  Maintaining the security of control systems in the presence of integrity
attacks is a significant challenge. In literature, several possible attacks
against control systems have been formulated including replay, false data
injection, and zero dynamics attacks. The detection and prevention of these
attacks may require the defender to possess a particular subset of trusted
communication channels. Alternatively, these attacks can be prevented by
keeping the system model secret from the adversary. In this paper, we consider
an adversary who has the ability to modify and read all sensor and actuator
channels. To thwart this adversary, we introduce external states dependent on
the state of the control system, with linear time-varying dynamics unknown to
the adversary. We also include sensors to measure these states. The presence of
unknown time-varying dynamics is leveraged to detect an adversary who
simultaneously aims to identify the system and inject stealthy outputs.
Potential attack strategies and bounds on the attacker's performance are
provided.
