Performance/Reliability Tradeoffs When Watermarking Cyber-Physical Systems

Abstract

Watermarking is an important security technique in cyber-physical systems. It is a process by which the user injects unexpected actuator inputs and then checks if the reported sensor inputs respond as they should to such inputs. Such an approach can detect the falsification, by an attacker, of sensor reports. Significant control-theoretic research has been reported on watermark performance and strategy. Missing in all this work, however, has been the impact of watermarking on the cyber platform of the CPS. This is the focus of the present paper. Watermarking creates pressure to reduce the control update period, which increases thermal stress on the cyber platform. This can dramatically increase the processor aging rate and significantly reduce mean processor lifetimes. The problem is especially acute when the controlled plant is close to the edge of its Safe State Space. One approach to mitigating cyber platform stress is to proactively skip control update calculations. At the price of some degradation in the plant Quality of Control, such proactive skips can meaningfully improve thermal stress and reduce processor aging.