'] = restore_stack(function() {

python/olm/__main__.py

@@ -328,7 +328,7 @@ def do_group_decrypt(args):
     session = InboundGroupSession()
     session.unpickle(args.key, read_base64_file(args.session_file))
     message = args.message_file.read()
-    plaintext = session.decrypt(message)
+    plaintext, message_index = session.decrypt(message)
     with open(args.session_file, "wb") as f:
         f.write(session.pickle(args.key))
     args.plaintext_file.write(plaintext)

python/olm/inbound_group_session.py

@@ -43,6 +43,7 @@ inbound_group_session_function(
     lib.olm_group_decrypt,
     c_void_p, c_size_t, # message
     c_void_p, c_size_t, # plaintext
+    POINTER(c_uint32), # message_index
 )
 
 inbound_group_session_function(lib.olm_inbound_group_session_id_length)
@@ -82,11 +83,14 @@ class InboundGroupSession(object):
         )
         plaintext_buffer = create_string_buffer(max_plaintext_length)
         message_buffer = create_string_buffer(message)
+
+        message_index = c_uint32()
         plaintext_length = lib.olm_group_decrypt(
             self.ptr, message_buffer, len(message),
-            plaintext_buffer, max_plaintext_length
+            plaintext_buffer, max_plaintext_length,
+            byref(message_index)
         )
-        return plaintext_buffer.raw[:plaintext_length]
+        return plaintext_buffer.raw[:plaintext_length], message_index
 
     def session_id(self):
         id_length = lib.olm_inbound_group_session_id_length(self.ptr)

src/inbound_group_session.c

@@ -263,7 +263,8 @@ size_t olm_group_decrypt_max_plaintext_length(
 static size_t _decrypt(
     OlmInboundGroupSession *session,
     uint8_t * message, size_t message_length,
-    uint8_t * plaintext, size_t max_plaintext_length
+    uint8_t * plaintext, size_t max_plaintext_length,
+    uint32_t * message_index
 ) {
     struct _OlmDecodeGroupMessageResults decoded_results;
     size_t max_length, r;
@@ -286,6 +287,8 @@ static size_t _decrypt(
         return (size_t)-1;
     }
 
+    *message_index = decoded_results.message_index;
+
     /* verify the signature. We could do this before decoding the message, but
      * we allow for the possibility of future protocol versions which use a
      * different signing mechanism; we would rather throw "BAD_MESSAGE_VERSION"
@@ -349,7 +352,8 @@ static size_t _decrypt(
 size_t olm_group_decrypt(
     OlmInboundGroupSession *session,
     uint8_t * message, size_t message_length,
-    uint8_t * plaintext, size_t max_plaintext_length
+    uint8_t * plaintext, size_t max_plaintext_length,
+    uint32_t * message_index
 ) {
     size_t raw_message_length;
 
@@ -361,7 +365,8 @@ size_t olm_group_decrypt(
 
     return _decrypt(
         session, message, raw_message_length,
-        plaintext, max_plaintext_length
+        plaintext, max_plaintext_length,
+        message_index
     );
 }
 

tests/test_group_session.cpp

@@ -161,8 +161,9 @@ int main() {
     memcpy(msgcopy, msg, msglen);
     size = olm_group_decrypt_max_plaintext_length(inbound_session, msgcopy, msglen);
     uint8_t plaintext_buf[size];
+    uint32_t message_index;
     res = olm_group_decrypt(inbound_session, msg, msglen,
-                            plaintext_buf, size);
+                            plaintext_buf, size, &message_index);
     assert_equals(plaintext_length, res);
     assert_equals(plaintext, plaintext_buf, res);
 }
@@ -208,8 +209,9 @@ int main() {
 
     memcpy(msgcopy, message, msglen);
     uint8_t plaintext_buf[size];
+    uint32_t message_index;
     res = olm_group_decrypt(
-        inbound_session, msgcopy, msglen, plaintext_buf, size
+        inbound_session, msgcopy, msglen, plaintext_buf, size, &message_index
     );
     assert_equals(plaintext_length, res);
     assert_equals(plaintext, plaintext_buf, res);
@@ -227,7 +229,7 @@ int main() {
     memcpy(msgcopy, message, msglen);
     res = olm_group_decrypt(
         inbound_session, msgcopy, msglen,
-        plaintext_buf, size
+        plaintext_buf, size, &message_index
     );
     assert_equals((size_t)-1, res);
     assert_equals(