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Diffstat (limited to 'libtommath/bn_mp_exptmod.c')
-rw-r--r-- | libtommath/bn_mp_exptmod.c | 112 |
1 files changed, 112 insertions, 0 deletions
diff --git a/libtommath/bn_mp_exptmod.c b/libtommath/bn_mp_exptmod.c new file mode 100644 index 0000000..714a96f --- /dev/null +++ b/libtommath/bn_mp_exptmod.c @@ -0,0 +1,112 @@ +#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://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: /root/tcl/repos-to-convert/tcl/libtommath/bn_mp_exptmod.c,v $ */ +/* $Revision: 1.1.1.4 $ */ +/* $Date: 2006/12/01 00:08:11 $ */ |