Mercurial > dropbear

annotate libtommath/bn_mp_montgomery_calc_normalization.c @ 1663:c795520269f9

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Fallback for key gen without hard link support (#89) Add a non-atomic fallback for key generation on platforms where link() is not permitted (such as most stock Android installs) or on filesystems without hard link support (such as FAT).
author Matt Robinson <git@nerdoftheherd.com>
date Sat, 14 Mar 2020 14:37:35 +0000
parents f52919ffd3b1
children 1051e4eea25a
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
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3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
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4 *
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5 * LibTomMath is a library that provides multiple-precision
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6 * integer arithmetic as well as number theoretic functionality.
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7 *
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8 * The library was designed directly after the MPI library by
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9 * Michael Fromberger but has been written from scratch with
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10 * additional optimizations in place.
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11 *
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12 * SPDX-License-Identifier: Unlicense
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13 */
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14
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15 /*
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16 * shifts with subtractions when the result is greater than b.
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17 *
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18 * The method is slightly modified to shift B unconditionally upto just under
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19 * the leading bit of b. This saves alot of multiple precision shifting.
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20 */
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21 int mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
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22 {
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23 int x, bits, res;
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24
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25 /* how many bits of last digit does b use */
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26 bits = mp_count_bits(b) % DIGIT_BIT;
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28 if (b->used > 1) {
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29 if ((res = mp_2expt(a, ((b->used - 1) * DIGIT_BIT) + bits - 1)) != MP_OKAY) {
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30 return res;
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31 }
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32 } else {
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33 mp_set(a, 1uL);
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34 bits = 1;
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35 }
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37
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38 /* now compute C = A * B mod b */
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39 for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
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40 if ((res = mp_mul_2(a, a)) != MP_OKAY) {
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41 return res;
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42 }
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43 if (mp_cmp_mag(a, b) != MP_LT) {
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44 if ((res = s_mp_sub(a, b, a)) != MP_OKAY) {
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45 return res;
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46 }
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47 }
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48 }
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49
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50 return MP_OKAY;
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51 }
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52 #endif
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53
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54 /* ref: HEAD -> master, tag: v1.1.0 */
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55 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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56 /* commit time: 2019-01-28 20:32:32 +0100 */