lib/nettle/ecc_projective_check_point.c

   1 /*
   2  * Copyright (C) 2011-2012 Free Software Foundation, Inc.
   3  *
   4  * This file is part of GNUTLS.
   5  *
   6  * The GNUTLS library is free software; you can redistribute it and/or
   7  * modify it under the terms of the GNU Lesser General Public License
   8  * as published by the Free Software Foundation; either version 3 of
   9  * the License, or (at your option) any later version.
  10  *
  11  * This library is distributed in the hope that it will be useful, but
  12  * WITHOUT ANY WARRANTY; without even the implied warranty of
  13  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14  * Lesser General Public License for more details.
  15  *
  16  * You should have received a copy of the GNU Lesser General Public License
  17  * along with this program.  If not, see <http://www.gnu.org/licenses/>
  18  *
  19  */
  20
  21 #include "ecc.h"
  22 #include <gnutls_errors.h>
  23
  24 #ifdef ECC_SECP_CURVES_ONLY
  25
  26 /*
  27   @file ecc_projective_check_point.c
  28 */
  29
  30 /*
  31    Checks whether a point lies on the curve y^2 = x^3 - 3x + b
  32    @param P   The point to check
  33    @param modulus  The modulus of the field the ECC curve is in
  34    @param b       The "B" value of the curve
  35    @return 0 on success
  36 */
  37 int ecc_projective_check_point (ecc_point * P, mpz_t b, mpz_t modulus)
  38 {
  39     mpz_t t1, t2, t3;
  40     int err;
  41
  42     if (P == NULL || b == NULL || modulus == NULL)
  43         return -1;
  44
  45     if (mpz_cmp_ui (P->z, 1) != 0)
  46       {
  47           gnutls_assert ();
  48           return -1;
  49       }
  50
  51     if ((err = mp_init_multi (&t1, &t2, &t3, NULL)) != 0)
  52       {
  53           return err;
  54       }
  55
  56     /* t1 = Z * Z */
  57     mpz_mul (t1, P->y, P->y);
  58     mpz_mod (t1, t1, modulus);  /* t1 = y^2 */
  59
  60     mpz_mul (t2, P->x, P->x);
  61     mpz_mod (t2, t2, modulus);
  62
  63     mpz_mul (t2, P->x, t2);
  64     mpz_mod (t2, t2, modulus);  /* t2 = x^3 */
  65
  66     mpz_add (t3, P->x, P->x);
  67     if (mpz_cmp (t3, modulus) >= 0)
  68       {
  69           mpz_sub (t3, t3, modulus);
  70       }
  71
  72     mpz_add (t3, t3, P->x);     /* t3 = 3x */
  73     if (mpz_cmp (t3, modulus) >= 0)
  74       {
  75           mpz_sub (t3, t3, modulus);
  76       }
  77
  78     mpz_sub (t1, t1, t2);       /* t1 = y^2 - x^3 */
  79     if (mpz_cmp_ui (t1, 0) < 0)
  80       {
  81           mpz_add (t1, t1, modulus);
  82       }
  83
  84     mpz_add (t1, t1, t3);       /* t1 = y^2 - x^3 + 3x */
  85     if (mpz_cmp (t1, modulus) >= 0)
  86       {
  87           mpz_sub (t1, t1, modulus);
  88       }
  89
  90     mpz_sub (t1, t1, b);        /* t1 = y^2 - x^3 + 3x - b */
  91     if (mpz_cmp_ui (t1, 0) < 0)
  92       {
  93           mpz_add (t1, t1, modulus);
  94       }
  95
  96     if (mpz_cmp_ui (t1, 0) != 0)
  97       {
  98           err = -1;
  99       }
 100     else
 101       {
 102           err = 0;
 103       }
 104
 105     mp_clear_multi(&t1, &t2, &t3, NULL);
 106
 107     return err;
 108 }
 109
 110 #endif