Remove session_id from group messages

Putting the session_id inside the packed message body makes it hard to extract
so that we can decide which session to use. We don't think there is any
advantage to having thes sesion_id protected by the HMACs, so we're going to
move it to the JSON framing.

3 files changed, 4 insertions, 21 deletions

diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index e171205..ce26033 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -231,9 +231,7 @@ static size_t _decrypt(
         return (size_t)-1;
     }
 
-    if (!decoded_results.has_message_index || !decoded_results.session_id
-        || !decoded_results.ciphertext
-    ) {
+    if (!decoded_results.has_message_index || !decoded_results.ciphertext ) {
         session->last_error = OLM_BAD_MESSAGE_FORMAT;
         return (size_t)-1;
     }
diff --git a/src/message.cpp b/src/message.cpp
index 2e841e5..ad26cb9 100644
--- a/src/message.cpp
+++ b/src/message.cpp
@@ -328,18 +328,15 @@ void olm::decode_one_time_key_message(
 
 
 
-static const std::uint8_t GROUP_SESSION_ID_TAG = 012;
-static const std::uint8_t GROUP_MESSAGE_INDEX_TAG = 020;
-static const std::uint8_t GROUP_CIPHERTEXT_TAG = 032;
+static const std::uint8_t GROUP_MESSAGE_INDEX_TAG = 010;
+static const std::uint8_t GROUP_CIPHERTEXT_TAG = 022;
 
 size_t _olm_encode_group_message_length(
-    size_t group_session_id_length,
     uint32_t message_index,
     size_t ciphertext_length,
     size_t mac_length
 ) {
     size_t length = VERSION_LENGTH;
-    length += 1 + varstring_length(group_session_id_length);
     length += 1 + varint_length(message_index);
     length += 1 + varstring_length(ciphertext_length);
     length += mac_length;
@@ -349,19 +346,14 @@ size_t _olm_encode_group_message_length(
 
 size_t _olm_encode_group_message(
     uint8_t version,
-    const uint8_t *session_id,
-    size_t session_id_length,
     uint32_t message_index,
     size_t ciphertext_length,
     uint8_t *output,
     uint8_t **ciphertext_ptr
 ) {
     std::uint8_t * pos = output;
-    std::uint8_t * session_id_pos;
 
     *(pos++) = version;
-    pos = encode(pos, GROUP_SESSION_ID_TAG, session_id_pos, session_id_length);
-    std::memcpy(session_id_pos, session_id, session_id_length);
     pos = encode(pos, GROUP_MESSAGE_INDEX_TAG, message_index);
     pos = encode(pos, GROUP_CIPHERTEXT_TAG, *ciphertext_ptr, ciphertext_length);
     return pos-output;
@@ -376,8 +368,6 @@ void _olm_decode_group_message(
     std::uint8_t const * end = input + input_length - mac_length;
     std::uint8_t const * unknown = nullptr;
 
-    results->session_id = nullptr;
-    results->session_id_length = 0;
     bool has_message_index = false;
     results->message_index = 0;
     results->ciphertext = nullptr;
@@ -389,10 +379,6 @@ void _olm_decode_group_message(
 
     while (pos != end) {
         pos = decode(
-            pos, end, GROUP_SESSION_ID_TAG,
-            results->session_id, results->session_id_length
-        );
-        pos = decode(
             pos, end, GROUP_MESSAGE_INDEX_TAG,
             results->message_index, has_message_index
         );
diff --git a/src/outbound_group_session.c b/src/outbound_group_session.c
index 9b2298a..2a6c220 100644
--- a/src/outbound_group_session.c
+++ b/src/outbound_group_session.c
@@ -187,7 +187,7 @@ static size_t raw_message_length(
     mac_length = megolm_cipher->ops->mac_length(megolm_cipher);
 
     return _olm_encode_group_message_length(
-        GROUP_SESSION_ID_LENGTH, session->ratchet.counter,
+        session->ratchet.counter,
         ciphertext_length, mac_length);
 }
 
@@ -220,7 +220,6 @@ static size_t _encrypt(
      */
     message_length = _olm_encode_group_message(
         OLM_PROTOCOL_VERSION,
-        session->session_id, GROUP_SESSION_ID_LENGTH,
         session->ratchet.counter,
         ciphertext_length,
         buffer,