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I think we have a difference in threat models. It sounds like you are dealing with a non-adversarial model in which neither party has much of an incentive to c

by evgen 17y ago

I think we have a difference in threat models. It sounds like you are dealing with a non-adversarial model in which neither party has much of an incentive to cheat. If this is the case then your system works fine. If either party gains much by cheating then your system is insufficient to solve the problem.

RiderOfGiraffes 17y ago
You might be right, but I don't understand how either party could cheat. Let's assume that the hash is computationally secure, so that it's effectively impossible to compute a file to match a given hash. SHA-256 seems to me to meet that assumption. I decide whether I want heads or tails, and create a file that starts with my choice, then has random padding. I compute the hash and send it to you. You now tell me openly your call. I believe you have no way to compute from my hash what I've chosen, so to you, my choice is hidden. Whether you guess right is effectively random. Now I reveal the file from which the hash was computed. Again, I can't compute a different file with the same hash, so I'm forced to reveal the choice I made. If you were right we both know it. If you were wrong we both know it. Can you explain how it's possible for either of us to cheat? I appreciate your time - thanks.