lib/nettle/ecc_map.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 /* Based on public domain code of LibTomCrypt by Tom St Denis.
  22  * Adapted to gmp and nettle by Nikos Mavrogiannopoulos.
  23  */
  24
  25 #include "ecc.h"
  26
  27 /*
  28   @file ecc_map.c
  29   ECC Crypto, Tom St Denis
  30 */
  31
  32 /*
  33   Map a projective jacobian point back to affine space
  34   @param P        [in/out] The point to map
  35   @param modulus  The modulus of the field the ECC curve is in
  36   @param mp       The "b" value from montgomery_setup()
  37   @return 0 on success
  38 */
  39 int
  40 ecc_map (ecc_point * P, mpz_t modulus)
  41 {
  42   mpz_t t1, t2;
  43   int err;
  44
  45   if (P == NULL)
  46     return -1;
  47
  48   if ((err = mp_init_multi (&t1, &t2, NULL)) != 0)
  49     {
  50       return -1;
  51     }
  52
  53   mpz_mod (P->z, P->z, modulus);
  54
  55   /* get 1/z */
  56   mpz_invert (t1, P->z, modulus);
  57
  58   /* get 1/z^2 and 1/z^3 */
  59   mpz_mul (t2, t1, t1);
  60   mpz_mod (t2, t2, modulus);
  61   mpz_mul (t1, t1, t2);
  62   mpz_mod (t1, t1, modulus);
  63
  64   /* multiply against x/y */
  65   mpz_mul (P->x, P->x, t2);
  66   mpz_mod (P->x, P->x, modulus);
  67   mpz_mul (P->y, P->y, t1);
  68   mpz_mod (P->y, P->y, modulus);
  69   mpz_set_ui (P->z, 1);
  70
  71   err = 0;
  72
  73   mp_clear_multi (&t1, &t2, NULL);
  74   return err;
  75 }
  76
  77 /* $Source: /cvs/libtom/libtomcrypt/src/pk/ecc/ecc_map.c,v $ */
  78 /* $Revision: 1.7 $ */
  79 /* $Date: 2007/05/12 14:32:35 $ */