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/*
* Copyright (C) 2011-2012 Free Software Foundation, Inc.
*
* Author: Ilya Tumaykin
*
* This file is part of GNUTLS.
*
* The GNUTLS library is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public License
* as published by the Free Software Foundation; either version 3 of
* the License, or (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>
*
*/
#include "ecc.h"
/*
Check if the given point is the neutral point
@param P The point to check
@param modulus The modulus of the field the ECC curve is in
@return 0 if given point is a neutral point
@return 1 if given point is not a neutral point
@return negative value in case of error
*/
int
ecc_projective_isneutral (ecc_point * P, mpz_t modulus)
{
mpz_t t1, t2;
int err;
if (P == NULL || modulus == NULL)
return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
/*
* neutral point is a point with projective
* coordinates (x,y,0) such that y^2 == x^3
* excluding point (0,0,0)
*/
if (mpz_sgn (P->z))
/* Z != 0 */
return 1;
if ((err = mp_init_multi (&t1, &t2, NULL)) != 0)
{
return err;
}
/* t1 == x^3 */
mpz_mul (t1, P->x, P->x);
mpz_mod (t1, t1, modulus);
mpz_mul (t1, t1, P->x);
mpz_mod (t1, t1, modulus);
/* t2 == y^2 */
mpz_mul (t2, P->y, P->y);
mpz_mod (t2, t2, modulus);
if ((!mpz_cmp (t1, t2)) && (mpz_sgn (t1)))
{
/* Z == 0 and X^3 == Y^2 != 0
* it is neutral */
err = 0;
goto done;
}
/* Z == 0 and X^3 != Y^2 or
* Z == X == Y == 0
* this should never happen */
err = GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER;
goto done;
done:
mp_clear_multi (&t1, &t2, NULL);
return err;
}
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