private String byteToHex(byte[] bytes) {
  return String.format("%040x", new BigInteger(1, bytes));
}

private byte[] hexToByte(String s) {
  final int len = s.length();
  if(len % 2 == 0) {
    byte[] data = new byte[len / 2];
    for (int i = 0; i < len; i += 2) {
      int h = Character.digit(s.charAt(i), 16);
      int l = Character.digit(s.charAt(i + 1), 16);
      if(h == -1 || l == -1) {
        throw new IllegalArgumentException("Illegal characters detected: " + s);
      }
      data[i / 2] = (byte)(h * 16 + l);
    }
    return data;
  }
  throw new IllegalArgumentException("Hex format must be even-length: " + s);
}