96 lines
2.4 KiB
C
96 lines
2.4 KiB
C
#include "tommath_private.h"
|
|
#ifdef BN_MP_EXPTMOD_C
|
|
/* LibTomMath, multiple-precision integer library -- Tom St Denis */
|
|
/* SPDX-License-Identifier: Unlicense */
|
|
|
|
/* this is a shell function that calls either the normal or Montgomery
|
|
* exptmod functions. Originally the call to the montgomery code was
|
|
* embedded in the normal function but that wasted alot of stack space
|
|
* for nothing (since 99% of the time the Montgomery code would be called)
|
|
*/
|
|
mp_err mp_exptmod(const mp_int *G, const mp_int *X, const mp_int *P, mp_int *Y)
|
|
{
|
|
int dr;
|
|
|
|
/* modulus P must be positive */
|
|
if (P->sign == MP_NEG) {
|
|
return MP_VAL;
|
|
}
|
|
|
|
/* if exponent X is negative we have to recurse */
|
|
if (X->sign == MP_NEG) {
|
|
#ifdef BN_MP_INVMOD_C
|
|
mp_int tmpG, tmpX;
|
|
mp_err err;
|
|
|
|
/* first compute 1/G mod P */
|
|
if ((err = mp_init(&tmpG)) != MP_OKAY) {
|
|
return err;
|
|
}
|
|
if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
|
|
mp_clear(&tmpG);
|
|
return err;
|
|
}
|
|
|
|
/* now get |X| */
|
|
if ((err = mp_init(&tmpX)) != MP_OKAY) {
|
|
mp_clear(&tmpG);
|
|
return err;
|
|
}
|
|
if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
|
|
mp_clear_multi(&tmpG, &tmpX, NULL);
|
|
return err;
|
|
}
|
|
|
|
/* and now compute (1/G)**|X| instead of G**X [X < 0] */
|
|
err = mp_exptmod(&tmpG, &tmpX, P, Y);
|
|
mp_clear_multi(&tmpG, &tmpX, NULL);
|
|
return err;
|
|
#else
|
|
/* no invmod */
|
|
return MP_VAL;
|
|
#endif
|
|
}
|
|
|
|
/* modified diminished radix reduction */
|
|
#if defined(BN_MP_REDUCE_IS_2K_L_C) && defined(BN_MP_REDUCE_2K_L_C) && defined(BN_S_MP_EXPTMOD_C)
|
|
if (mp_reduce_is_2k_l(P) == MP_YES) {
|
|
return s_mp_exptmod(G, X, P, Y, 1);
|
|
}
|
|
#endif
|
|
|
|
#ifdef BN_MP_DR_IS_MODULUS_C
|
|
/* is it a DR modulus? */
|
|
dr = (mp_dr_is_modulus(P) == MP_YES) ? 1 : 0;
|
|
#else
|
|
/* default to no */
|
|
dr = 0;
|
|
#endif
|
|
|
|
#ifdef BN_MP_REDUCE_IS_2K_C
|
|
/* if not, is it a unrestricted DR modulus? */
|
|
if (dr == 0) {
|
|
dr = (mp_reduce_is_2k(P) == MP_YES) ? 2 : 0;
|
|
}
|
|
#endif
|
|
|
|
/* if the modulus is odd or dr != 0 use the montgomery method */
|
|
#ifdef BN_S_MP_EXPTMOD_FAST_C
|
|
if (MP_IS_ODD(P) || (dr != 0)) {
|
|
return s_mp_exptmod_fast(G, X, P, Y, dr);
|
|
} else {
|
|
#endif
|
|
#ifdef BN_S_MP_EXPTMOD_C
|
|
/* otherwise use the generic Barrett reduction technique */
|
|
return s_mp_exptmod(G, X, P, Y, 0);
|
|
#else
|
|
/* no exptmod for evens */
|
|
return MP_VAL;
|
|
#endif
|
|
#ifdef BN_S_MP_EXPTMOD_FAST_C
|
|
}
|
|
#endif
|
|
}
|
|
|
|
#endif
|