fix(core/thp): correct Failure_InvalidProtocol constant message

[no changelog]

Author: romanz

core/src/trezor/wire/thp/interface_context.py

@@ -172,7 +172,7 @@ async def _handle_codec_v1(self, packet: AnyBytes) -> None:
             response = bytearray(self._iface.TX_PACKET_LEN)
             # Codec_v1 magic constant:
             # "?##" + Failure message type + msg_size + msg_data (code = "Failure_InvalidProtocol")
-            utils.memcpy(response, 0, b"?##\x00\x03\x00\x00\x00\x14\x08\x11", 0)
+            utils.memcpy(response, 0, b"?##\x00\x03\x00\x00\x00\x02\x08\x11", 0)
             await self._write_packets([response])
 
     async def _handle_broadcast(self, packet: AnyBytes) -> None:

tests/device_tests/thp/test_basic.py

@@ -24,8 +24,11 @@ def test_v1(client: Client):
     # There should be a failure response to received init packet (starts with "?##")
     write_padded(client.transport, b"?## Init packet")
     res_id, res_data = protocol_v1.read(client.transport)
+    expected = messages.Failure(code=messages.FailureType.InvalidProtocol)
     res = DEFAULT_MAPPING.decode(res_id, res_data)
-    assert res == messages.Failure(code=messages.FailureType.InvalidProtocol)
+    assert res == expected
+    # make sure the constant "InvalidProtocol" response doesn't have trailing bytes (#6549)
+    assert (res_id, res_data) == DEFAULT_MAPPING.encode(expected)
 
     # There should be no response for continuation packet (starts with "?" only)
     write_padded(client.transport, b"? Cont packet")