code: public static Token parse(String block, byte[] key)
  	  throws GeneralSecurityException
    {
   	  byte[] decoded = Modhex.decode(block);
   	  byte[] b = decrypt(key, decoded);
  	  return new Token(b);
    }
docs: <p>
  Decrypt and parse YubiKey OTP.
  </p>
  @param block {@link Modhex} encoded representation of the YubiKey OTP.
  @param key   128 bit AES key used to encrypt {@code block}, now will be
  used to ecrypt.
  @throws GeneralSecurityException If decryption fails.