security/nss/lib/freebl/ecl/ec_naf.c

   1 /*
   2  * ***** BEGIN LICENSE BLOCK *****
   3  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
   4  *
   5  * The contents of this file are subject to the Mozilla Public License Version
   6  * 1.1 (the "License"); you may not use this file except in compliance with
   7  * the License. You may obtain a copy of the License at
   8  * http://www.mozilla.org/MPL/
   9  *
  10  * Software distributed under the License is distributed on an "AS IS" basis,
  11  * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
  12  * for the specific language governing rights and limitations under the
  13  * License.
  14  *
  15  * The Original Code is the elliptic curve math library.
  16  *
  17  * The Initial Developer of the Original Code is
  18  * Sun Microsystems, Inc.
  19  * Portions created by the Initial Developer are Copyright (C) 2003
  20  * the Initial Developer. All Rights Reserved.
  21  *
  22  * Contributor(s):
  23  *   Stephen Fung <fungstep@hotmail.com>, Sun Microsystems Laboratories
  24  *
  25  * Alternatively, the contents of this file may be used under the terms of
  26  * either the GNU General Public License Version 2 or later (the "GPL"), or
  27  * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
  28  * in which case the provisions of the GPL or the LGPL are applicable instead
  29  * of those above. If you wish to allow use of your version of this file only
  30  * under the terms of either the GPL or the LGPL, and not to allow others to
  31  * use your version of this file under the terms of the MPL, indicate your
  32  * decision by deleting the provisions above and replace them with the notice
  33  * and other provisions required by the GPL or the LGPL. If you do not delete
  34  * the provisions above, a recipient may use your version of this file under
  35  * the terms of any one of the MPL, the GPL or the LGPL.
  36  *
  37  * ***** END LICENSE BLOCK ***** */
  38
  39 #include "ecl-priv.h"
  40
  41 /* Returns 2^e as an integer. This is meant to be used for small powers of
  42  * two. */
  43 int
  44 ec_twoTo(int e)
  45 {
  46         int a = 1;
  47         int i;
  48
  49         for (i = 0; i < e; i++) {
  50                 a *= 2;
  51         }
  52         return a;
  53 }
  54
  55 /* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
  56  * be an array of signed char's to output to, bitsize should be the number
  57  * of bits of out, in is the original scalar, and w is the window size.
  58  * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
  59  * Menezes, "Software implementation of elliptic curve cryptography over
  60  * binary fields", Proc. CHES 2000. */
  61 mp_err
  62 ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
  63 {
  64         mp_int k;
  65         mp_err res = MP_OKAY;
  66         int i, twowm1, mask;
  67
  68         twowm1 = ec_twoTo(w - 1);
  69         mask = 2 * twowm1 - 1;
  70
  71         MP_DIGITS(&k) = 0;
  72         MP_CHECKOK(mp_init_copy(&k, in));
  73
  74         i = 0;
  75         /* Compute wNAF form */
  76         while (mp_cmp_z(&k) > 0) {
  77                 if (mp_isodd(&k)) {
  78                         out[i] = MP_DIGIT(&k, 0) & mask;
  79                         if (out[i] >= twowm1)
  80                                 out[i] -= 2 * twowm1;
  81
  82                         /* Subtract off out[i].  Note mp_sub_d only works with
  83                          * unsigned digits */
  84                         if (out[i] >= 0) {
  85                                 mp_sub_d(&k, out[i], &k);
  86                         } else {
  87                                 mp_add_d(&k, -(out[i]), &k);
  88                         }
  89                 } else {
  90                         out[i] = 0;
  91                 }
  92                 mp_div_2(&k, &k);
  93                 i++;
  94         }
  95         /* Zero out the remaining elements of the out array. */
  96         for (; i < bitsize + 1; i++) {
  97                 out[i] = 0;
  98         }
  99   CLEANUP:
 100         mp_clear(&k);
 101         return res;
 102
 103 }