April 30, 2010
We have now discussed several different factorization algorithms: trial division, wheel factorization, Fermat’s method, Pollard’s rho method, Pollard’s p-1 method in both one-stage and two-stage variants, and the elliptic curve method in both one–stage and two–stage variants. So, if you must factor a large integer, where do you start?
The basic idea is to perform several methods, starting with simple methods and working up to more complicated ones until the factorization is determined. The exact choice of the methods to be used and the decision on when to abandon one method in favor of another is idiosyncratic at best. There are many possibilities, and many degrees of freedom, that suggest one alternative or another.
Your task is to write a program that factors large integers using some mix of the factorization methods we have studied. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.