Unformatted text preview: ce signs the second letter of her name and passes the contract to Bob. (4) Bob signs the second letter of his name and passes the contract to Alice. (5) This continues until both Alice and Bob have signed their entire names. If you ignore the obvious problem with this protocol (Alice has a longer name than Bob), it works just fine. After signing only one letter, Alice knows that no judge will bind her to the terms of the contract. But the letter is an act of good faith, and Bob responds with a similar act of good faith. After each party has signed several letters, a judge could probably be convinced that both parties had signed the contract. The details are murky, though. Surely they are not bound after only the first letter; just as surely they are bound after they sign their entire names. At what point in the protocol do they become bound? After signing one-half of their names? Two-thirds of their names? Three-quarters? Since neither Alice nor Bob is certain of the exact point at which she or he is bound, each has at least some fear that she or he is bound throughout the protocol. At no point can Bob say: “You signed four letters and I only signed three. You are bound but I am not.” Bob has no reason not to continue with the protocol. Furthermore, the longer they continue, the greater the probability that a judge will rule that they are bound. Again, there is no reason not to continue with the protocol. After all, they both wanted to sign the contract; they just didn’t want to sign before the other one. Simultaneous Contract Signing without an Arbitrator (Not Face-to-Face)
This protocol uses the same sort of uncertainty . Alice and Bob alternate taking baby steps toward signing until both have signed. In the protocol, Alice and Bob exchange a series of signed messages of the form: “I agree that with probability p, I am bound by this contract.” The recipient of this message can take it to a judge and, with probability p, the judge will consider the contract to be...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips