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104 lines
3.1 KiB
C
104 lines
3.1 KiB
C
/*
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* ***** BEGIN LICENSE BLOCK *****
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* Version: MPL 1.1/GPL 2.0/LGPL 2.1
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*
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* The contents of this file are subject to the Mozilla Public License Version
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* 1.1 (the "License"); you may not use this file except in compliance with
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* the License. You may obtain a copy of the License at
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* http://www.mozilla.org/MPL/
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*
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* Software distributed under the License is distributed on an "AS IS" basis,
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* WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
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* for the specific language governing rights and limitations under the
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* License.
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*
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* The Original Code is the elliptic curve math library.
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*
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* The Initial Developer of the Original Code is
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* Sun Microsystems, Inc.
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* Portions created by the Initial Developer are Copyright (C) 2003
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* the Initial Developer. All Rights Reserved.
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*
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* Contributor(s):
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* Stephen Fung <fungstep@hotmail.com>, Sun Microsystems Laboratories
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*
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* Alternatively, the contents of this file may be used under the terms of
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* either the GNU General Public License Version 2 or later (the "GPL"), or
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* the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
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* in which case the provisions of the GPL or the LGPL are applicable instead
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* of those above. If you wish to allow use of your version of this file only
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* under the terms of either the GPL or the LGPL, and not to allow others to
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* use your version of this file under the terms of the MPL, indicate your
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* decision by deleting the provisions above and replace them with the notice
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* and other provisions required by the GPL or the LGPL. If you do not delete
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* the provisions above, a recipient may use your version of this file under
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* the terms of any one of the MPL, the GPL or the LGPL.
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*
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* ***** END LICENSE BLOCK ***** */
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#include "ecl-priv.h"
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/* Returns 2^e as an integer. This is meant to be used for small powers of
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* two. */
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int
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ec_twoTo(int e)
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{
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int a = 1;
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int i;
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for (i = 0; i < e; i++) {
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a *= 2;
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}
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return a;
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}
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/* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
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* be an array of signed char's to output to, bitsize should be the number
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* of bits of out, in is the original scalar, and w is the window size.
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* NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
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* Menezes, "Software implementation of elliptic curve cryptography over
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* binary fields", Proc. CHES 2000. */
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mp_err
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ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
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{
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mp_int k;
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mp_err res = MP_OKAY;
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int i, twowm1, mask;
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twowm1 = ec_twoTo(w - 1);
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mask = 2 * twowm1 - 1;
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MP_DIGITS(&k) = 0;
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MP_CHECKOK(mp_init_copy(&k, in));
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i = 0;
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/* Compute wNAF form */
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while (mp_cmp_z(&k) > 0) {
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if (mp_isodd(&k)) {
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out[i] = MP_DIGIT(&k, 0) & mask;
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if (out[i] >= twowm1)
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out[i] -= 2 * twowm1;
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/* Subtract off out[i]. Note mp_sub_d only works with
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* unsigned digits */
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if (out[i] >= 0) {
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mp_sub_d(&k, out[i], &k);
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} else {
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mp_add_d(&k, -(out[i]), &k);
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}
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} else {
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out[i] = 0;
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}
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mp_div_2(&k, &k);
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i++;
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}
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/* Zero out the remaining elements of the out array. */
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for (; i < bitsize + 1; i++) {
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out[i] = 0;
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}
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CLEANUP:
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mp_clear(&k);
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return res;
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}
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