50 lines
998 B
C
50 lines
998 B
C
#include "tommath_private.h"
|
|
#ifdef BN_MP_REDUCE_2K_L_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
|
|
/* SPDX-License-Identifier: Unlicense */
|
|
|
|
/* reduces a modulo n where n is of the form 2**p - d
|
|
This differs from reduce_2k since "d" can be larger
|
|
than a single digit.
|
|
*/
|
|
mp_err mp_reduce_2k_l(mp_int *a, const mp_int *n, const mp_int *d)
|
|
{
|
|
mp_int q;
|
|
mp_err err;
|
|
int p;
|
|
|
|
if ((err = mp_init(&q)) != MP_OKAY) {
|
|
return err;
|
|
}
|
|
|
|
p = mp_count_bits(n);
|
|
top:
|
|
/* q = a/2**p, a = a mod 2**p */
|
|
if ((err = mp_div_2d(a, p, &q, a)) != MP_OKAY) {
|
|
goto LBL_ERR;
|
|
}
|
|
|
|
/* q = q * d */
|
|
if ((err = mp_mul(&q, d, &q)) != MP_OKAY) {
|
|
goto LBL_ERR;
|
|
}
|
|
|
|
/* a = a + q */
|
|
if ((err = s_mp_add(a, &q, a)) != MP_OKAY) {
|
|
goto LBL_ERR;
|
|
}
|
|
|
|
if (mp_cmp_mag(a, n) != MP_LT) {
|
|
if ((err = s_mp_sub(a, n, a)) != MP_OKAY) {
|
|
goto LBL_ERR;
|
|
}
|
|
goto top;
|
|
}
|
|
|
|
LBL_ERR:
|
|
mp_clear(&q);
|
|
return err;
|
|
}
|
|
|
|
#endif
|