public static String bitvector_to_hex(BitVector bv) {
   String binary = "";
   for (int i = 0; i < bv.size(); i++) {
     if (bv.getBit(i)) binary += "1";
     else binary += "0";
   }
   return binary_to_hex(binary);
 }
 public static BitVector hex_to_bitvector(String hex) {
   String ff = hex_to_binary(hex);
   BitVector bv = new BitVector(ff.length());
   for (int i = 0; i < ff.length(); i++) {
     Boolean val = false;
     if (ff.charAt(i) == '1') val = true;
     bv.setBit(i, val);
   }
   return bv;
 }