requesting another ECC function...

[Werner Koch]

On Thu, 21 Mar 2013 13:39, grothoff at in.tum.de said:

> I can manipulate 'Q' freely) to an S-expression.  So what I need is
> something like a function "gcry_sexp_from_ec_context", to be used as
> follows:

Here we go:  Instead of

>   if (0 != (rc = gcry_sexp_from_ec_context (&pk_sexpr, ctx)))
>   {
>     LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_sexp_from_context", rc);
>     gcry_ctx_release (ctx);
>     gcry_sexp_release (data);
>     gcry_sexp_release (sig_sexpr);
>     return GNUNET_SYSERR;
>   }

use this:

   if (0 != (rc = gcry_pubkey_get_sexp (&pk_sexpr, GCRY_PK_GET_PUBKEY, ctx)))
   {
     LOG_GCRY (GNUNET_ERROR_TYPE_ERROR, "gcry_pubkey_get_sexp", rc);
     gcry_ctx_release (ctx);
     gcry_sexp_release (data);
     gcry_sexp_release (sig_sexpr);
     return GNUNET_SYSERR;
   }

Instead of GCRY_PK_GET_PUBKEY you might simply use 0, but that would
return the private key if D is available in the contetx.  Thus better
request the public key explicit.  Note that there are a few new error
code, thus a gpg_strerror would return "unknown error code".  If in
doubt look at the number:

#define GPG_ERR_NO_CRYPT_CTX	   191
#define GPG_ERR_WRONG_CRYPT_CTX    192
#define GPG_ERR_BAD_CRYPT_CTX	   193
#define GPG_ERR_CRYPT_CTX_CONFLICT 194
#define GPG_ERR_BROKEN_PUBKEY      195
#define GPG_ERR_BROKEN_SECKEY      196

You also need libgpg-error 1.11 now.  Check out tests/t-mpi-point.c if
you need examples.

> Also, I noticed that there is point_get_affine, but no point_set_affine.
> As creating an MPI with value "1" is inconvenient, it might be nice
> (also for symmetry) to have a gcry_point_set_affine (x,y) API as well.

I'll look at this later.


Salam-Shalom,

   Werner

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