Use the ed22519 public key as the group session id.

Some clients expect the session id to be globally unique,
so allowing the end devices to pick the session id will cause
problems.

Include the current ratchet index with the initial keys, this decreases
the risk that the client will supply the wrong index causing problems.

Sign the initial keys with the ratchet ed25519 key, this reduces the
risk of a client claiming a session that they didn't create.

1 files changed, 20 insertions, 7 deletions

diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index 11e3dbe..f2a310a 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -30,7 +30,7 @@
 
 #define OLM_PROTOCOL_VERSION     3
 #define PICKLE_VERSION           1
-#define SESSION_KEY_VERSION      1
+#define SESSION_KEY_VERSION      2
 
 struct OlmInboundGroupSession {
     /** our earliest known ratchet value */
@@ -71,12 +71,12 @@ size_t olm_clear_inbound_group_session(
 }
 
 #define SESSION_KEY_RAW_LENGTH \
-    (1 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH)
+    (1 + 4 + MEGOLM_RATCHET_LENGTH + ED25519_PUBLIC_KEY_LENGTH\
+        + ED25519_SIGNATURE_LENGTH)
 
 /** init the session keys from the un-base64-ed session keys */
 static size_t _init_group_session_keys(
     OlmInboundGroupSession *session,
-    uint32_t message_index,
     const uint8_t *key_buf
 ) {
     const uint8_t *ptr = key_buf;
@@ -87,13 +87,27 @@ static size_t _init_group_session_keys(
         return (size_t)-1;
     }
 
-    megolm_init(&session->initial_ratchet, ptr, message_index);
-    megolm_init(&session->latest_ratchet, ptr, message_index);
+    uint32_t counter = 0;
+    counter <<= 8; counter |= *ptr++;
+    counter <<= 8; counter |= *ptr++;
+    counter <<= 8; counter |= *ptr++;
+    counter <<= 8; counter |= *ptr++;
+
+    megolm_init(&session->initial_ratchet, ptr, counter);
+    megolm_init(&session->latest_ratchet, ptr, counter);
+
     ptr += MEGOLM_RATCHET_LENGTH;
     memcpy(
         session->signing_key.public_key, ptr, ED25519_PUBLIC_KEY_LENGTH
     );
     ptr += ED25519_PUBLIC_KEY_LENGTH;
+
+    if (!_olm_crypto_ed25519_verify(
+        &session->signing_key, key_buf, ptr - key_buf, ptr
+    )) {
+        session->last_error = OLM_BAD_SIGNATURE;
+        return (size_t)-1;
+    }
     return 0;
 }
 
@@ -117,7 +131,7 @@ size_t olm_init_inbound_group_session(
     }
 
     _olm_decode_base64(session_key, session_key_length, key_buf);
-    result = _init_group_session_keys(session, message_index, key_buf);
+    result = _init_group_session_keys(session, key_buf);
     _olm_unset(key_buf, SESSION_KEY_RAW_LENGTH);
     return result;
 }
@@ -288,7 +302,6 @@ static size_t _decrypt(
         return (size_t)-1;
     }
 
-
     max_length = megolm_cipher->ops->decrypt_max_plaintext_length(
         megolm_cipher,
         decoded_results.ciphertext_length