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annotate libtommath/bn_mp_montgomery_setup.c @ 1684:d5d25ce2a2ed

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cast to fix warning
author Matt Johnston <matt@ucc.asn.au>
date Tue, 26 May 2020 19:57:28 +0800
parents f52919ffd3b1
children 1051e4eea25a
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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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29
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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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45
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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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48
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49 return MP_OKAY;
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50 }
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51 #endif
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52
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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 */