Detection and isolation of replay attacks through sensor watermarking

This paper addresses the detection and isolation of replay attacks on sensor measurements. As opposed to previously proposed additive watermarking, we propose a multiplicative watermarking scheme, where each sensor's output is separately watermarked by being fed to a SISO watermark generator. Additionally, a set of equalizing filters is placed at the controller's side, which reconstructs the original output signals from the received watermarked data. We show that the proposed scheme has several advantages over existing approaches: it has no detrimental effects on the closed-loop performance in the absence of attacks; it can be designed in a modular fashion, independently of the design of the controller and anomaly detector; it facilitates the detection of replay attacks and the isolation of the time at which the replayed data was recorded. These properties are discussed in detail and the results are illustrated through a numerical example.