Well, running an ECM with a comparable chance of success takes longer than P-1 factoring. Hence, as long as there are enough candidates, it's best to stay at P-1. ECM comes in handy when a particular candidate has to be factored, especially when looking for a 40+ digit factor. In this range, P-1 factoring gets less efficient...
If you want to try ECM factoring nevertheless, there are basically two (well, three) options to do it:
1a. Use Prime95
This should be the easiest one. Just use the command "ECM2=" in the worktodo.ini. The syntax is1b. Use gmp-ecmECM2=k,b,n,c,B1,B2,curves_to_do[,specific_sigma]
I haven't tested this program with numbers that high, though. And I think that it will be slower, because base 2 factoring attempts are pretty optimized in Prime95. Maybe the better stage2 implementation can gain some performance points again, but this usually levels the playing field at high digit searches (guesstimate: 35+ digits) only.
Syntax:
ecm <number> <B1> [<B2>]
You can also add parameters such as -c <curveCount>.
2. Mix: Prime95 for stage1, gmp-ecm for stage2
The most efficient, but also most complex way. I don't go into further details, as knowing how to do 1a and 1b is recommended...
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Originally Posted by Keroberts1
