  A state-feedback watermarking signal design for the detection of replay
attacks in linear systems is proposed. The control input is augmented with a
random time-delayed term of the system state estimate, in order to secure the
system against attacks of replay type. We outline the basic analysis of the
closed-loop response of the state-feedback watermarking in a LQG controlled
system. Our theoretical results are applied on a temperature process control
example. While the proposed secure control scheme requires very involved
analysis, it, nevertheless, holds promise of being superior to conventional,
feed-forward, watermarking schemes, in both its ability to detect attacks as
well as the secured system performance.
