| /* 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_CHECKOK(mp_sub_d(&k, out[i], &k)); |
| } else { |
| MP_CHECKOK(mp_add_d(&k, -(out[i]), &k)); |
| } |
| } else { |
| out[i] = 0; |
| } |
| MP_CHECKOK(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; |
| |
| } |