Mercurial > dropbear

annotate libtommath/bn_mp_reduce_2k.c @ 1831:0a3d02c66bf6

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Comment on reason for DROPBEAR_MAX_PASSWORD_LEN limit
author Matt Johnston <matt@codeconstruct.com.au>
date Tue, 12 Oct 2021 21:29:25 +0800
parents 1051e4eea25a
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1655
f52919ffd3b1 update ltm to 1.1.0 and enable FIPS 186.4 compliant key-generation (#79)
Steffen Jaeckel <s_jaeckel@gmx.de>
parents: 1470
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1 #include "tommath_private.h"
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2 #ifdef BN_MP_REDUCE_2K_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 mp_err mp_reduce_2k(mp_int *a, const mp_int *n, mp_digit d)
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8 {
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9 mp_int q;
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10 mp_err err;
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11 int p;
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12
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13 if ((err = mp_init(&q)) != MP_OKAY) {
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14 return err;
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15 }
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16
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17 p = mp_count_bits(n);
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18 top:
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19 /* q = a/2**p, a = a mod 2**p */
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20 if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
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21 goto LBL_ERR;
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22 }
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23
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24 if (d != 1u) {
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25 /* q = q * d */
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26 if ((err = mp_mul_d(&q, d, &q)) != MP_OKAY) {
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27 goto LBL_ERR;
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28 }
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29 }
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30
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31 /* a = a + q */
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32 if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
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33 goto LBL_ERR;
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34 }
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35
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36 if (mp_cmp_mag(a, n) != MP_LT) {
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37 if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
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38 goto LBL_ERR;
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39 }
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40 goto top;
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41 }
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42
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43 LBL_ERR:
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44 mp_clear(&q);
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45 return err;
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46 }
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47
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48 #endif