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

 *

 * Sun elects to have this file available under and governed by the

 * Mozilla Public License Version 1.1 ("MPL") (see

 * http://www.mozilla.org/MPL/ for full license text). For the avoidance

 * of doubt and subject to the following, Sun also elects to allow

 * licensees to use this file under the MPL, the GNU General Public

 * License version 2 only or the Lesser General Public License version

 * 2.1 only. Any references to the "GNU General Public License version 2

 * or later" or "GPL" in the following shall be construed to mean the

 * GNU General Public License version 2 only. Any references to the "GNU

 * Lesser General Public License version 2.1 or later" or "LGPL" in the

 * following shall be construed to mean the GNU Lesser General Public

 * License version 2.1 only. However, the following notice accompanied

 * the original version of this file:

 *

 * Version: MPL 1.1/GPL 2.0/LGPL 2.1

 *

 * The contents of this file are subject to the Mozilla Public License Version

 * 1.1 (the "License"); you may not use this file except in compliance with

 * the License. You may obtain a copy of the License at

 * http://www.mozilla.org/MPL/

 *

 * Software distributed under the License is distributed on an "AS IS" basis,

 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License

 * for the specific language governing rights and limitations under the

 * License.

 *

 * 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

 *

 * Alternatively, the contents of this file may be used under the terms of

 * either the GNU General Public License Version 2 or later (the "GPL"), or

 * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),

 * in which case the provisions of the GPL or the LGPL are applicable instead

 * of those above. If you wish to allow use of your version of this file only

 * under the terms of either the GPL or the LGPL, and not to allow others to

 * use your version of this file under the terms of the MPL, indicate your

 * decision by deleting the provisions above and replace them with the notice

 * and other provisions required by the GPL or the LGPL. If you do not delete

 * the provisions above, a recipient may use your version of this file under

 * the terms of any one of the MPL, the GPL or the LGPL.

 *

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

/*

 * Use is subject to license terms.

 */

#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;

}