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/* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "ecl-priv.h"
/* Returns 2^e as an integer. This is meant to be used for small powers of
* two. */
int
ec_twoTo(int e)
{
int a = 1;
int i;
for (i = 0; i < e; i++) {
a *= 2;
}
return a;
}
/* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should
* be an array of signed char's to output to, bitsize should be the number
* of bits of out, in is the original scalar, and w is the window size.
* NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.
* Menezes, "Software implementation of elliptic curve cryptography over
* binary fields", Proc. CHES 2000. */
mp_err
ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)
{
mp_int k;
mp_err res = MP_OKAY;
int i, twowm1, mask;
twowm1 = ec_twoTo(w - 1);
mask = 2 * twowm1 - 1;
MP_DIGITS(&k) = 0;
MP_CHECKOK(mp_init_copy(&k, in));
i = 0;
/* Compute wNAF form */
while (mp_cmp_z(&k) > 0) {
if (mp_isodd(&k)) {
out[i] = MP_DIGIT(&k, 0) & mask;
if (out[i] >= twowm1)
out[i] -= 2 * twowm1;
/* Subtract off out[i]. Note mp_sub_d only works with
* unsigned digits */
if (out[i] >= 0) {
MP_CHECKOK(mp_sub_d(&k, out[i], &k));
} else {
MP_CHECKOK(mp_add_d(&k, -(out[i]), &k));
}
} else {
out[i] = 0;
}
MP_CHECKOK(mp_div_2(&k, &k));
i++;
}
/* Zero out the remaining elements of the out array. */
for (; i < bitsize + 1; i++) {
out[i] = 0;
}
CLEANUP:
mp_clear(&k);
return res;
}
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