libtommath/mp_montgomery_setup.c
Steffen Jaeckel 7a68f12873 Execute move.sh - Rename files from bn_* to match the function names.
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2019-10-19 16:24:39 +02:00

41 lines
1.1 KiB
C

#include "tommath_private.h"
#ifdef 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 */
x *= 2u - (b * x); /* here x*a==1 mod 2**16 */
#if defined(MP_64BIT) || !(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