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

This assumes the existence of a symmetric cipher that

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Unformatted text preview: ature. 21.3 Schnorr Claus Schnorr’s authentication and signature scheme [1396,1397] gets its security from the difficulty of calculating discrete logarithms. To generate a key pair, first choose two primes, p and q, such that q is a prime factor of p - 1. Then, choose an a not equal to 1, such that aq a 1 (mod p). All these numbers can be common to a group of users and can be freely published. To generate a particular public-key/private-key key pair, choose a random number less than q. This is the private key, s. Then calculate v = a-s mod p. This is the public key. Authentication Protocol (1) Peggy picks a random number, r, less than q, and computes x = ar mod p. This is the preprocessing stage and can be done long before Victor is present. (2) Peggy sends x to Victor. (3) Victor sends Peggy a random number, e, between 0 and 2t - 1. (I’ll discuss t in a moment.) (4) Peggy computes y = (r + se) mod q and sends y to Victor. (5) Victor verifies that x = ayve mod p. The security is based on the parameter t. The difficulty of breaking the algorithm is about 2t. Schnorr recommended that p be about 512 bits, q be about 140 bits, and t be 72. Digital Signature Protocol Schnorr can also be used as a digital signature protocol on a message, M. The public-key/private-key key pair is the same, but we’re now adding a one-way hash function, H(M). (1) Alice picks a random number, r, less than q, and computes x = ar mod p. This computation is the preprocessing stage. (2) Alice concatenates M and x, and hashes the result: e = H(M,x) (3) Alice computes y = (r + se) mod q. The signature is e and y; she sends these to Bob. (4) Bob computes x´ = ayve mod p. He then confirms that the concatenation of M and x´ hashes to e. e = H(M,x´) If it does, he accepts the signature as valid. In his paper, Schnorr cites these novel features of his algorithm: Most of the computation for signature generation can be completed in a preprocessing stage, independent of the message being signed. Hence, it can be done during idle time and not affect the signature speed. An attack...
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