Zero-Dynamics Attack, Variations, and Countermeasures

This chapter presents an overview of actuator attacks that exploit zero dynamics, and countermeasures against them. First, zero-dynamics attack is reintroduced based on a canonical representation called normal form. Then it is shown that the target dynamic system is at elevated risk if the associated zero dynamics is unstable. From there on, several questions are raised in series to ensure when the target system is immune to an attack of this kind. The first question is: Is the target system secure from zero-dynamics attack if it does not have any unstable zeros? An answer provided for this question is: No, the target system may still be at risk due to another attack surface emerging in the process of implementation. This is followed by a series of questions, and in the course of providing answers, variants of the classic zero-dynamics attack are presented, from which the vulnerability of the target system is explored in depth. In the end, countermeasures are proposed to render the attack ineffective. Because it is known that zero dynamics in continuous-time systems cannot be modified by feedback, the main idea of the countermeasure is to relocate any unstable zero to a stable region in the stage of digital implementation through modified digital samplers and holders. Adversaries can still attack actuators, but due to the relocated zeros, they are of little use in damaging the target system.