lib/nettle/ecc_projective_negate_point.c

   1 /*
   2  * Copyright (C) 2011-2012 Free Software Foundation, Inc.
   3  *
   4  * Author: Ilya Tumaykin
   5  *
   6  * This file is part of GNUTLS.
   7  *
   8  * The GNUTLS library is free software; you can redistribute it and/or
   9  * modify it under the terms of the GNU Lesser General Public License
  10  * as published by the Free Software Foundation; either version 3 of
  11  * the License, or (at your option) any later version.
  12  *
  13  * This library is distributed in the hope that it will be useful, but
  14  * WITHOUT ANY WARRANTY; without even the implied warranty of
  15  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  16  * Lesser General Public License for more details.
  17  *
  18  * You should have received a copy of the GNU Lesser General Public License
  19  * along with this program.  If not, see <http://www.gnu.org/licenses/>
  20  *
  21  */
  22
  23 #include "ecc.h"
  24
  25 /*
  26    Negate an ECC point
  27    @param P        The point to negate
  28    @param R        [out] The destination of the negate
  29    @param modulus  The modulus of the field the ECC curve is in
  30    @return         GNUTLS_E_SUCCESS on success
  31 */
  32 int
  33 ecc_projective_negate_point (ecc_point * P, ecc_point * R, mpz_t modulus)
  34 {
  35
  36   if (P == NULL || R == NULL)
  37     return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
  38
  39   if (ecc_projective_isneutral (P, modulus))
  40     {
  41       /* we set R.y to (modulus - P.y) to avoid negative coordinates */
  42       mpz_set (R->x, P->x);
  43       mpz_sub (R->y, modulus, P->y);
  44       mpz_mod (R->y, R->y, modulus);
  45       mpz_set (R->z, P->z);
  46     }
  47   else
  48     {
  49       /* -neutral = neutral */
  50       mpz_set_ui (R->x, 1);
  51       mpz_set_ui (R->y, 1);
  52       mpz_set_ui (R->z, 0);
  53     }
  54
  55   return GNUTLS_E_SUCCESS;
  56 }