/* 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_sub_d(&k, out[i], &k);

			} else {

				mp_add_d(&k, -(out[i]), &k);

			}

		} else {

			out[i] = 0;

		}

		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;

}