applied cryptography - protocols, algorithms, and source code in c

# If alice were very clever she could only disrupt half

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Unformatted text preview: e that she is bound with 2 percent probability. Bob might respond that he is bound with 3 percent probability. Alice’s next message might state that she is bound with 5 percent probability and so on, until both are bound with 100 percent probability. If both Alice and Bob complete the protocol by the completion date, all is well. Otherwise, either party can take the contract to the judge, along with the other party’s last signed message. The judge then randomly chooses a value between 0 and 1 before seeing the contract. If the value is less than the probability the other party signed, then both parties are bound. If the value is greater than the probability, then both parties are not bound. (The judge then saves the value, in case he has to rule on another matter regarding the same contract.) This is what is meant by being bound to the contract with probability p. That’s the basic protocol, but it can have more complications. The judge can rule in the absence of one of the parties. The judge’s ruling either binds both or neither party; in no situation is one party bound and the other one not. Furthermore, as long as one party is willing to have a slightly higher probability of being bound than the other (no matter how small), the protocol will terminate. Simultaneous Contract Signing without an Arbitrator (Using Cryptography) This cryptographic protocol uses the same baby-step approach [529]. DES is used in the description, although any symmetric algorithm will do. (1) Both Alice and Bob randomly select 2n DES keys, grouped in pairs. The pairs are nothing special; they are just grouped that way for the protocol. (2) Both Alice and Bob generate n pairs of messages, Li and Ri: “This is the left half of my ith signature” and “This is the right half of my ith signature,” for example. The identifier, i, runs from 1 to n. Each message will probably also include a digital signature of the contract and a timestamp. The contract is considered signed if the other party can produce both halves, Li and Ri,...
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