[PATCH] Add gcry_mpi_ec_sub.

[Ian Goldberg]

On Tue, Jul 22, 2014 at 10:50:43AM +0200, Markus Teich wrote:
> Ian Goldberg wrote:
> > Wait: isn't the negation of a point (x,y) on a twisted Edwards curve
> > the point (-x,y)?  You're calculating (x,-y) above, it seems to me.
> 
> Heyho,
> 
> indeed I am calculating (x,-y). I took Appendix A from the Curve 25519 paper[0]
> as reference. If that is indeed wrong, I will of course amend the patch.

To be sure, give it a try: use your code to subtract a random point from
itself, and see what you get?

   - Ian