code: public synchronized byte[] response(byte[] token) {
      try {
        return saslServer != null ? saslServer.evaluateResponse(token) : new byte[0];
      } catch (SaslException e) {
        throw Throwables.propagate(e);
      }
    }
docs: Used to respond to server SASL tokens.
  @param token Server's SASL token
  @return response to send back to the server.