test: align API crypto coverage with the AES-GCM protocol

backend/tests/test_api_crypto.py

@@ -33,7 +33,7 @@ class ApiCryptoTests(unittest.TestCase):
         plain = api_crypto.decrypt_json(cipher, key)
         self.assertEqual(plain, payload)
 
-    def test_encrypt_decrypt_roundtrip_cbc(self):
+    def test_decrypt_rejects_tampered_gcm_ciphertext(self):
         key = api_crypto.derive_key(
             fixed_secret="fixed-secret",
             user_id="u1",
@@ -41,9 +41,13 @@ class ApiCryptoTests(unittest.TestCase):
             nonce="nonce-2",
         )
         payload = {"list": [1, 2, 3], "ok": True}
-        cipher = api_crypto.encrypt_json(payload, key, alg="cbc")
-        plain = api_crypto.decrypt_json(cipher, key, alg="cbc")
-        self.assertEqual(plain, payload)
+        cipher = api_crypto.encrypt_json(payload, key)
+        raw = bytearray(api_crypto._b64url_decode(cipher))
+        raw[-1] ^= 1
+        tampered = api_crypto._b64url_encode(bytes(raw))
+
+        with self.assertRaises(api_crypto.ApiCryptoError):
+            api_crypto.decrypt_json(tampered, key)
 
 
 if __name__ == "__main__":