1/* $NetBSD: bn_mp_exptmod.c,v 1.2 2017/01/28 21:31:47 christos Exp $ */ 2 3#include <tommath.h> 4#ifdef BN_MP_EXPTMOD_C 5/* LibTomMath, multiple-precision integer library -- Tom St Denis 6 * 7 * LibTomMath is a library that provides multiple-precision 8 * integer arithmetic as well as number theoretic functionality. 9 * 10 * The library was designed directly after the MPI library by 11 * Michael Fromberger but has been written from scratch with 12 * additional optimizations in place. 13 * 14 * The library is free for all purposes without any express 15 * guarantee it works. 16 * 17 * Tom St Denis, tomstdenis@gmail.com, http://libtom.org 18 */ 19 20 21/* this is a shell function that calls either the normal or Montgomery 22 * exptmod functions. Originally the call to the montgomery code was 23 * embedded in the normal function but that wasted alot of stack space 24 * for nothing (since 99% of the time the Montgomery code would be called) 25 */ 26int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y) 27{ 28 int dr; 29 30 /* modulus P must be positive */ 31 if (P->sign == MP_NEG) { 32 return MP_VAL; 33 } 34 35 /* if exponent X is negative we have to recurse */ 36 if (X->sign == MP_NEG) { 37#ifdef BN_MP_INVMOD_C 38 mp_int tmpG, tmpX; 39 int err; 40 41 /* first compute 1/G mod P */ 42 if ((err = mp_init(&tmpG)) != MP_OKAY) { 43 return err; 44 } 45 if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) { 46 mp_clear(&tmpG); 47 return err; 48 } 49 50 /* now get |X| */ 51 if ((err = mp_init(&tmpX)) != MP_OKAY) { 52 mp_clear(&tmpG); 53 return err; 54 } 55 if ((err = mp_abs(X, &tmpX)) != MP_OKAY) { 56 mp_clear_multi(&tmpG, &tmpX, NULL); 57 return err; 58 } 59 60 /* and now compute (1/G)**|X| instead of G**X [X < 0] */ 61 err = mp_exptmod(&tmpG, &tmpX, P, Y); 62 mp_clear_multi(&tmpG, &tmpX, NULL); 63 return err; 64#else 65 /* no invmod */ 66 return MP_VAL; 67#endif 68 } 69 70/* modified diminished radix reduction */ 71#if defined(BN_MP_REDUCE_IS_2K_L_C) && defined(BN_MP_REDUCE_2K_L_C) && defined(BN_S_MP_EXPTMOD_C) 72 if (mp_reduce_is_2k_l(P) == MP_YES) { 73 return s_mp_exptmod(G, X, P, Y, 1); 74 } 75#endif 76 77#ifdef BN_MP_DR_IS_MODULUS_C 78 /* is it a DR modulus? */ 79 dr = mp_dr_is_modulus(P); 80#else 81 /* default to no */ 82 dr = 0; 83#endif 84 85#ifdef BN_MP_REDUCE_IS_2K_C 86 /* if not, is it a unrestricted DR modulus? */ 87 if (dr == 0) { 88 dr = mp_reduce_is_2k(P) << 1; 89 } 90#endif 91 92 /* if the modulus is odd or dr != 0 use the montgomery method */ 93#ifdef BN_MP_EXPTMOD_FAST_C 94 if (mp_isodd (P) == 1 || dr != 0) { 95 return mp_exptmod_fast (G, X, P, Y, dr); 96 } else { 97#endif 98#ifdef BN_S_MP_EXPTMOD_C 99 /* otherwise use the generic Barrett reduction technique */ 100 return s_mp_exptmod (G, X, P, Y, 0); 101#else 102 /* no exptmod for evens */ 103 return MP_VAL; 104#endif 105#ifdef BN_MP_EXPTMOD_FAST_C 106 } 107#endif 108} 109 110#endif 111 112/* Source: /cvs/libtom/libtommath/bn_mp_exptmod.c,v */ 113/* Revision: 1.5 */ 114/* Date: 2006/12/28 01:25:13 */ 115