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# hw2sol - CS 170 Fall 2009 1(10 pts RSA Problem 1.42...

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CS 170 Algorithms Fall 2009 Christos Papadimitriou HW 2 Solutions 1. (10 pts.) RSA Problem 1.42 Solution We know that p is a prime and gcd ( e , p - 1 ) = 1. Using the extended Euclidean algorithm find integers x , y such that xe + y ( p - 1 ) = 1. We then have that m xe + y ( p - 1 ) = m 1 ( mod p ) and using Fermat’s little theorem we get m xe = m ( mod p ) , i.e. ( m e ) x = m ( mod p ) . So the algorithm is as follows: Input: p , e , m e mod p Compute x , y such that xe + y ( p - 1 ) = 1 using extended euclidean algorithm (in Θ ( log ( p - 1 )) steps). Compute and output ( m e ) x mod p (in Θ ( log 3 p ) steps). The correctness of the algorithm follows from the previous discussion and its running time is Θ ( log 3 p ) , clearly polynomial. 2. (15 pts.) RSA Problem 1.43 Solution We first note that we must have 3 d = 1 ( mod ( p - 1 )( q - 1 )) or equivalently 3 d - 1 = k ( p - 1 )( q - 1 ) for some k Z ; we will try to find this k . First note that d < ( p - 1 )( q - 1 ) (since it is the inverse of 3 modulo ( p - 1 )( q - 1 ) ); hence kd < k ( p - 1 )( q - 1 ) = 3 d - 1 < 3 d k < 3. Moreover we have that k > 0 because 3 d - 1 > 0. So the only possible values for k are 1 and 2. Notice now that p 6 = 0 ( mod 3 ) (since p is prime) and p 6 = 1 ( mod 3 ) since 3 has an inverse modulo ( p - 1 )( q - 1 ) and therefore it has to be relatively prime to p - 1. So the only option left is p = 2 ( mod 3 ) . Likewise q = 2 ( mod 3 ) , so we have ( p - 1 )( q - 1 ) = 1 ( mod 3 ) . Suppose now that k = 1; then ( p - 1 )( q - 1 ) = 3 d - 1 = 2 ( mod 3 ) , a contradiction. Hence k = 2. So now we have two equations involving the two unknowns p , q , namely: 3 d - 1 = 2 ( p - 1 )( q - 1 ) and pq = N . We can now solve the first one to get p + q = N + 1 - 3 d - 1 2 , S and end up with two formulas of the form p + q = S and pq = N . We can now find p , q as the roots to the equation x 2 - Sx + N = 0. 3. (10 pts.) Attacking RSA Consider the RSA cryptosystem with N = pq , e , d as usual. Let x = m e ( mod N ) . An RSA-cycle for x is: x , x e , x e 2 , x e 3 ,..., x e t = x and t > 0 is the length of the cycle.

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