1 /* *********************************************************************
2 *
3 * Sun elects to have this file available under and governed by the
4 * Mozilla Public License Version 1.1 ("MPL") (see
5 * http://www.mozilla.org/MPL/ for full license text). For the avoidance
6 * of doubt and subject to the following, Sun also elects to allow
7 * licensees to use this file under the MPL, the GNU General Public
8 * License version 2 only or the Lesser General Public License version
9 * 2.1 only. Any references to the "GNU General Public License version 2
10 * or later" or "GPL" in the following shall be construed to mean the
11 * GNU General Public License version 2 only. Any references to the "GNU
12 * Lesser General Public License version 2.1 or later" or "LGPL" in the
13 * following shall be construed to mean the GNU Lesser General Public
14 * License version 2.1 only. However, the following notice accompanied
15 * the original version of this file:
16 *
17 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
18 *
19 * The contents of this file are subject to the Mozilla Public License Version
20 * 1.1 (the "License"); you may not use this file except in compliance with
21 * the License. You may obtain a copy of the License at
22 * http://www.mozilla.org/MPL/
23 *
24 * Software distributed under the License is distributed on an "AS IS" basis,
25 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
26 * for the specific language governing rights and limitations under the
27 * License.
28 *
29 * The Original Code is the elliptic curve math library.
30 *
31 * The Initial Developer of the Original Code is
32 * Sun Microsystems, Inc.
33 * Portions created by the Initial Developer are Copyright (C) 2003
34 * the Initial Developer. All Rights Reserved.
35 *
36 * Contributor(s):
37 * Stephen Fung <fungstep@hotmail.com>, Sun Microsystems Laboratories
38 *
39 * Alternatively, the contents of this file may be used under the terms of
40 * either the GNU General Public License Version 2 or later (the "GPL"), or
41 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
42 * in which case the provisions of the GPL or the LGPL are applicable instead
43 * of those above. If you wish to allow use of your version of this file only
44 * under the terms of either the GPL or the LGPL, and not to allow others to
45 * use your version of this file under the terms of the MPL, indicate your
46 * decision by deleting the provisions above and replace them with the notice
47 * and other provisions required by the GPL or the LGPL. If you do not delete
48 * the provisions above, a recipient may use your version of this file under
49 * the terms of any one of the MPL, the GPL or the LGPL.
50 *
51 *********************************************************************** */
52 /*
53 * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
54 * Use is subject to license terms.
55 */
56
57 #include "ecl-priv.h"
58
59 /* Returns 2^e as an integer. This is meant to be used for small powers of
60 * two. */
61 int
62 ec_twoTo(int e)
63 {
64 int a = 1;
65 int i;
66
67 for (i = 0; i < e; i++) {
68 a *= 2;
69 }
70 return a;
71 }
72
73 /* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
74 * be an array of signed char's to output to, bitsize should be the number
75 * of bits of out, in is the original scalar, and w is the window size.
76 * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
77 * Menezes, "Software implementation of elliptic curve cryptography over
78 * binary fields", Proc. CHES 2000. */
79 mp_err
80 ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
81 {
82 mp_int k;
83 mp_err res = MP_OKAY;
84 int i, twowm1, mask;
85
86 twowm1 = ec_twoTo(w - 1);
87 mask = 2 * twowm1 - 1;
88
89 MP_DIGITS(&k) = 0;
90 MP_CHECKOK(mp_init_copy(&k, in));
91
92 i = 0;
93 /* Compute wNAF form */
94 while (mp_cmp_z(&k) > 0) {
95 if (mp_isodd(&k)) {
96 out[i] = MP_DIGIT(&k, 0) & mask;
97 if (out[i] >= twowm1)
98 out[i] -= 2 * twowm1;
99
100 /* Subtract off out[i]. Note mp_sub_d only works with
101 * unsigned digits */
102 if (out[i] >= 0) {
103 mp_sub_d(&k, out[i], &k);
104 } else {
105 mp_add_d(&k, -(out[i]), &k);
106 }
107 } else {
108 out[i] = 0;
109 }
110 mp_div_2(&k, &k);
111 i++;
112 }
113 /* Zero out the remaining elements of the out array. */
114 for (; i < bitsize + 1; i++) {
115 out[i] = 0;
116 }
117 CLEANUP:
118 mp_clear(&k);
119 return res;
120
121 }