This paper presents a replay attack detection method that addresses the performance loss of watermarkingbased approaches. The proposed method injects a sinusoidal signal that affects a subset, chosen at random, of the system outputs. The presence of the signal in each one of the outputs is estimated by means of independent observers and its effect is compensated in the control loop. When a system output is affected by a replay attack, the loss of feedback of the associated observer destabilizes the signal estimation, leading to an exponential increase of the estimation error up to a threshold, above which the estimated signal compensation in the control loop is disabled. This event triggers the detection of a replay attack over the output corresponding to the disrupted observer. The effectiveness of the method is demonstrated using results obtained with a quadruple-tank system simulator.
This paper presents a watermarking signal injection method that compensates its effect in the loop, avoiding thus the signal reinjection. Similar to a virtual actuator scheme, the proposed methodology masks the presence of the authentication signal to the system controller, that do not need to be retuned as it remains immunized. Furthermore, a set-based analysis concerning the effect that the performance loss imposed by a watermarking signal has in the detectability of a replay attack is performed for the stationary, assuming that a standard state observer is used in order to monitor the plant. Finally, a numerical application example is used to illustrate the proposed approach.
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