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

# Not all hard problems can be used for zero knowledge

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Unformatted text preview: the protocol is zero-knowledge. In the case where Peggy did not know the magic words, Victor will obviously not learn anything from watching the recording. But since there is no way to distinguish a real recording from a faked recording, Victor cannot learn anything from the real proof—it must be zero knowledge. The technique used in this protocol is called cut and choose, because of its similarity to the classic protocol for dividing anything fairly: (1) Alice cuts the thing in half. (2) Bob chooses one of the halves for himself. (3) Alice takes the remaining half. It is in Alice’s best interest to divide fairly in step (1), because Bob will choose whichever half he wants in step (2). Michael Rabin was the first person to use the cut-and-choose technique in cryptography [1282]. The concepts of interactive protocol and zero-knowledge were formalized later [626,627]. The cut-and-choose protocol works because there is no way Peggy can repeatedly guess which side Victor will ask her to come out of. If Peggy doesn’t know the secret, she can only come out the way she came in. She has a 50 percent chance of guessing which side Victor will ask in each round (sometimes called an accreditation) of the protocol, so she has a 50 percent chance of fooling him. The chance of her fooling him in two rounds is 25 percent, and the chance of her fooling him all n times is 1 in 2n. After 16 rounds, Peggy has a 1 in 65,536 chance of fooling Victor. Victor can safely assume that if all 16 of Peggy’s proofs are valid, then she must know the secret words to open the door between points C and D. (The cave analogy isn’t perfect. Peggy can simply walk in one side and out the other; there’s no need for any cut-and-choose protocol. However, mathematical zero knowledge requires it.) Assume that Peggy knows some information, and furthermore that the information is the solution to a hard problem. The basic zero-knowledge protocol consists of several rounds. (1) Peggy uses her information and a random number to transform the hard problem into another hard problem, one t...
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