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 * Copyright 2007 Sun Microsystems, Inc. All rights reserved.
40 * Use is subject to license terms.
41 *
42 * Sun elects to use this software under the MPL license.
43 */
44
45 #include "ecl-priv.h"
46
47 /* Returns 2^e as an integer. This is meant to be used for small powers of
48 * two. */
49 int
ec_twoTo(int e)50 ec_twoTo(int e)
51 {
52 int a = 1;
53 int i;
54
55 for (i = 0; i < e; i++) {
56 a *= 2;
57 }
58 return a;
59 }
60
61 /* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
62 * be an array of signed char's to output to, bitsize should be the number
63 * of bits of out, in is the original scalar, and w is the window size.
64 * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
65 * Menezes, "Software implementation of elliptic curve cryptography over
66 * binary fields", Proc. CHES 2000. */
67 mp_err
ec_compute_wNAF(signed char * out,int bitsize,const mp_int * in,int w)68 ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
69 {
70 mp_int k;
71 mp_err res = MP_OKAY;
72 int i, twowm1, mask;
73
74 twowm1 = ec_twoTo(w - 1);
75 mask = 2 * twowm1 - 1;
76
77 MP_DIGITS(&k) = 0;
78 MP_CHECKOK(mp_init_copy(&k, in));
79
80 i = 0;
81 /* Compute wNAF form */
82 while (mp_cmp_z(&k) > 0) {
83 if (mp_isodd(&k)) {
84 out[i] = MP_DIGIT(&k, 0) & mask;
85 if (out[i] >= twowm1)
86 out[i] -= 2 * twowm1;
87
88 /* Subtract off out[i]. Note mp_sub_d only works with
89 * unsigned digits */
90 if (out[i] >= 0) {
91 mp_sub_d(&k, out[i], &k);
92 } else {
93 mp_add_d(&k, -(out[i]), &k);
94 }
95 } else {
96 out[i] = 0;
97 }
98 mp_div_2(&k, &k);
99 i++;
100 }
101 /* Zero out the remaining elements of the out array. */
102 for (; i < bitsize + 1; i++) {
103 out[i] = 0;
104 }
105 CLEANUP:
106 mp_clear(&k);
107 return res;
108
109 }
110