text: #include <tommath.h>
  #ifdef BN_MP_EXPTMOD_C
  /* LibTomMath, multiple-precision integer library -- Tom St Denis
   *
   * LibTomMath is a library that provides multiple-precision
   * integer arithmetic as well as number theoretic functionality.
   *
   * The library was designed directly after the MPI library by
   * Michael Fromberger but has been written from scratch with
   * additional optimizations in place.
   *
   * The library is free for all purposes without any express
   * guarantee it works.
   *
   * Tom St Denis, tomstdenis@gmail.com, http://math.libtomcrypt.com
   */
  
  
  /* this is a shell function that calls either the normal or Montgomery
   * exptmod functions.  Originally the call to the montgomery code was
   * embedded in the normal function but that wasted alot of stack space
   * for nothing (since 99% of the time the Montgomery code would be called)
   */
  int mp_exptmod (mp_int * G, mp_int * X, mp_int * P, mp_int * Y)
  {
    int dr;
  
    /* modulus P must be positive */
    if (P->sign == MP_NEG) {
       return MP_VAL;
    }
  
    /* if exponent X is negative we have to recurse */
    if (X->sign == MP_NEG) {
  #ifdef BN_MP_INVMOD_C
       mp_int tmpG, tmpX;
       int err;
  
       /* first compute 1/G mod P */
       if ((err = mp_init(&tmpG)) != MP_OKAY) {
          return err;
       }
       if ((err = mp_invmod(G, P, &tmpG)) != MP_OKAY) {
          mp_clear(&tmpG);
          return err;
       }
  
       /* now get |X| */
       if ((err = mp_init(&tmpX)) != MP_OKAY) {
          mp_clear(&tmpG);
          return err;
       }
       if ((err = mp_abs(X, &tmpX)) != MP_OKAY) {
          mp_clear_multi(&tmpG, &tmpX, NULL);
          return err;
       }
  
       /* and now compute (1/G)**|X| instead of G**X [X < 0] */
       err = mp_exptmod(&tmpG, &tmpX, P, Y);
       mp_clear_multi(&tmpG, &tmpX, NULL);
       return err;
  #else 
       /* no invmod */
       return MP_VAL;
  #endif
    }
  
  /* modified diminished radix reduction */
  #if defined(BN_MP_REDUCE_IS_2K_L_C) && defined(BN_MP_REDUCE_2K_L_C) && defined(BN_S_MP_EXPTMOD_C)
    if (mp_reduce_is_2k_l(P) == MP_YES) {
       return s_mp_exptmod(G, X, P, Y, 1);
    }
  #endif
  
  #ifdef BN_MP_DR_IS_MODULUS_C
    /* is it a DR modulus? */
    dr = mp_dr_is_modulus(P);
  #else
    /* default to no */
    dr = 0;
  #endif
  
  #ifdef BN_MP_REDUCE_IS_2K_C
    /* if not, is it a unrestricted DR modulus? */
    if (dr == 0) {
       dr = mp_reduce_is_2k(P) << 1;
    }
  #endif
      
    /* if the modulus is odd or dr != 0 use the montgomery method */
  #ifdef BN_MP_EXPTMOD_FAST_C
    if (mp_isodd (P) == 1 || dr !=  0) {
      return mp_exptmod_fast (G, X, P, Y, dr);
    } else {
  #endif
  #ifdef BN_S_MP_EXPTMOD_C
      /* otherwise use the generic Barrett reduction technique */
      return s_mp_exptmod (G, X, P, Y, 0);
  #else
      /* no exptmod for evens */
      return MP_VAL;
  #endif
  #ifdef BN_MP_EXPTMOD_FAST_C
    }
  #endif
  }
  
  #endif
  
  /* $Source$ */
  /* $Revision: 24838 $ */
  /* $Date: 2007-01-23 23:16:57 -0600 (Tue, 23 Jan 2007) $ */
  