feat: Provide descriptive fallback messages for OMEMO decryption failures

If an incoming OMEMO message failed to decrypt (due to missing session
keys or untrusted identities), Profanity would fall back to displaying
the raw XMPP body. This usually contained a generic string like "This
message is encrypted with OMEMO,".

We wrote the detailed reason in the debug logs but the user only saw the
fallback message and probably wondered *why* the message wasn't
displayed properly.

I'm unsure if we should display the fallback message as well though.

src/omemo/omemo.c

@@ -1014,13 +1014,21 @@ out:
 char*
 omemo_on_message_recv(const char* const from_jid, uint32_t sid,
                       const unsigned char* const iv, size_t iv_len, GList* keys,
-                      const unsigned char* const payload, size_t payload_len, gboolean muc, gboolean* trusted)
+                      const unsigned char* const payload, size_t payload_len, gboolean muc, gboolean* trusted, omemo_error_t* error)
 {
     unsigned char* plaintext = NULL;
     auto_jid Jid* sender = NULL;
     auto_jid Jid* from = jid_create(from_jid);
+
+    if (error) {
+        *error = OMEMO_ERR_NONE;
+    }
+
     if (!from) {
         log_error("[OMEMO][RECV] Invalid jid %s", from_jid);
+        if (error) {
+            *error = OMEMO_ERR_INVALID_JID;
+        }
         return NULL;
     }
 
@@ -1036,6 +1044,9 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
 
     if (!key) {
         log_warning("[OMEMO][RECV] received a message with no corresponding key");
+        if (error) {
+            *error = OMEMO_ERR_NO_KEY;
+        }
         return NULL;
     }
 
@@ -1052,6 +1063,9 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
         g_list_free(roster);
         if (!sender) {
             log_warning("[OMEMO][RECV] cannot find MUC message sender fulljid");
+            if (error) {
+                *error = OMEMO_ERR_MUC_SENDER_NOT_FOUND;
+            }
             return NULL;
         }
     } else {
@@ -1069,6 +1083,9 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
     res = session_cipher_create(&cipher, omemo_ctx.store, &address, omemo_ctx.signal);
     if (res != 0) {
         log_error("[OMEMO][RECV] cannot create session cipher");
+        if (error) {
+            *error = OMEMO_ERR_NO_SESSION;
+        }
         return NULL;
     }
 
@@ -1127,13 +1144,23 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
 
     session_cipher_free(cipher);
     if (res != 0) {
-        log_error("[OMEMO][RECV] cannot decrypt message key");
+        log_error("[OMEMO][RECV] cannot decrypt message key: %d", res);
+        if (error) {
+            if (!*trusted) {
+                *error = OMEMO_ERR_NOT_TRUSTED;
+            } else {
+                *error = OMEMO_ERR_DECRYPT_FAILED;
+            }
+        }
         return 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);
+        if (error) {
+            *error = OMEMO_ERR_DECRYPT_FAILED;
+        }
         return NULL;
     }
 
@@ -1146,6 +1173,9 @@ omemo_on_message_recv(const char* const from_jid, uint32_t sid,
     if (res != 0) {
         log_error("[OMEMO][RECV] cannot decrypt message: %s", gcry_strerror(res));
         free(plaintext);
+        if (error) {
+            *error = OMEMO_ERR_DECRYPT_FAILED;
+        }
         return NULL;
     }
 

src/omemo/omemo.h

@@ -98,8 +98,9 @@ void omemo_start_muc_sessions(const char* const roomjid);
 void omemo_start_device_session(const char* const jid, uint32_t device_id, GList* prekeys, uint32_t signed_prekey_id, const unsigned char* const signed_prekey, size_t signed_prekey_len, const unsigned char* const signature, size_t signature_len, const unsigned char* const identity_key, size_t identity_key_len);
 
 gboolean omemo_loaded(void);
+
 char* omemo_on_message_send(ProfWin* win, const char* const message, gboolean request_receipt, gboolean muc, const char* const replace_id);
-char* omemo_on_message_recv(const char* const from, uint32_t sid, const unsigned char* const iv, size_t iv_len, GList* keys, const unsigned char* const payload, size_t payload_len, gboolean muc, gboolean* trusted);
+char* omemo_on_message_recv(const char* const from, uint32_t sid, const unsigned char* const iv, size_t iv_len, GList* keys, const unsigned char* const payload, size_t payload_len, gboolean muc, gboolean* trusted, omemo_error_t* error);
 
 char* omemo_encrypt_file(FILE* in, FILE* out, off_t file_size, int* gcry_res);
 gcry_error_t omemo_decrypt_file(FILE* in, FILE* out, off_t file_size, const char* fragment);