fix: fix connection drop with aes-gcm ciphers

CHANGELOG.md

@@ -3,6 +3,7 @@
 - Added chunked transfer-encoding decoding for HTTP response bodies according to RFC 7230 §4.1, improving interoperability with HTTP/1.1 servers [#171]. Thanks [@GT-610].
 - Added support for OpenSSH's `posix-rename@openssh.com` SFTP extension to perform atomic renames with POSIX semantics (replace destination if it exists) when advertised by the server [#172]. Thanks [@GT-610].
 - Added `SftpFile.downloadToRandomAccess` to download a remote file directly into a `dart:io` `RandomAccessFile` using out-of-order pipelined writes, maximizing download performance on high-latency links [#173]. Thanks [@GT-610].
+- Fixed a connection drop bug during AEAD (AES-GCM) decryption caused by incorrect padding length validation offset calculation [#168]. Thanks [@nuclear06].
 
 ## [2.18.0] - 2026-05-18
 - Fixed AES-GCM cipher encryption and decryption sequence number/nonce counter resetting during key exchanges [#165]. Thanks [@vicajilau].

lib/src/ssh_transport.dart

@@ -758,7 +758,15 @@ class SSHTransport {
 
     final paddingLength = plaintext[0];
     final payloadLength = packetLength - paddingLength - 1;
-    _verifyPacketPadding(payloadLength, paddingLength);
+
+    final minPaddingLength =
+        _alignedPaddingLength(payloadLength, cipherType.blockSize);
+    if (paddingLength < minPaddingLength) {
+      throw SSHPacketError(
+        'Invalid padding length: $paddingLength, expected: $minPaddingLength',
+      );
+    }
+
     return Uint8List.sublistView(plaintext, 1, 1 + payloadLength);
   }
 

test/src/ssh_transport_aead_test.dart

@@ -39,53 +39,57 @@ void main() {
         iv[i] = i + 16;
       }
 
-      final senderSocket = _CaptureSSHSocket();
-      final sender = SSHTransport(
-        senderSocket,
-        algorithms: const SSHAlgorithms(
-          cipher: [SSHCipherType.aes128gcm],
-        ),
-      );
-
-      setPrivate(sender, '_clientCipherType', SSHCipherType.aes128gcm);
-      setPrivate(sender, '_localCipherKey', key);
-      setPrivate(sender, '_localIV', iv);
-      setPrivate(sender, '_kexInProgress', false);
-      setSequenceValue(sender, '_localPacketSN', 0);
-
-      final payload = Uint8List.fromList([250, 1, 2, 3, 4, 5]);
-      sender.sendPacket(payload);
-
-      final encryptedPacket = senderSocket.packets.last;
-
-      final receiverSocket = _CaptureSSHSocket();
-      final receivedPacket = Completer<Uint8List>();
-      final receiver = SSHTransport(
-        receiverSocket,
-        algorithms: const SSHAlgorithms(
-          cipher: [SSHCipherType.aes128gcm],
-        ),
-        onPacket: (packet) {
-          if (!receivedPacket.isCompleted) {
-            receivedPacket.complete(packet);
-          }
-        },
-      );
-
-      setPrivate(receiver, '_remoteVersion', 'SSH-2.0-test');
-      setPrivate(receiver, '_serverCipherType', SSHCipherType.aes128gcm);
-      setPrivate(receiver, '_remoteCipherKey', key);
-      setPrivate(receiver, '_remoteIV', iv);
-      setSequenceValue(receiver, '_remotePacketSN', 0);
-
-      receiverSocket.addIncomingBytes(encryptedPacket);
-
-      final received =
-          await receivedPacket.future.timeout(const Duration(seconds: 2));
-      expect(received, payload);
-
-      sender.close();
-      receiver.close();
+      for (final payloadLength in [1, 5, 6, 10, 11, 15, 16, 20, 31, 32]) {
+        final senderSocket = _CaptureSSHSocket();
+        final sender = SSHTransport(
+          senderSocket,
+          algorithms: const SSHAlgorithms(
+            cipher: [SSHCipherType.aes128gcm],
+          ),
+        );
+
+        setPrivate(sender, '_clientCipherType', SSHCipherType.aes128gcm);
+        setPrivate(sender, '_localCipherKey', key);
+        setPrivate(sender, '_localIV', iv);
+        setPrivate(sender, '_kexInProgress', false);
+        setSequenceValue(sender, '_localPacketSN', 0);
+
+        final payload = Uint8List.fromList(
+          List.generate(payloadLength, (index) => index % 256),
+        );
+        sender.sendPacket(payload);
+
+        final encryptedPacket = senderSocket.packets.last;
+
+        final receiverSocket = _CaptureSSHSocket();
+        final receivedPacket = Completer<Uint8List>();
+        final receiver = SSHTransport(
+          receiverSocket,
+          algorithms: const SSHAlgorithms(
+            cipher: [SSHCipherType.aes128gcm],
+          ),
+          onPacket: (packet) {
+            if (!receivedPacket.isCompleted) {
+              receivedPacket.complete(packet);
+            }
+          },
+        );
+
+        setPrivate(receiver, '_remoteVersion', 'SSH-2.0-test');
+        setPrivate(receiver, '_serverCipherType', SSHCipherType.aes128gcm);
+        setPrivate(receiver, '_remoteCipherKey', key);
+        setPrivate(receiver, '_remoteIV', iv);
+        setSequenceValue(receiver, '_remotePacketSN', 0);
+
+        receiverSocket.addIncomingBytes(encryptedPacket);
+
+        final received =
+            await receivedPacket.future.timeout(const Duration(seconds: 2));
+        expect(received, payload);
+
+        sender.close();
+        receiver.close();
+      }
     });
 
     test('reports AEAD authentication failure when packet is tampered',