#include <tommath.h>
#ifdef BN_MP_EXPTMOD_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, tomstdenis@gmail.com, http://libtom.org
 */

/* 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)
 */
int mp_exptmod(mp_int * G, mp_int * X, 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;
		int 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);
#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) << 1;
	}
#endif

	/* if the modulus is odd or dr != 0 use the montgomery method */
#ifdef BN_MP_EXPTMOD_FAST_C
	if (mp_isodd(P) == 1 || dr != 0) {
		return 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_MP_EXPTMOD_FAST_C
	}
#endif
}

#endif

/* $Source: /cvs/libtom/libtommath/bn_mp_exptmod.c,v $ */
/* $Revision: 1.5 $ */
/* $Date: 2006/12/28 01:25:13 $ */