Raise KasaException on decryption errors (#1078)

Currently if the library encounters an invalid decryption error it
raises a value error. This PR wraps it in a KasaException so consumers
such as HA can catch an expected library exception.

kasa/tests/test_klapprotocol.py

@@ -238,6 +238,71 @@ def test_encrypt_unicode():
     assert d == decrypted
 
 
+async def test_transport_decrypt(mocker):
+    """Test transport decryption."""
+    d = {"great": "success"}
+
+    seed = secrets.token_bytes(16)
+    auth_hash = KlapTransport.generate_auth_hash(Credentials("foo", "bar"))
+    encryption_session = KlapEncryptionSession(seed, seed, auth_hash)
+
+    transport = KlapTransport(config=DeviceConfig(host="127.0.0.1"))
+    transport._handshake_done = True
+    transport._session_expire_at = time.monotonic() + 60
+    transport._encryption_session = encryption_session
+
+    async def _return_response(url: URL, params=None, data=None, *_, **__):
+        encryption_session = KlapEncryptionSession(
+            transport._encryption_session.local_seed,
+            transport._encryption_session.remote_seed,
+            transport._encryption_session.user_hash,
+        )
+        seq = params.get("seq")
+        encryption_session._seq = seq - 1
+        encrypted, seq = encryption_session.encrypt(json.dumps(d))
+        seq = seq
+        return 200, encrypted
+
+    mocker.patch.object(HttpClient, "post", side_effect=_return_response)
+
+    resp = await transport.send(json.dumps({}))
+    assert d == resp
+
+
+async def test_transport_decrypt_error(mocker, caplog):
+    """Test that a decryption error raises a kasa exception."""
+    d = {"great": "success"}
+
+    seed = secrets.token_bytes(16)
+    auth_hash = KlapTransport.generate_auth_hash(Credentials("foo", "bar"))
+    encryption_session = KlapEncryptionSession(seed, seed, auth_hash)
+
+    transport = KlapTransport(config=DeviceConfig(host="127.0.0.1"))
+    transport._handshake_done = True
+    transport._session_expire_at = time.monotonic() + 60
+    transport._encryption_session = encryption_session
+
+    async def _return_response(url: URL, params=None, data=None, *_, **__):
+        encryption_session = KlapEncryptionSession(
+            secrets.token_bytes(16),
+            transport._encryption_session.remote_seed,
+            transport._encryption_session.user_hash,
+        )
+        seq = params.get("seq")
+        encryption_session._seq = seq - 1
+        encrypted, seq = encryption_session.encrypt(json.dumps(d))
+        seq = seq
+        return 200, encrypted
+
+    mocker.patch.object(HttpClient, "post", side_effect=_return_response)
+
+    with pytest.raises(
+        KasaException,
+        match="Error trying to decrypt device 127.0.0.1 response: Invalid padding bytes.",
+    ):
+        await transport.send(json.dumps({}))
+
+
 @pytest.mark.parametrize(
     "device_credentials, expectation",
     [