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Pollard's rho algorithm for logarithms

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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 , by substituting as and noting that two powers are equal if and only if the exponents are equivalent modulo the order of the base, in this case modulo , we get that 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 of approximate length after steps. One way to define such a function is to use the following rules: Partition into three disjoint subsets , , and of approximately equal size using a hash function. If is in then double both and ; if then increment , if then increment .

Algorithm

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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 i ← 0, a0 ← 0, b0 ← 0, x0 ← 1 ∈ G

loop
    ii + 1

    xif(xi−1),
    aig(xi−1, ai−1),
    bih(xi−1, bi−1)

    x2i−1f(x2i−2),
    a2i−1g(x2i−2, a2i−2),
    b2i−1h(x2i−2, b2i−2)
    x2if(x2i−1),
    a2ig(x2i−1, a2i−1),
    b2ih(x2i−1, b2i−1)
while xix2i

rbib2i
if r = 0 return failure
return r−1(a2iai) mod n

Example

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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.

Complexity

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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 .

References

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