Pollard's rho algorithm for logarithms

Pollard's rho algorithm for logarithms is an algorithm introduced by John Pollard in 1978 to solve the discrete logarithm problem, analogous to Pollard's rho algorithm to solve the integer factorization problem.

The goal is to compute such that , where belongs to a cyclic group generated by . The algorithm computes integers , , , and such that . If the underlying group is cyclic of order , is one of the solutions of the equation . Solutions to this equation are easily obtained using the Extended Euclidean algorithm.

To find the needed , , , and the algorithm uses Floyd's cycle-finding algorithm to find a cycle in the sequence , where the function is assumed to be random-looking and thus is likely to enter into a loop after approximately steps. One way to define such a function is to use the following rules: Divide into three disjoint subsets of approximately equal size: , , and . If is in then double both and ; if then increment , if then increment .

AlgorithmEdit

Let   be a cyclic group of order  , and given  , and a partition  , let   be the map and define maps   and   by

 
input: a: a generator of G
       b: an element of G
output: An integer x such that ax = b, or failure

Initialise a0 ← 0, b0 ← 0, x0 ← 1 ∈ G

i ← 1
loop
    xif(xi-1), 
    aig(xi-1, ai-1), 
    bih(xi-1, bi-1)

    x2if(f(x2i-2)), 
    a2ig(f(x2i-2), g(x2i-2, a2i-2)), 
    b2ih(f(x2i-2), h(x2i-2, b2i-2))

    if xi = x2i then
        rbi - b2i
        if r = 0 return failure
        xr−1(a2i - ai) mod p
        return x
    else // xix2i
        ii + 1
    end if
end loop

ExampleEdit

Consider, for example, the group generated by 2 modulo   (the order of the group is  , 2 generates the group of units modulo 1019). The algorithm is implemented by the following C++ program:

#include <stdio.h>

const int n = 1018, N = n + 1;  /* N = 1019 -- prime     */
const int alpha = 2;            /* generator             */
const int beta = 5;             /* 2^{10} = 1024 = 5 (N) */

void new_xab(int& x, int& a, int& b) {
  switch (x % 3) {
  case 0: x = x * x     % N;  a =  a*2  % n;  b =  b*2  % n;  break;
  case 1: x = x * alpha % N;  a = (a+1) % n;                  break;
  case 2: x = x * beta  % N;                  b = (b+1) % n;  break;
  }
}

int main(void) {
  int x = 1, a = 0, b = 0;
  int X = x, A = a, B = b;
  for (int i = 1; i < n; ++i) {
    new_xab(x, a, b);
    new_xab(X, A, B);
    new_xab(X, A, B);
    printf("%3d  %4d %3d %3d  %4d %3d %3d\n", i, x, a, b, X, A, B);
    if (x == X) break;
  }
  return 0;
}

The results are as follows (edited):

 i     x   a   b     X   A   B
------------------------------
 1     2   1   0    10   1   1
 2    10   1   1   100   2   2
 3    20   2   1  1000   3   3
 4   100   2   2   425   8   6
 5   200   3   2   436  16  14
 6  1000   3   3   284  17  15
 7   981   4   3   986  17  17
 8   425   8   6   194  17  19
..............................
48   224 680 376    86 299 412
49   101 680 377   860 300 413
50   505 680 378   101 300 415
51  1010 681 378  1010 301 416

That is   and so  , for which   is a solution as expected. As   is not prime, there is another solution  , for which   holds.

ComplexityEdit

The running time is approximately  . If used together with the Pohlig–Hellman algorithm, the running time of the combined algorithm is  , where   is the largest prime factor of  .

ReferencesEdit

  • Pollard, J. M. (1978). "Monte Carlo methods for index computation (mod p)". Mathematics of Computation. 32 (143): 918–924. doi:10.2307/2006496.
  • Menezes, Alfred J.; van Oorschot, Paul C.; Vanstone, Scott A. (2001). "Chapter 3" (PDF). Handbook of Applied Cryptography.