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The Coin Tossing Experiment
Topic Started: May 9 2008, 08:06 AM (133 Views)
Revelation May 9 2008, 08:06 AM Post #1
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 After reading a bit in the book Modern Cryptography: Theory and Practice, I've come across a protocol that I find very cool: Coin Flipping Over Telephone.

Imagine Alice wants to flip a coin and Bob may choose if it will be head or tail. If Bob can see the coin, he will know if it is a fair game.

But what would you do when you want to do the same game over the phone? Think about it before reading the answer!



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The problem is, is that you don't know if Alice is lying about the results. So what we have to do is mark the result of the coin flip, but Bob shouldn't be able to see if it was head or tail from that mark. That's when a one-way hash comes in.

1. Alice flips the coin: if it's head, she will use a large even number and encrypt in with the one-way hash function f(x). She gives the result to Bob.
2. Bob, unable to decrypt f(x), makes a guess.
3. Alice tells if Bob is right. If her number was even, and Bob says head, he was right.
4. Alice gives Bob her key, so he can check if she didn't manipulate the experiment.

Of course this one-way hash should be really one way, and the hash should be unique. If an uneven and an even number encrypt to the same hash, Alice can cheat.


I found this a cool experiment, and I doubt that I would have made this up on my own.
RRRREJMEEEEEPVKLWENFNVJKEEEEEAOLKAFKLXCFZAASDJXZTTTTTTTLSIOWJXMOKLAFJNNKFNXN
RAGRBAQEMHIGDJVDSEOXVIYCELFHWLELJFIENXLRATALSJFSLCYTKLASJDKMHGOVOKAJDNMNUITN
RRRRLJVEEEEECLYVYHNVPFTAEEEEEMWLMEIRNGLARWJAKJDFLWNTIERJMIPQWOTZEOCXKNUBNXCN
RJIRPOWEANFUSNCZVDVZNMSFEKLOEPZLDKDJWSAAAAAAAOERHJCTNCKFRIMVKSOFOMKMANREWNBN
RZUDRGXEEEEENFQIDVLQNCKNEEEEEDGLLLLLLAWIOSNCDARLODMTOEJXMILDFJROTKJSDNLVCZNN
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