libs/libtommath/bn_mp_exptmod.c

   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  */
  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) && 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: /cvs/libtom/libtommath/bn_mp_exptmod.c,v $ */
 111 /* $Revision: 1.5 $ */
 112 /* $Date: 2006/12/28 01:25:13 $ */