Adding JSON handling as fallback of EBTF decode failure

The Cryptic engine is using Erlang Binary Term Format for
encoding, for example, metadata in X3DH messages. However,
with the introduction of a Mobila App written in Dart
it is easer to just encode it as JSON. Hence, we introduce
this fallback mechanism, so that if the term_to_binary fail,
we will try to decode the term as JSON.

Author: etnt

src/cryptic_engine.erl

@@ -2217,8 +2217,10 @@ initialize_receiver_session_from_x3dh(FromUsername, MessagePayload, State) ->
             maps:get(<<"metadata">>, MessagePayload)
         ),
 
-        % Decode metadata to extract sender identity and other info
-        Metadata = erlang:binary_to_term(EncodedMetadata),
+        % Decode metadata - try ETF first, then JSON for mobile clients
+        {Metadata, MetadataFormat} = decode_x3dh_metadata(EncodedMetadata),
+        ?dbg("Decoded X3DH metadata (format: ~p): ~p~n", [MetadataFormat, Metadata]),
+        
         SenderIdPub = maps:get(sender_identity_sign_public, Metadata),
 
         % Extract receiver keys from state
@@ -2262,6 +2264,8 @@ initialize_receiver_session_from_x3dh(FromUsername, MessagePayload, State) ->
 
                 MessageBlob = #{
                     metadata => Metadata,
+                    metadata_bytes => EncodedMetadata,  % Original bytes for signature verification
+                    metadata_format => MetadataFormat,   % 'etf' or 'json'
                     signature => Signature,
                     ciphertext => Ciphertext,
                     nonce => Nonce
@@ -2342,6 +2346,41 @@ initialize_receiver_session_from_x3dh(FromUsername, MessagePayload, State) ->
             {error, {exception, ErrorClass, ErrorReason}}
     end.
 
+%% @private
+%% @doc Decode X3DH metadata from either ETF (Erlang clients) or JSON (mobile clients)
+%% Returns {Metadata, Format} where Format is 'etf' or 'json'
+decode_x3dh_metadata(EncodedMetadata) ->
+    % Try ETF first (Erlang native format)
+    try
+        EtfMetadata = erlang:binary_to_term(EncodedMetadata),
+        {EtfMetadata, etf}
+    catch
+        error:badarg ->
+            % Try JSON (mobile client format)
+            JsonMap = jsx:decode(EncodedMetadata, [return_maps]),
+            JsonMetadata = convert_json_metadata_to_erlang(JsonMap),
+            {JsonMetadata, json}
+    end.
+
+%% @private
+%% @doc Convert JSON metadata map with binary keys to Erlang metadata with atom keys
+convert_json_metadata_to_erlang(JsonMap) ->
+    #{
+        version => maps:get(<<"version">>, JsonMap),
+        type => maps:get(<<"type">>, JsonMap),
+        sender_id => base64:decode(maps:get(<<"sender_id">>, JsonMap)),
+        sender_identity_dh_public => base64:decode(maps:get(<<"sender_identity_dh_public">>, JsonMap)),
+        sender_identity_sign_public => base64:decode(maps:get(<<"sender_identity_sign_public">>, JsonMap)),
+        recipient_id => base64:decode(maps:get(<<"recipient_id">>, JsonMap)),
+        ephemeral_public => base64:decode(maps:get(<<"ephemeral_public">>, JsonMap)),
+        otpk_id => case maps:get(<<"otpk_id">>, JsonMap, null) of
+            null -> undefined;
+            OtpkIdB64 -> base64:decode(OtpkIdB64)
+        end,
+        message_id => base64:decode(maps:get(<<"message_id">>, JsonMap)),
+        timestamp => maps:get(<<"timestamp">>, JsonMap)
+    }.
+
 %% @private
 %% @doc Initialize ratchet session as receiver after X3DH
 %% If SenderRatchetDHPub is provided, the receiver can immediately send (bidirectional)

src/cryptic_lib.erl

@@ -1460,7 +1460,11 @@ x3dh_receiver_decrypt(
         %% This ensures we verify against exactly what Alice signed
 
         %% Verify message signature using complete metadata
-        MetadataBin = erlang:term_to_binary(CompleteMetadata),
+        %% Use original metadata bytes if available (for mobile clients with JSON format)
+        MetadataBin = case maps:get(metadata_bytes, MessageBlob, undefined) of
+            undefined -> erlang:term_to_binary(CompleteMetadata);
+            OriginalBytes -> OriginalBytes
+        end,
         ?dbg("X3DH signature verification - Complete Metadata map: ~p", [
             CompleteMetadata
         ]),
@@ -1580,7 +1584,11 @@ x3dh_receiver_decrypt_with_session_key(
         } = Metadata,
 
         %% Verify message signature (use provided SenderIdPub parameter)
-        MetadataBin = erlang:term_to_binary(Metadata),
+        %% Use original metadata bytes if available (for mobile clients with JSON format)
+        MetadataBin = case maps:get(metadata_bytes, MessageBlob, undefined) of
+            undefined -> erlang:term_to_binary(Metadata);
+            OriginalBytes -> OriginalBytes
+        end,
         case verify_signature(MetadataBin, Signature, SenderIdPub) of
             false ->
                 {error, invalid_message_signature};