Mercurial > dropbear

annotate libtommath/bn_mp_montgomery_calc_normalization.c @ 1711:e9dba7abd939

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Merge libtomcrypt v1.18.2
author Matt Johnston <matt@ucc.asn.au>
date Wed, 10 Jun 2020 23:16:13 +0800
parents 1051e4eea25a
children
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f52919ffd3b1 update ltm to 1.1.0 and enable FIPS 186.4 compliant key-generation (#79)
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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 /* SPDX-License-Identifier: Unlicense */
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6 /*
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7 * shifts with subtractions when the result is greater than b.
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8 *
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9 * The method is slightly modified to shift B unconditionally upto just under
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10 * the leading bit of b. This saves alot of multiple precision shifting.
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11 */
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12 mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
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13 {
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14 int x, bits;
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15 mp_err err;
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16
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17 /* how many bits of last digit does b use */
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18 bits = mp_count_bits(b) % MP_DIGIT_BIT;
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19
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20 if (b->used > 1) {
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21 if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
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22 return err;
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23 }
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24 } else {
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25 mp_set(a, 1uL);
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26 bits = 1;
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27 }
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29
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30 /* now compute C = A * B mod b */
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31 for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
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32 if ((err = mp_mul_2(a, a)) != MP_OKAY) {
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33 return err;
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34 }
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35 if (mp_cmp_mag(a, b) != MP_LT) {
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36 if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
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37 return err;
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38 }
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39 }
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40 }
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41
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42 return MP_OKAY;
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43 }
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44 #endif