The Tor Browser: security/nss/lib/freebl/ecl/ec

Cloned upstream origin tor-browser at tor-browser-31.3.0esr-4.5-1-build1
revision ID fc1c9ff7c1b2defdbc039f12214767608f46423f for hacking purpose.

     1 /* This Source Code Form is subject to the terms of the Mozilla Public

     2  * License, v. 2.0. If a copy of the MPL was not distributed with this

     3  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

     5 #include "ecl-priv.h"

     7 /* Returns 2^e as an integer. This is meant to be used for small powers of

     8  * two. */

     9 int

    10 ec_twoTo(int e)

    11 {

    12 	int a = 1;

    13 	int i;

    15 	for (i = 0; i < e; i++) {

    16 		a *= 2;

    17 	}

    18 	return a;

    19 }

    21 /* Computes the windowed non-adjacent-form (NAF) of a scalar. Out should

    22  * be an array of signed char's to output to, bitsize should be the number

    23  * of bits of out, in is the original scalar, and w is the window size.

    24  * NAF is discussed in the paper: D. Hankerson, J. Hernandez and A.

    25  * Menezes, "Software implementation of elliptic curve cryptography over

    26  * binary fields", Proc. CHES 2000. */

    27 mp_err

    28 ec_compute_wNAF(signed char *out, int bitsize, const mp_int *in, int w)

    29 {

    30 	mp_int k;

    31 	mp_err res = MP_OKAY;

    32 	int i, twowm1, mask;

    34 	twowm1 = ec_twoTo(w - 1);

    35 	mask = 2 * twowm1 - 1;

    37 	MP_DIGITS(&k) = 0;

    38 	MP_CHECKOK(mp_init_copy(&k, in));

    40 	i = 0;

    41 	/* Compute wNAF form */

    42 	while (mp_cmp_z(&k) > 0) {

    43 		if (mp_isodd(&k)) {

    44 			out[i] = MP_DIGIT(&k, 0) & mask;

    45 			if (out[i] >= twowm1)

    46 				out[i] -= 2 * twowm1;

    48 			/* Subtract off out[i].  Note mp_sub_d only works with

    49 			 * unsigned digits */

    50 			if (out[i] >= 0) {

    51 				mp_sub_d(&k, out[i], &k);

    52 			} else {

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

    54 			}

    55 		} else {

    56 			out[i] = 0;

    57 		}

    58 		mp_div_2(&k, &k);

    59 		i++;

    60 	}

    61 	/* Zero out the remaining elements of the out array. */

    62 	for (; i < bitsize + 1; i++) {

    63 		out[i] = 0;

    64 	}

    65   CLEANUP:

    66 	mp_clear(&k);

    67 	return res;

    69 }

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