3 /* LibTomMath, multiple-precision integer library -- Tom St Denis
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
8 * The library was designed directly after the MPI library by
9 * Michael Fromberger but has been written from scratch with
10 * additional optimizations in place.
12 * The library is free for all purposes without any express
15 * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
19 /* this is a shell function that calls either the normal or Montgomery
20 * exptmod functions. Originally the call to the montgomery code was
21 * embedded in the normal function but that wasted alot of stack space
22 * for nothing (since 99% of the time the Montgomery code would be called)
24 int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
28 /* modulus P must be positive */
29 if (P->sign == MP_NEG) {
33 /* if exponent X is negative we have to recurse */
34 if (X->sign == MP_NEG) {
39 /* first compute 1/G mod P */
40 if ((err = mp_init(&tmpG)) != MP_OKAY) {
43 if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
49 if ((err = mp_init(&tmpX)) != MP_OKAY) {
53 if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
54 mp_clear_multi(&tmpG, &tmpX, NULL);
58 /* and now compute (1/G)**|X| instead of G**X [X < 0] */
59 err = mp_exptmod(&tmpG, &tmpX, P, Y);
60 mp_clear_multi(&tmpG, &tmpX, NULL);
68 /* modified diminished radix reduction */
69 #if defined(BN_MP_REDUCE_IS_2K_L_C) && defined(BN_MP_REDUCE_2K_L_C) && defined(BN_S_MP_EXPTMOD_C)
70 if (mp_reduce_is_2k_l(P) == MP_YES) {
71 return s_mp_exptmod(G, X, P, Y, 1);
75 #ifdef BN_MP_DR_IS_MODULUS_C
76 /* is it a DR modulus? */
77 dr = mp_dr_is_modulus(P);
83 #ifdef BN_MP_REDUCE_IS_2K_C
84 /* if not, is it a unrestricted DR modulus? */
86 dr = mp_reduce_is_2k(P) << 1;
90 /* if the modulus is odd or dr != 0 use the montgomery method */
91 #ifdef BN_MP_EXPTMOD_FAST_C
92 if (mp_isodd (P) == 1 || dr != 0) {
93 return mp_exptmod_fast (G, X, P, Y, dr);
96 #ifdef BN_S_MP_EXPTMOD_C
97 /* otherwise use the generic Barrett reduction technique */
98 return s_mp_exptmod (G, X, P, Y, 0);
100 /* no exptmod for evens */
103 #ifdef BN_MP_EXPTMOD_FAST_C
110 /* $Source: /cvs/libtom/libtommath/bn_mp_exptmod.c,v $ */
111 /* $Revision: 1.4 $ */
112 /* $Date: 2006/03/31 14:18:44 $ */