Zero-dynamics Attack, Variations, and Countermeasures

Abstract

This chapter presents an overview on 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 the 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 next 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. At the end, countermeasures are proposed to render the attack ineffective. Because it is known that the zero-dynamics in continuoustime 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 re-located zeros, they are of little use in damaging the target system.

A Generalized Hold Based Countermeasure Against Zero-Dynamics Attack With Application to DC-DC Converter

“…If the system has unstable zero-dynamics, then the attack signal will drive the internal state of the system unbounded while the presence of attack signal cannot be detected by monitoring the system output. In this respect, ZDA is classified as a stealthy actuator attack [12], [20]. It is noted that robust versions of ZDA for linear systems [21] and nonlinear systems [22] have been proposed, which do not require complete knowledge on the system dynamics and increased the threat of ZDA significantly.…”