  Dynamic watermarking, as an active intrusion detection technique, can
potentially detect replay attacks, spoofing attacks, and deception attacks in
the feedback channel for control systems. In this paper, we develop a novel
dynamic watermarking algorithm for finite-state finite-action Markov decision
processes and present upper bounds on the mean time between false alarms, and
the mean delay between the time an attack occurs and when it is detected. We
further compute the sensitivity of the performance of the control system as a
function of the watermark. We demonstrate the effectiveness of the proposed
dynamic watermarking algorithm by detecting a spoofing attack in a sensor
network system.
