Detection of replay attacks in cyber-physical systems

A new approach for detecting replay attacks on cyber-physical systems is presented in this paper. The diagnosis system under a replay attack will not trigger an alarm as long as the received sensor readings are plausible. Since an adversary is not able to react on system changes due to the simplicity of the attack and a lack of system knowledge, our goal is to excite the system in non-regular time intervals. Subsequently we can check by means of signal processing whether the received sensor readings are valid. To excite harmonic oscillations, we insert a nonlinear element in the control loop and exploit the theory of describing functions. Since the nonlinear function is a design parameter, we are able to design robust harmonic oscillations. Another advantage of the proposed approach is the possibility of elegant implementation into existing control systems.