Mercurial > dropbear

annotate libtommath/bn_mp_reduce_2k_l.c @ 1734:73646de50f13 DROPBEAR_2020.80

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version 2020.80
author Matt Johnston <matt@ucc.asn.au>
date Fri, 26 Jun 2020 21:45:59 +0800
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
rev   line source
1655
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_REDUCE_2K_L_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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5
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6 /* reduces a modulo n where n is of the form 2**p - d
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7 This differs from reduce_2k since "d" can be larger
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8 than a single digit.
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9 */
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10 mp_err mp_reduce_2k_l(mp_int *a, const mp_int *n, const mp_int *d)
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11 {
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12 mp_int q;
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13 mp_err err;
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14 int p;
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15
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16 if ((err = mp_init(&q)) != MP_OKAY) {
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17 return err;
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18 }
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19
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20 p = mp_count_bits(n);
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21 top:
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22 /* q = a/2**p, a = a mod 2**p */
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23 if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
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24 goto LBL_ERR;
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25 }
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26
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27 /* q = q * d */
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28 if ((err = mp_mul(&q, d, &q)) != MP_OKAY) {
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29 goto LBL_ERR;
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30 }
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31
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32 /* a = a + q */
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33 if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
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34 goto LBL_ERR;
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35 }
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36
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37 if (mp_cmp_mag(a, n) != MP_LT) {
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38 if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
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39 goto LBL_ERR;
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40 }
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41 goto top;
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42 }
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43
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44 LBL_ERR:
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45 mp_clear(&q);
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46 return err;
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47 }
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48
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49 #endif