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 #include #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://math.libtomcrypt.com */ /* 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: 1.4 \$ */ /* \$Date: 2006/03/31 14:18:44 \$ */