release/src/router/dropbear/libtommath/bn_mp_expt_d.c

   1 #include <tommath.h>
   2 #ifdef BN_MP_EXPT_D_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://math.libtomcrypt.com
  16  */
  17
  18 /* calculate c = a**b  using a square-multiply algorithm */
  19 int mp_expt_d (mp_int * a, mp_digit b, mp_int * c)
  20 {
  21   int     res, x;
  22   mp_int  g;
  23
  24   if ((res = mp_init_copy (&g, a)) != MP_OKAY) {
  25     return res;
  26   }
  27
  28   /* set initial result */
  29   mp_set (c, 1);
  30
  31   for (x = 0; x < (int) DIGIT_BIT; x++) {
  32     /* square */
  33     if ((res = mp_sqr (c, c)) != MP_OKAY) {
  34       mp_clear (&g);
  35       return res;
  36     }
  37
  38     /* if the bit is set multiply */
  39     if ((b & (mp_digit) (((mp_digit)1) << (DIGIT_BIT - 1))) != 0) {
  40       if ((res = mp_mul (c, &g, c)) != MP_OKAY) {
  41          mp_clear (&g);
  42          return res;
  43       }
  44     }
  45
  46     /* shift to next bit */
  47     b <<= 1;
  48   }
  49
  50   mp_clear (&g);
  51   return MP_OKAY;
  52 }
  53 #endif
  54
  55 /* $Source: /cvs/libtom/libtommath/bn_mp_expt_d.c,v $ */
  56 /* $Revision: 1.3 $ */
  57 /* $Date: 2006/03/31 14:18:44 $ */