ecc_decrypt_raw (gcry_sexp_t *r_plain, gcry_sexp_t s_data, gcry_sexp_t keyparms)
{
  unsigned int nbits;
  gpg_err_code_t rc;
  struct pk_encoding_ctx ctx;
  gcry_sexp_t l1 = NULL;
  gcry_mpi_t data_e = NULL;
  ECC_secret_key sk;
  gcry_mpi_t mpi_g = NULL;
  char *curvename = NULL;
  mpi_ec_t ec = NULL;
  mpi_point_struct kG;
  mpi_point_struct R;
  gcry_mpi_t r = NULL;
  int flags = 0;

  memset (&sk, 0, sizeof sk);
  point_init (&kG);
  point_init (&R);

  _gcry_pk_util_init_encoding_ctx (&ctx, PUBKEY_OP_DECRYPT,
                                   (nbits = ecc_get_nbits (keyparms)));

  /* Look for flags. */
  l1 = sexp_find_token (keyparms, "flags", 0);
  if (l1)
    {
      rc = _gcry_pk_util_parse_flaglist (l1, &flags, NULL);
      if (rc)
        goto leave;
    }
  sexp_release (l1);
  l1 = NULL;

  /*
   * Extract the data.
   */
  rc = _gcry_pk_util_preparse_encval (s_data, ecc_names, &l1, &ctx);
  if (rc)
    goto leave;
  rc = sexp_extract_param (l1, NULL, "e", &data_e, NULL);
  if (rc)
    goto leave;
  if (DBG_CIPHER)
    log_printmpi ("ecc_decrypt  d_e", data_e);
  if (mpi_is_opaque (data_e))
    {
      rc = GPG_ERR_INV_DATA;
      goto leave;
    }

  /*
   * Extract the key.
   */
  rc = sexp_extract_param (keyparms, NULL, "-p?a?b?g?n?h?+d",
                           &sk.E.p, &sk.E.a, &sk.E.b, &mpi_g, &sk.E.n,
                           &sk.E.h, &sk.d, NULL);
  if (rc)
    goto leave;
  if (mpi_g)
    {
      point_init (&sk.E.G);
      rc = _gcry_ecc_os2ec (&sk.E.G, mpi_g);
      if (rc)
        goto leave;
    }
  /* Add missing parameters using the optional curve parameter.  */
  sexp_release (l1);
  l1 = sexp_find_token (keyparms, "curve", 5);
  if (l1)
    {
      curvename = sexp_nth_string (l1, 1);
      if (curvename)
        {
          rc = _gcry_ecc_fill_in_curve (0, curvename, &sk.E, NULL);
          if (rc)
            goto leave;
        }
    }
  /* Guess required fields if a curve parameter has not been given.  */
  if (!curvename)
    {
      sk.E.model = MPI_EC_WEIERSTRASS;
      sk.E.dialect = ECC_DIALECT_STANDARD;
      if (!sk.E.h)
	sk.E.h = mpi_const (MPI_C_ONE);
    }
  if (DBG_CIPHER)
    {
      log_debug ("ecc_decrypt info: %s/%s\n",
                 _gcry_ecc_model2str (sk.E.model),
                 _gcry_ecc_dialect2str (sk.E.dialect));
      if (sk.E.name)
        log_debug  ("ecc_decrypt name: %s\n", sk.E.name);
      log_printmpi ("ecc_decrypt    p", sk.E.p);
      log_printmpi ("ecc_decrypt    a", sk.E.a);
      log_printmpi ("ecc_decrypt    b", sk.E.b);
      log_printpnt ("ecc_decrypt  g",   &sk.E.G, NULL);
      log_printmpi ("ecc_decrypt    n", sk.E.n);
      log_printmpi ("ecc_decrypt    h", sk.E.h);
      if (!fips_mode ())
        log_printmpi ("ecc_decrypt    d", sk.d);
    }
  if (!sk.E.p || !sk.E.a || !sk.E.b || !sk.E.G.x || !sk.E.n || !sk.E.h || !sk.d)
    {
      rc = GPG_ERR_NO_OBJ;
      goto leave;
    }


  ec = _gcry_mpi_ec_p_internal_new (sk.E.model, sk.E.dialect, flags,
                                    sk.E.p, sk.E.a, sk.E.b);

  /*
   * Compute the plaintext.
   */
  if (ec->model == MPI_EC_MONTGOMERY)
    rc = _gcry_ecc_mont_decodepoint (data_e, ec, &kG);
  else
    rc = _gcry_ecc_os2ec (&kG, data_e);
  if (rc)
    goto leave;

  if (DBG_CIPHER)
    log_printpnt ("ecc_decrypt    kG", &kG, NULL);

  if (!(flags & PUBKEY_FLAG_DJB_TWEAK)
      /* For X25519, by its definition, validation should not be done.  */
      && !_gcry_mpi_ec_curve_point (&kG, ec))
    {
      rc = GPG_ERR_INV_DATA;
      goto leave;
    }

  /* R = dkG */
  _gcry_mpi_ec_mul_point (&R, sk.d, &kG, ec);

  /* The following is false: assert( mpi_cmp_ui( R.x, 1 )==0 );, so:  */
  {
    gcry_mpi_t x, y;

    x = mpi_new (0);
    if (ec->model == MPI_EC_MONTGOMERY)
      y = NULL;
    else
      y = mpi_new (0);

    if (_gcry_mpi_ec_get_affine (x, y, &R, ec))
      {
        rc = GPG_ERR_INV_DATA;
        goto leave;
        /*
         * Note for X25519.
         *
         * By the definition of X25519, this is the case where X25519
         * returns 0, mapping infinity to zero.  However, we
         * deliberately let it return an error.
         *
         * For X25519 ECDH, comming here means that it might be
         * decrypted by anyone with the shared secret of 0 (the result
         * of this function could be always 0 by other scalar values,
         * other than the private key of SK.D).
         *
         * So, it looks like an encrypted message but it can be
         * decrypted by anyone, or at least something wrong
         * happens.  Recipient should not proceed as if it were
         * properly encrypted message.
         *
         * This handling is needed for our major usage of GnuPG,
         * where it does the One-Pass Diffie-Hellman method,
         * C(1, 1, ECC CDH), with an ephemeral key.
         */
      }

    if (y)
      r = _gcry_ecc_ec2os (x, y, sk.E.p);
    else
      {
        unsigned char *rawmpi;
        unsigned int rawmpilen;

        rawmpi = _gcry_mpi_get_buffer_extra (x, nbits/8, -1,
                                             &rawmpilen, NULL);
        if (!rawmpi)
          {
            rc = gpg_err_code_from_syserror ();
            goto leave;
          }
        else
          {
            rawmpi[0] = 0x40;
            rawmpilen++;
            r = mpi_new (0);
            mpi_set_opaque (r, rawmpi, rawmpilen*8);
          }
      }
    if (!r)
      rc = gpg_err_code_from_syserror ();
    else
      rc = 0;
    mpi_free (x);
    mpi_free (y);
  }
  if (DBG_CIPHER)
    log_printmpi ("ecc_decrypt  res", r);

  if (!rc)
    rc = sexp_build (r_plain, NULL, "(value %m)", r);

 leave:
  point_free (&R);
  point_free (&kG);
  _gcry_mpi_release (r);
  _gcry_mpi_release (sk.E.p);
  _gcry_mpi_release (sk.E.a);
  _gcry_mpi_release (sk.E.b);
  _gcry_mpi_release (mpi_g);
  point_free (&sk.E.G);
  _gcry_mpi_release (sk.E.n);
  _gcry_mpi_release (sk.E.h);
  _gcry_mpi_release (sk.d);
  _gcry_mpi_release (data_e);
  xfree (curvename);
  sexp_release (l1);
  _gcry_mpi_ec_free (ec);
  _gcry_pk_util_free_encoding_ctx (&ctx);
  if (DBG_CIPHER)
    log_debug ("ecc_decrypt    => %s\n", gpg_strerror (rc));
  return rc;
}