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comparison libtommath/bn_mp_montgomery_setup.c @ 1739:13d834efc376 fuzz

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merge from main
author Matt Johnston <matt@ucc.asn.au>
date Thu, 15 Oct 2020 19:55:15 +0800
parents 1051e4eea25a
children
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1562:768ebf737aa0 1739:13d834efc376
1 #include <tommath_private.h> 1 #include "tommath_private.h"
2 #ifdef BN_MP_MONTGOMERY_SETUP_C 2 #ifdef BN_MP_MONTGOMERY_SETUP_C
3 /* LibTomMath, multiple-precision integer library -- Tom St Denis 3 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
4 * 4 /* SPDX-License-Identifier: Unlicense */
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, [email protected], http://libtom.org
16 */
17 5
18 /* setups the montgomery reduction stuff */ 6 /* setups the montgomery reduction stuff */
19 int 7 mp_err mp_montgomery_setup(const mp_int *n, mp_digit *rho)
20 mp_montgomery_setup (mp_int * n, mp_digit * rho)
21 { 8 {
22 mp_digit x, b; 9 mp_digit x, b;
23 10
24 /* fast inversion mod 2**k 11 /* fast inversion mod 2**k
25 * 12 *
26 * Based on the fact that 13 * Based on the fact that
27 * 14 *
28 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n) 15 * XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
29 * => 2*X*A - X*X*A*A = 1 16 * => 2*X*A - X*X*A*A = 1
30 * => 2*(1) - (1) = 1 17 * => 2*(1) - (1) = 1
31 */ 18 */
32 b = n->dp[0]; 19 b = n->dp[0];
33 20
34 if ((b & 1) == 0) { 21 if ((b & 1u) == 0u) {
35 return MP_VAL; 22 return MP_VAL;
36 } 23 }
37 24
38 x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */ 25 x = (((b + 2u) & 4u) << 1) + b; /* here x*a==1 mod 2**4 */
39 x *= 2 - (b * x); /* here x*a==1 mod 2**8 */ 26 x *= 2u - (b * x); /* here x*a==1 mod 2**8 */
40 #if !defined(MP_8BIT) 27 #if !defined(MP_8BIT)
41 x *= 2 - (b * x); /* here x*a==1 mod 2**16 */ 28 x *= 2u - (b * x); /* here x*a==1 mod 2**16 */
42 #endif 29 #endif
43 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT)) 30 #if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
44 x *= 2 - (b * x); /* here x*a==1 mod 2**32 */ 31 x *= 2u - (b * x); /* here x*a==1 mod 2**32 */
45 #endif 32 #endif
46 #ifdef MP_64BIT 33 #ifdef MP_64BIT
47 x *= 2 - (b * x); /* here x*a==1 mod 2**64 */ 34 x *= 2u - (b * x); /* here x*a==1 mod 2**64 */
48 #endif 35 #endif
49 36
50 /* rho = -1/m mod b */ 37 /* rho = -1/m mod b */
51 *rho = (mp_digit)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK; 38 *rho = (mp_digit)(((mp_word)1 << (mp_word)MP_DIGIT_BIT) - x) & MP_MASK;
52 39
53 return MP_OKAY; 40 return MP_OKAY;
54 } 41 }
55 #endif 42 #endif
56
57 /* ref: $Format:%D$ */
58 /* git commit: $Format:%H$ */
59 /* commit time: $Format:%ai$ */