Cyber-physical systems consist of interconnected physical processes and computational re-sources. Because the physical world is connected to the cyber world, cyber-attacks can result indamage to the physical system. If an attacker could access control inputs and mask measure-ments, a cyber-attack could damage the system while remaining undetected by plant operatorsor control systems. By masking certain sets of measurements, an attacker may cause a portionof the state space to become unobservable, meaning that it is impossible to reconstruct thosestates. This is called an observability attack.A sequential game-theoretic approach is presented to analyze observability attacks. Thesequential game consists of alternating defense and attack stages. In each defense stage, the de-fender's strategy set consists of reinforcing all possible combinations of system measurements. Ineach attack stage, the attacker's strategy set has two components: a reconnaissance componentand a measurement-masking component. The attacker's and defender's payoffs are quantified atthe end of each defense-attack sequence using the responses of the observable and unobservablestates. The observability attack game is analyzed for two defense-attack rounds for a nuclearbalance of plant system. A mixed-strategy Nash equilibrium is identified.
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