lib/nettle/ecc_projective_isneutral.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    Check if the given point is the neutral point
  27    @param P        The point to check
  28    @param modulus  The modulus of the field the ECC curve is in
  29    @return         0 if given point is a neutral point
  30    @return         1 if given point is not a neutral point
  31    @return         negative value in case of error
  32 */
  33 int
  34 ecc_projective_isneutral (ecc_point * P, mpz_t modulus)
  35 {
  36   mpz_t t1, t2;
  37   int err;
  38
  39   if (P == NULL || modulus == NULL)
  40     return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
  41
  42   /*
  43    * neutral point is a point with projective
  44    * coordinates (x,y,0) such that y^2 == x^3
  45    * excluding point (0,0,0)
  46    */
  47   if (mpz_sgn (P->z))
  48     /* Z != 0 */
  49     return 1;
  50
  51   if ((err = mp_init_multi (&t1, &t2, NULL)) != 0)
  52     {
  53       return err;
  54     }
  55
  56   /* t1 == x^3 */
  57   mpz_mul (t1, P->x, P->x);
  58   mpz_mod (t1, t1, modulus);
  59   mpz_mul (t1, t1, P->x);
  60   mpz_mod (t1, t1, modulus);
  61   /* t2 == y^2 */
  62   mpz_mul (t2, P->y, P->y);
  63   mpz_mod (t2, t2, modulus);
  64
  65   if ((!mpz_cmp (t1, t2)) && (mpz_sgn (t1)))
  66     {
  67       /* Z == 0 and X^3 == Y^2 != 0
  68        * it is neutral */
  69       err = 0;
  70       goto done;
  71     }
  72
  73   /* Z == 0 and X^3 != Y^2 or
  74    * Z == X == Y == 0
  75    * this should never happen */
  76   err = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
  77   goto done;
  78 done:
  79   mp_clear_multi (&t1, &t2, NULL);
  80   return err;
  81 }