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annotate libtommath/bn_mp_montgomery_setup.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_SETUP_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 /* setups the montgomery reduction stuff */
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16 int mp_montgomery_setup(const mp_int *n, mp_digit *rho)
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17 {
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18 mp_digit x, b;
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19
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20 /* fast inversion mod 2**k
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21 *
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22 * Based on the fact that
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23 *
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24 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
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25 * => 2*X*A - X*X*A*A = 1
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26 * => 2*(1) - (1) = 1
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27 */
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28 b = n->dp[0];
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30 if ((b & 1u) == 0u) {
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31 return MP_VAL;
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32 }
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33
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34 x = (((b + 2u) & 4u) << 1) + b; /* here x*a==1 mod 2**4 */
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35 x *= 2u - (b * x); /* here x*a==1 mod 2**8 */
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36 #if !defined(MP_8BIT)
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37 x *= 2u - (b * x); /* here x*a==1 mod 2**16 */
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38 #endif
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39 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
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40 x *= 2u - (b * x); /* here x*a==1 mod 2**32 */
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41 #endif
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42 #ifdef MP_64BIT
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43 x *= 2u - (b * x); /* here x*a==1 mod 2**64 */
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44 #endif
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46 /* rho = -1/m mod b */
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47 *rho = (mp_digit)(((mp_word)1 << (mp_word)DIGIT_BIT) - x) & MP_MASK;
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49 return MP_OKAY;
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50 }
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51 #endif
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53 /* ref: HEAD -> master, tag: v1.1.0 */
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54 /* git commit: 08549ad6bc8b0cede0b357a9c341c5c6473a9c55 */
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55 /* commit time: 2019-01-28 20:32:32 +0100 */