code: public synchronized byte[] response(byte[] token) {
      try {
        return saslClient != null ? saslClient.evaluateChallenge(token) : new byte[0];
      } catch (SaslException e) {
        throw Throwables.propagate(e);
      }
    }
docs: Respond to server's SASL token.
  @param token contains server's SASL token
  @return client's response SASL token