1#include <tommath.h>
2#ifdef BN_MP_EXPTMOD_C
3/* LibTomMath, multiple-precision integer library -- Tom St Denis
4 *
5 * LibTomMath is a library that provides multiple-precision
6 * integer arithmetic as well as number theoretic functionality.
7 *
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.
11 *
12 * The library is free for all purposes without any express
13 * guarantee it works.
14 *
15 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org
16 */
17
18
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)
23 */
24int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
25{
26  int dr;
27
28  /* modulus P must be positive */
29  if (P->sign == MP_NEG) {
30     return MP_VAL;
31  }
32
33  /* if exponent X is negative we have to recurse */
34  if (X->sign == MP_NEG) {
35#ifdef BN_MP_INVMOD_C
36     mp_int tmpG, tmpX;
37     int err;
38
39     /* first compute 1/G mod P */
40     if ((err = mp_init(&tmpG)) != MP_OKAY) {
41        return err;
42     }
43     if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
44        mp_clear(&tmpG);
45        return err;
46     }
47
48     /* now get |X| */
49     if ((err = mp_init(&tmpX)) != MP_OKAY) {
50        mp_clear(&tmpG);
51        return err;
52     }
53     if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
54        mp_clear_multi(&tmpG, &tmpX, NULL);
55        return err;
56     }
57
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);
61     return err;
62#else
63     /* no invmod */
64     return MP_VAL;
65#endif
66  }
67
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);
72  }
73#endif
74
75#ifdef BN_MP_DR_IS_MODULUS_C
76  /* is it a DR modulus? */
77  dr = mp_dr_is_modulus(P);
78#else
79  /* default to no */
80  dr = 0;
81#endif
82
83#ifdef BN_MP_REDUCE_IS_2K_C
84  /* if not, is it a unrestricted DR modulus? */
85  if (dr == 0) {
86     dr = mp_reduce_is_2k(P) << 1;
87  }
88#endif
89
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);
94  } else {
95#endif
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);
99#else
100    /* no exptmod for evens */
101    return MP_VAL;
102#endif
103#ifdef BN_MP_EXPTMOD_FAST_C
104  }
105#endif
106}
107
108#endif
109
110/* $Source$ */
111/* $Revision$ */
112/* $Date$ */
113