Validate AES-GCM payload length

src/password_manager/encryption.py

@@ -89,6 +89,9 @@ class EncryptionManager:
             try:
                 nonce = encrypted_data[3:15]
                 ciphertext = encrypted_data[15:]
+                if len(ciphertext) < 16:
+                    logger.error("AES-GCM payload too short")
+                    raise InvalidToken("AES-GCM payload too short")
                 return self.cipher.decrypt(nonce, ciphertext, None)
             except InvalidTag as e:
                 logger.error("AES-GCM decryption failed: Invalid authentication tag.")

src/tests/test_v2_prefix_fallback.py

@@ -1,6 +1,10 @@
 import logging
+import os
 from pathlib import Path
 
+import pytest
+from cryptography.fernet import InvalidToken
+
 from helpers import TEST_SEED
 from utils.key_derivation import derive_index_key
 from password_manager.encryption import EncryptionManager
@@ -19,3 +23,16 @@ def test_v2_prefix_fernet_fallback(tmp_path: Path, caplog) -> None:
 
     assert decrypted == original
     assert "incorrect 'V2:' header" in caplog.text
+
+
+def test_aesgcm_payload_too_short(tmp_path: Path, caplog) -> None:
+    key = derive_index_key(TEST_SEED)
+    manager = EncryptionManager(key, tmp_path)
+
+    payload = b"V2:" + os.urandom(12) + b"short"
+
+    caplog.set_level(logging.ERROR, logger="password_manager.encryption")
+    with pytest.raises(InvalidToken, match="AES-GCM payload too short"):
+        manager.decrypt_data(payload)
+
+    assert "AES-GCM payload too short" in caplog.text