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