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. */ 51 int 52 ec_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. */ 69 mp_err 70 ec_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