Opacity of Networked Supervisory Control Systems over Insecure Multiple Channel Networks

In this paper, we investigate the security issue in networked supervisory control systems over multiple channel networks. We consider a networked discrete-event system controlled by a supervisor that receives information from sensors and sends control decisions to actuators via observations channels and control channels, respectively. The security problem is studied for the scenario where some of the communication channels are insecure in the sense there exists a passive intruder (eavesdropper) that can access the information-flow in those insecure communication channels. We adopt the concept of opacity, an information flow security property, to characterize the security status of the supervisory control system. Specifically, we assume that system has a secret and the system is said to be opaque if the intruder can never determine the secret of the system unambiguously based on the information-flow in the insecure channels. A new network observer is proposed to estimate the state of the system with two-side incomparable channel information. We show that the opacity verification problem for the networked setting can be effectively solved using the proposed network observer.