Cost-effective watermark based detector for replay attacks on cyber-physical systems

Security issues of Cyber-Physical Systems (CPS) are of great significance since cyber attacks may cause catastrophic effects on such systems. Due to the events of Stuxnet, replay attacks have gained increasing attention in recent years. To counter for the replay attacks, it has been proposed to impose additional watermarking signals to nominal control inputs. However, such detector may result in the waste of control cost especially when the attack is absent. In this paper, we firstly formulate an attack duration model for the replay attack. By exploiting the attack model, we propose a periodic watermarking strategy which aims to reduce the costs while guaranteeing the detection performance. Furthermore, we obtain an optimal periodic watermarking strategy for the approximated detection performance. Extensive simulations over quadruple water tank are conducted to demonstrate the system performance.