[PATCH] Add gcry_mpi_ec_sub.

[Markus Teich]

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.

--Markus

[0] http://cr.yp.to/ecdh/curve25519-20060209.pdf