*

 *

 *

 *

 * The Original Code is the elliptic curve math library.

 *

 * The Initial Developer of the Original Code is

 * Sun Microsystems, Inc.

 * Portions created by the Initial Developer are Copyright (C) 2003

 * the Initial Developer. All Rights Reserved.

 *

 * Contributor(s):

 *   Stephen Fung <fungstep@hotmail.com>, Sun Microsystems Laboratories

 *

 *********************************************************************** */

#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;

}