59 lines
1.6 KiB
C
59 lines
1.6 KiB
C
#include <tommath_private.h>
|
|
#ifdef BN_MP_MONTGOMERY_SETUP_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis
|
|
*
|
|
* LibTomMath is a library that provides multiple-precision
|
|
* integer arithmetic as well as number theoretic functionality.
|
|
*
|
|
* The library was designed directly after the MPI library by
|
|
* Michael Fromberger but has been written from scratch with
|
|
* additional optimizations in place.
|
|
*
|
|
* The library is free for all purposes without any express
|
|
* guarantee it works.
|
|
*
|
|
* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
|
|
*/
|
|
|
|
/* setups the montgomery reduction stuff */
|
|
int mp_montgomery_setup(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 & 1) == 0) {
|
|
return MP_VAL;
|
|
}
|
|
|
|
x = (((b + 2) & 4) << 1) + b; /* here x*a==1 mod 2**4 */
|
|
x *= 2 - (b * x); /* here x*a==1 mod 2**8 */
|
|
#if !defined(MP_8BIT)
|
|
x *= 2 - (b * x); /* here x*a==1 mod 2**16 */
|
|
#endif
|
|
#if defined(MP_64BIT) || !(defined(MP_8BIT) || defined(MP_16BIT))
|
|
x *= 2 - (b * x); /* here x*a==1 mod 2**32 */
|
|
#endif
|
|
#ifdef MP_64BIT
|
|
x *= 2 - (b * x); /* here x*a==1 mod 2**64 */
|
|
#endif
|
|
|
|
/* rho = -1/m mod b */
|
|
*rho = (mp_digit)(((mp_word)1 << ((mp_word) DIGIT_BIT)) - x) & MP_MASK;
|
|
|
|
return MP_OKAY;
|
|
}
|
|
#endif
|
|
|
|
/* ref: $Format:%D$ */
|
|
/* git commit: $Format:%H$ */
|
|
/* commit time: $Format:%ai$ */
|