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, 21 insertions, 0 deletions

diff --git a/mpi/ec.c b/mpi/ec.c
index 40e09bed..346e5f1e 100644
--- a/mpi/ec.c
+++ b/mpi/ec.c
@@ -589,6 +589,27 @@ _gcry_mpi_ec_set_point (const char *name, gcry_mpi_point_t newvalue,
 }
 
 
+/* Given an encoded point in the MPI VALUE and a context EC, decode
+ * the point according to the context and store it in RESULT.  On
+ * error an error code is return but RESULT might have been changed.
+ * If no context is given the function tries to decode VALUE by
+ * assuming a 0x04 prefixed uncompressed encoding.  */
+gpg_err_code_t
+_gcry_mpi_ec_decode_point (mpi_point_t result, gcry_mpi_t value, mpi_ec_t ec)
+{
+  gcry_err_code_t rc;
+
+  if (ec && ec->dialect == ECC_DIALECT_ED25519)
+    rc = _gcry_ecc_eddsa_decodepoint (value, ec, result, NULL, NULL);
+  else if (ec && ec->model == MPI_EC_MONTGOMERY)
+    rc = _gcry_ecc_mont_decodepoint (value, ec, result);
+  else
+    rc = _gcry_ecc_os2ec (result, value);
+
+  return rc;
+}
+
+
 /* Compute the affine coordinates from the projective coordinates in
    POINT.  Set them into X and Y.  If one coordinate is not required,
    X or Y may be passed as NULL.  CTX is the usual context. Returns: 0