ec_naf.c revision 5697:324be5104707 
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#pragma ident	"%Z%%M%	%I%	%E% SMI"
46
47#include "ecl-priv.h"
48
49/* Returns 2^e as an integer. This is meant to be used for small powers of
50 * two. */
51int
52ec_twoTo(int e)
53{
54	int a = 1;
55	int i;
56
57	for (i = 0; i < e; i++) {
58		a *= 2;
59	}
60	return a;
61}
62
63/* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
64 * be an array of signed char's to output to, bitsize should be the number
65 * of bits of out, in is the original scalar, and w is the window size.
66 * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
67 * Menezes, "Software implementation of elliptic curve cryptography over
68 * binary fields", Proc. CHES 2000. */
69mp_err
70ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
71{
72	mp_int k;
73	mp_err res = MP_OKAY;
74	int i, twowm1, mask;
75
76	twowm1 = ec_twoTo(w - 1);
77	mask = 2 * twowm1 - 1;
78
79	MP_DIGITS(&k) = 0;
80	MP_CHECKOK(mp_init_copy(&k, in));
81
82	i = 0;
83	/* Compute wNAF form */
84	while (mp_cmp_z(&k) > 0) {
85		if (mp_isodd(&k)) {
86			out[i] = MP_DIGIT(&k, 0) & mask;
87			if (out[i] >= twowm1)
88				out[i] -= 2 * twowm1;
89
90			/* Subtract off out[i].  Note mp_sub_d only works with
91			 * unsigned digits */
92			if (out[i] >= 0) {
93				mp_sub_d(&k, out[i], &k);
94			} else {
95				mp_add_d(&k, -(out[i]), &k);
96			}
97		} else {
98			out[i] = 0;
99		}
100		mp_div_2(&k, &k);
101		i++;
102	}
103	/* Zero out the remaining elements of the out array. */
104	for (; i < bitsize + 1; i++) {
105		out[i] = 0;
106	}
107  CLEANUP:
108	mp_clear(&k);
109	return res;
110
111}
112