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70 lines
1.6 KiB
C
70 lines
1.6 KiB
C
/* This Source Code Form is subject to the terms of the Mozilla Public
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* License, v. 2.0. If a copy of the MPL was not distributed with this
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* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
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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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