ec subtract

[NIIBE Yutaka]

Hello,

On 2014-07-10 at 19:35 +0200, Christian Grothoff wrote:
> Sounds like a question for libgcrypt hackers.  Extending the API to
> make it trivial to invert a point would make sense IMO.

As I only have experience of GnuPG, I'm not sure what kind of API is
better for libgcrypt, but, I'm just considering making
gcry_mpi_ec_add_points to private function (i.e., opposite direction).

So, I'm interested in your situation.

On 07/10/2014 07:14 PM, Markus Teich wrote:
> for the ECBD I need to subtract two points on the curve (Z_{i+1} - Z_{i-1}). I
> found out[0], that I have to invert the y value of the second point to achieve
> this.

I wonder if you can explain ECBD.  If its some crypto protocol, isn't
it better to be implemented within libgcrypt?

My background is supporting Curve25519, ECDH on Montgomery curve.  In
its computation, we don't care Y-coordinate, and gcry_mpi_ec_add_points
has to return an error as "not supported by the curve".
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