/************************************************************************
  Computes base^exp mod (mod) (modular exponentiation) using the
  algorithm square-and-multiply. Returns a 16 bit number.

  base   16 bits positive integer
  exp    16 bits positive integer
  mod    16 bits positive integer

*************************************************************************/
unsigned short power(unsigned short base, unsigned short exp,
                                          unsigned short mod) {

   unsigned long result=1;

   while (exp>0) {
      /* Exp is odd, bitwise test */
      if (exp & 1) {
         result=(base*result) % mod;
      }
      base=(base*base) % mod;
      exp/=2;     /* Integer division, rounds down */
   }
   /* Will not truncate since mod is unsigned short */
   return (unsigned short)result;
}