From 0826d043d5511c23878725742765da5687546723 Mon Sep 17 00:00:00 2001
From: "Alexander \"PapaTutuWawa" <papatutuwawa@polynom.me>
Date: Sun, 2 Oct 2022 17:03:39 +0200
Subject: [PATCH] feat: Attempt to detect already decrypted messages

---
 lib/src/errors.dart               | 6 ++++++
 lib/src/omemo/sessionmanager.dart | 6 ++++++
 test/omemo_test.dart              | 4 +++-
 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/lib/src/errors.dart b/lib/src/errors.dart
index 3a2cf70..a2672f4 100644
--- a/lib/src/errors.dart
+++ b/lib/src/errors.dart
@@ -40,3 +40,9 @@ class InvalidKeyExchangeException implements Exception {
   final int actualPn;
   String errMsg() => 'The pn attribute of the key exchange is invalid. Expected $expectedPn, got $actualPn';
 }
+
+/// Triggered by the Session Manager when a message's sequence number is smaller than we
+/// expect it to be.
+class MessageAlreadyDecryptedException implements Exception {
+  String errMsg() => 'The message has already been decrypted';
+}
diff --git a/lib/src/omemo/sessionmanager.dart b/lib/src/omemo/sessionmanager.dart
index 3fbd160..cf06607 100644
--- a/lib/src/omemo/sessionmanager.dart
+++ b/lib/src/omemo/sessionmanager.dart
@@ -401,6 +401,12 @@ class OmemoSessionManager {
     final ratchet = (await _getRatchet(ratchetKey))!;
     oldRatchet ??= ratchet.clone();
 
+    if (!rawKey.kex) {
+      if (message.n! < ratchet.nr - 1) {
+        throw MessageAlreadyDecryptedException();
+      }
+    }
+    
     try {
       if (rawKey.kex) {
         keyAndHmac = await ratchet.ratchetDecrypt(message, authMessage.writeToBuffer());
diff --git a/test/omemo_test.dart b/test/omemo_test.dart
index cdaf4fc..3858960 100644
--- a/test/omemo_test.dart
+++ b/test/omemo_test.dart
@@ -730,7 +730,9 @@ void main() {
           msg.encryptedKeys,
         );
         expect(true, false);
-      } catch (_) {
+      } on MessageAlreadyDecryptedException catch (_) {
+        errorCounter++;
+      } on InvalidKeyExchangeException catch (_) {
         errorCounter++;
       }
     }