ecc: New API function gcry_mpi_ec_decode_point.

* mpi/ec.c (_gcry_mpi_ec_decode_point): New.
* cipher/ecc-common.h: Move two prototypes to ...
* src/ec-context.h: here.
* src/gcrypt.h.in (gcry_mpi_ec_decode_point): New.
* src/libgcrypt.def (gcry_mpi_ec_decode_point): New.
* src/libgcrypt.vers (gcry_mpi_ec_decode_point): New.
* src/visibility.c (gcry_mpi_ec_decode_point): New.
* src/visibility.h: Add new function.
--

This new function make the use of the gcry_mpi_ec_curve_point function
possible in many contexts.  Here is a code snippet which could be used
in gpg to check a point:

static gpg_error_t
check_point (PKT_public_key *pk, gcry_mpi_t m_point)
{
  gpg_error_t err;
  char *curve;
  gcry_ctx_t gctx = NULL;
  gcry_mpi_point_t point = NULL;

  /* Get the curve name from the first OpenPGP key parameter.  */
  curve = openpgp_oid_to_str (pk->pkey[0]);
  if (!curve)
    {
      err = gpg_error_from_syserror ();
      goto leave;
    }

  point = gcry_mpi_point_new (0);
  if (!point)
    {
      err = gpg_error_from_syserror ();
      goto leave;
    }

  err = gcry_mpi_ec_new (&gctx, NULL, curve);
  if (err)
    goto leave;

  err = gcry_mpi_ec_decode_point (point, m_point, gctx);
  if (err)
    goto leave;

  if (!gcry_mpi_ec_curve_point (point, gctx))
    err = gpg_error (GPG_ERR_BAD_DATA);

 leave:
  gcry_ctx_release (gctx);
  gcry_mpi_point_release (point);
  xfree (curve);
  return err;
}

Signed-off-by: Werner Koch <wk@gnupg.org>

1 files changed, 13 insertions, 0 deletions

diff --git a/doc/gcrypt.texi b/doc/gcrypt.texi
index 39c7c9f5..bbaaac62 100644
--- a/doc/gcrypt.texi
+++ b/doc/gcrypt.texi
@@ -4882,6 +4882,19 @@ Valid names are the point parameters of an elliptic curve
 (@pxref{ecc_keyparam}).
 @end deftypefun
 
+@deftypefun gpg_err_code_t gcry_mpi_ec_decode_point ( @
+ @w{mpi_point_t @var{result}}, @w{gcry_mpi_t @var{value}}, @
+ @w{gcry_ctx_t @var{ctx}})
+
+Decode the point given as an MPI in @var{value} and store at
+@var{result}.  To decide which encoding is used the function takes a
+context @var{ctx} which can be created with @code{gcry_mpi_ec_new}.
+If @code{NULL} is given for the context the function assumes a 0x04
+prefixed uncompressed encoding.  On error an error code is returned
+and @var{result} might be changed.
+@end deftypefun
+
+
 @deftypefun int gcry_mpi_ec_get_affine ( @
  @w{gcry_mpi_t @var{x}}, @w{gcry_mpi_t @var{y}}, @
  @w{gcry_mpi_point_t @var{point}}, @w{gcry_ctx_t @var{ctx}})