mpi: EC point operations [discussion]

[Werner Koch]

On Fri,  8 Mar 2013 23:46, christian at grothoff.org said:

> using libgcrypt; however, the mismatch between the s-expression based public key
> APIs to create signatures (ECDSA) and the low-level MPI-API is pretty rough from a
> user's perspective. 

Agreed.  Right now we only have a low level API for some math fucntions.
This is similar to the RSA and the raw MPI functions.

> +  ctx = gcry_mpi_ec_p_new (P, A);

I had the idea to add some more optional arguments to that function but
then decided against it.  As of now, this function is basically the same
as the internally used one.  A useful and more generic additional
function would be

  ctx = gcry_mpi_ec_new ("(curve \"NIST P256\")");

and for ease of use would also accept the genkey parameter set like

  ctx = gcry_mpi_ec_new ("(genkey(ecdsa(curve \"NIST P256\")))")

this solves parsing the s-exp for P and A.  To retrieve the key
parameters (or whatever) from the context I can image these functions:

  mpi   = gcry_mpi_ec_get_mpi ("d", ctx);
  point = gcry_mpi_ec_get_point ("G", ctx)

This requires additionalallocation etc. but it is a quite generic way to
retrieve the needed values.

> So having a higher-level API that can --- instead of a manually constructed 'ctx' --- take
> the "s_key" S-expression and 'd' and returns 'Q' would certainly be a nice extension.

See above.

> but in cases where they are already in use (i.e. because of gcry_pk_*-operations), it
> would make sense to me to make the transition between the high- and low-level APIs
> as painless as possible.

We have to see how we can optimize that further.

> I wonder if you'd not want some "other" means to initialize that 'ctx', say with a bunch of
> function pointers that point to curve-specific optimized operations.  Right now, the

That would be an internal operation and invisible to the API.  Thus it
can be done at any time without changing the API.

  gcry_mpi_ec_set_mpi ("d", mpi, ctx);
  gcry_mpi_ec_set_point ("G", point, ctx)

might be auseful extension as well.

> p.s.: I didn't see these in Git master; is there a branch I should be
> looking 

I have a private branch for now.  If you think the current interface and
my proposed extensions are okay I can push that to master.


Salam-Shalom,

   Werner

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