omemo: ignore key contents if there is no payload

For key transport messages, Monal at least doesn't append the
authentication tag to the plaintext key contents as that only makes
sense if the key was used to encrypt something. This causes the key
length check to fail and show the

	OMEMO message received but decryption failed.

error to the user which is confusing because there is no user-originated
message involved.

Skip the length check for key transport messages as profanity only uses
these to advance the ratchet and makes no use of the decrypted contents.

Signed-off-by: Karel Balej <balejk@matfyz.cz>

Author: balejk

src/omemo/omemo.c

@@ -1141,16 +1141,16 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
         return NULL;
     }
 
-    if (signal_buffer_len(plaintext_key) != AES128_GCM_KEY_LENGTH + AES128_GCM_TAG_LENGTH) {
-        log_error("[OMEMO][RECV] invalid key length");
+    if (payload == NULL) {
         signal_buffer_free(plaintext_key);
-        *error = OMEMO_ERR_DECRYPT_FAILED;
+        *error = OMEMO_ERR_KEY_TRANSPORT;
         return NULL;
     }
 
-    if (payload == NULL) {
+    if (signal_buffer_len(plaintext_key) != AES128_GCM_KEY_LENGTH + AES128_GCM_TAG_LENGTH) {
+        log_error("[OMEMO][RECV] invalid key length");
         signal_buffer_free(plaintext_key);
-        *error = OMEMO_ERR_KEY_TRANSPORT;
+        *error = OMEMO_ERR_DECRYPT_FAILED;
         return NULL;
     }