Tuesday, November 17, 2015

factoring - Applying the idea of Dixon's method to discrete logs

I want to see if a technique like Dixon's method for integer factorization would work for discrete logs. If the base is 2 (which could be generalized), to find the log of r, set up equations like the following (in which the first 7 primes are the factor base):



rAi12Ai23Ai35Ai47Ai511Ai613Ai717Ai81 mod p



Then use row operations on the rows of the matrix A to find a linear combination of these rows with 0s in every entry other than the first two. This gives an equation of the form rx2y1 mod p, from which it's easy to obtain the log of r base 2.




The question I have is, whether the row operations can be performed so x and y are non-zero mod p1. I don't have much idea how to guarantee this, and there is a strong tendency to get 0s. For example, I picked p=4327 and r=1277, and generated a matrix A in which each row satisfies



1277Ai12Ai23Ai35Ai47Ai511Ai613Ai717Ai81 mod 4327



A=[11113000132101101111001111010020011501011120301111320111111222011]



Lo and behold, the determinant of A is 0 mod p1 (-4326 to be exact). Had this not been the case, an absurd result would have ensued: We could have shown that 1277x2y1 mod p for any ratio x/y of our choosing, by inverting A. So the determinant is bound to be 0 mod p1.

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