Asking with a hash, not for a hash, if we're going that way... I'm surprised to read replies like this here. The question is about possible protocols, devising which is a part of the problem, and you're asking me if I mean HTTP or FTP. Maybe try thinking this way: client has a hypothesis, sends a hash of it to the server. Server confirms. Then the client sends a hash of the hypothesis but reversed to avoid hash collision, and so on until certainty. Of course hashing is just, again, only symbolic here. It would have to be something more intelligent and tailor made for this problem in particular.