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annotate libtommath/bn_mp_montgomery_calc_normalization.c @ 1671:5c8913b7464c

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Mention Ed25519 in dropbearkey man (#94)
author Vladislav Grishenko <themiron@users.noreply.github.com>
date Mon, 25 May 2020 18:28:27 +0500
parents f52919ffd3b1
children 1051e4eea25a
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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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27
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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 */