The "stuff behind the door" is conveniently uploaded with the secret knock. It's not there and it will never be because it's remotely executed without getting w

The "stuff behind the door" is conveniently uploaded with the secret knock. It's not there and it will never be because it's remotely executed without getting written down. The attacker does send executable code, singed and encrypted (or only one of them? It does not matter) with their private key. The door checks anything incoming for a match with the public key it has and executes when happy. C++ has nothing to do with this, it's the dynamic linking mechanism that allows trusted code the things it allows trusted code to do (talking about the hooking that makes the key check possible, not about the execution that comes after - that is even more mundane, code can execute code, it's a von Neumann architecture after all).