blob: 39f6e9d24a8f13ef12668b089ae1d8e1a1ead931 (
plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
|
#include "tommath_private.h"
#ifdef BN_MP_MONTGOMERY_SETUP_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
/* SPDX-License-Identifier: Unlicense */
/* setups the montgomery reduction stuff */
mp_err mp_montgomery_setup(const mp_int *n, mp_digit *rho)
{
mp_digit x, b;
/* fast inversion mod 2**k
*
* Based on the fact that
*
* XA = 1 (mod 2**n) => (X(2-XA)) A = 1 (mod 2**2n)
* => 2*X*A - X*X*A*A = 1
* => 2*(1) - (1) = 1
*/
b = n->dp[0];
if ((b & 1u) == 0u) {
return MP_VAL;
}
x = (((b + 2u) & 4u) << 1) + b; /* here x*a==1 mod 2**4 */
x *= 2u - (b * x); /* here x*a==1 mod 2**8 */
#if !defined(MP_8BIT)
x *= 2u - (b * x); /* here x*a==1 mod 2**16 */
#endif
#if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
x *= 2u - (b * x); /* here x*a==1 mod 2**32 */
#endif
#ifdef MP_64BIT
x *= 2u - (b * x); /* here x*a==1 mod 2**64 */
#endif
/* rho = -1/m mod b */
*rho = (mp_digit)(((mp_word)1 << (mp_word)MP_DIGIT_BIT) - x) & MP_MASK;
return MP_OKAY;
}
#endif
|
