1/*
2 * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
3 * Use is subject to license terms.
4 *
5 * This library is free software; you can redistribute it and/or
6 * modify it under the terms of the GNU Lesser General Public
7 * License as published by the Free Software Foundation; either
8 * version 2.1 of the License, or (at your option) any later version.
9 *
10 * This library is distributed in the hope that it will be useful,
11 * but WITHOUT ANY WARRANTY; without even the implied warranty of
12 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
13 * Lesser General Public License for more details.
14 *
15 * You should have received a copy of the GNU Lesser General Public License
16 * along with this library; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 */
23
24/* *********************************************************************
25 *
26 * The Original Code is the elliptic curve math library.
27 *
28 * The Initial Developer of the Original Code is
29 * Sun Microsystems, Inc.
30 * Portions created by the Initial Developer are Copyright (C) 2003
31 * the Initial Developer. All Rights Reserved.
32 *
33 * Contributor(s):
34 *   Stephen Fung <fungstep@hotmail.com>, Sun Microsystems Laboratories
35 *
36 *********************************************************************** */
37
38#include "ecl-priv.h"
39
40/* Returns 2^e as an integer. This is meant to be used for small powers of
41 * two. */
42int
43ec_twoTo(int e)
44{
45        int a = 1;
46        int i;
47
48        for (i = 0; i < e; i++) {
49                a *= 2;
50        }
51        return a;
52}
53
54/* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
55 * be an array of signed char's to output to, bitsize should be the number
56 * of bits of out, in is the original scalar, and w is the window size.
57 * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
58 * Menezes, "Software implementation of elliptic curve cryptography over
59 * binary fields", Proc. CHES 2000. */
60mp_err
61ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
62{
63        mp_int k;
64        mp_err res = MP_OKAY;
65        int i, twowm1, mask;
66
67        twowm1 = ec_twoTo(w - 1);
68        mask = 2 * twowm1 - 1;
69
70        MP_DIGITS(&k) = 0;
71        MP_CHECKOK(mp_init_copy(&k, in));
72
73        i = 0;
74        /* Compute wNAF form */
75        while (mp_cmp_z(&k) > 0) {
76                if (mp_isodd(&k)) {
77                        out[i] = MP_DIGIT(&k, 0) & mask;
78                        if (out[i] >= twowm1)
79                                out[i] -= 2 * twowm1;
80
81                        /* Subtract off out[i].  Note mp_sub_d only works with
82                         * unsigned digits */
83                        if (out[i] >= 0) {
84                                mp_sub_d(&k, out[i], &k);
85                        } else {
86                                mp_add_d(&k, -(out[i]), &k);
87                        }
88                } else {
89                        out[i] = 0;
90                }
91                mp_div_2(&k, &k);
92                i++;
93        }
94        /* Zero out the remaining elements of the out array. */
95        for (; i < bitsize + 1; i++) {
96                out[i] = 0;
97        }
98  CLEANUP:
99        mp_clear(&k);
100        return res;
101
102}
103