xref: /illumos-gate/usr/src/common/crypto/ecc/ec_naf.c (revision c40a6cd785e883b3f052b122c332e21174fc1871)
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