bits codeword(char *attr_value, int m, int k)
{
   int  nbits = 0;   // count of set bits
   bits cword = 0;   // assuming m <= 32 bits
   srandom(hash(attr_value));
   while (nbits < k) {
      int i = random() % m;
      if (((1 << i) & cword) == 0) {
         cword |= (1 << i);
         nbits++;
      }
   }
   return cword;  // m-bits with k 1-bits and m-k 0-bits
}