# The running time for the recursive calls depends on

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The running time for the recursive calls depends on the strategy used to choose f . If we always choose f = 1 or f = e - 1, then the running time is for all the recursive calls is O ( e 2 log q · L ( p ) 2 ). However, if we use a “balanced” divide-and-conquer strategy, choosing f e/ 2, then we get O ( e log e log q · L ( p ) 2 ). In summary, the total running time is: O (( eq 1 / 2 + e log e log q ) · L ( p ) 2 ) . 54

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8.2.4 Discrete logarithms in Z * p Suppose that we are given a prime p , along with the prime factorization p - 1 = r Y i =1 q e i i , a generator γ for Z * p , and α Z * p . We wish to compute log γ α . Suppose that α = γ x , where 0 x < p - 1. Then for 1 i r , α ( p - 1) /q e i i = γ ( p - 1) /q e i i x . Note that γ ( p - 1) /q e i i has order q e i i , and if x i is the discrete logarithm of α ( p - 1) /q e i i to the base γ ( p - 1) /q e i i , then we have 0 x i < q e i i and x x i (mod q e i i ). Thus, if we compute the values x 1 , . . . , x r , using the algorithm in § 8.2.3, we can obtain x using the algorithm of the Chinese Remainder Theorem. If we define q := max { q i : 1 i r } , then the running time of this algorithm will be bounded by q 1 / 2 L ( p ) O (1) . 8.3 Further remarks One conclusion to be drawn from the observations in this cahpter is that if all the prime factors of p - 1 are “small,” then the discrete logarithm problem in Z * p is “easy.” The algorithm we have presented here is by no means the fastest. The fastest known algorithm for this problem is based on a technique called the number field sieve , and runs in time exp( L ( P ) 1 / 3 (log L ( P )) 2 / 3 ) . While this running time is still larger than any polynomial in L ( P ), it is still much smaller than that of the simple algorithm presented above. Finally, we remark that all of the algorithms presented in this chapter work in any finite cyclic group — we really did not exploit any properties about Z * p other than the fact that it is a cyclic group. However, faster discrete logarithm algorithms, like those mentioned above based on the number field sieve, do not work in an arbitrary finite cyclic group; these algorithms only work for Z * p , and more generally, for K * , where K is a finite field. 55
Chapter 9 Quadratic Residues and Quadratic Reciprocity 9.1 Quadratic Residues For positive integer n , an integer a is called a quadratic residue modulo n if gcd( a, n ) = 1 and x 2 a (mod n ) for some integer x ; in this case, we say that x is a square root of a modulo n . The quadratic residues modulo n correspond exactly to the subgroup of squares ( Z * n ) 2 of Z * n ; that is, a is a quadratic residue modulo n if and only if [ a mod n ] ( Z * n ) 2 . Let us first consider the case where n = p , where p is an odd prime. In this case, we know that Z * p is cyclic of order p - 1. Recall that the subgroups any finite cyclic group are in one-to-one correspondence with the divisors of the order of the group.

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