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comparison libtommath/bn_mp_exptmod.c @ 284:eed26cff980b

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propagate from branch 'au.asn.ucc.matt.ltm.dropbear' (head 6c790cad5a7fa866ad062cb3a0c279f7ba788583) to branch 'au.asn.ucc.matt.dropbear' (head fff0894a0399405a9410ea1c6d118f342cf2aa64)
author Matt Johnston <matt@ucc.asn.au>
date Wed, 08 Mar 2006 13:23:49 +0000
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children cd14c94fe89c 5ff8218bcee9
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283:bd240aa12ba7 284:eed26cff980b
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, [email protected], http://math.libtomcrypt.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 */
24 int 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)
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