### view libtommath/bn_mp_montgomery_setup.c @ 1785:9026f976eee8

fuzz: work around fuzz_connect_remote() limitations
author Matt Johnston Sun, 06 Dec 2020 21:27:25 +0800 1051e4eea25a
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```#include "tommath_private.h"
#ifdef BN_MP_MONTGOMERY_SETUP_C
/* LibTomMath, multiple-precision integer library -- Tom St Denis */

/* 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
```