  As networked control systems continue to evolve, ensuring the privacy of
sensitive data becomes an increasingly pressing concern, especially in
situations where the controller is physically separated from the plant. In this
paper, we propose a secure control scheme for computing linear quadratic
control in a networked control system utilizing two networked controllers, a
privacy encoder and a control restorer. Specifically, the encoder generates two
state signals blurred with random noise and sends them to the controllers,
while the restorer reconstructs the correct control signal. The proposed design
effectively preserves the privacy of the control system's state without
sacrificing the control performance. We theoretically quantify the
privacy-preserving performance in terms of the state estimation error of the
controllers and the disclosure probability. Additionally, the proposed
privacy-preserving scheme is also proven to satisfy differential privacy.
Moreover, we extend the proposed privacy-preserving scheme and evaluation
method to cases where collusion between two controllers occurs. Finally, we
verify the validity of our proposed scheme through simulations.
