Return the message index when decrypting group messages.

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

1 files changed, 8 insertions, 3 deletions

diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index bf00008..ed313a0 100644
--- a/src/inbound_group_session.c
+++ b/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
     );
 }