  Cyber-Physical Systems (CPSs) play an increasingly significant role in many
critical applications. These valuable applications attract various
sophisticated attacks. This paper considers a stealthy estimation attack, which
aims to modify the state estimation of the CPSs. The intelligent attackers can
learn defense strategies and use clandestine attack strategies to avoid
detection. To address the issue, we design a Chi-square detector in a Digital
Twin (DT), which is an online digital model of the physical system. We use a
Signaling Game with Evidence (SGE) to find the optimal attack and defense
strategies. Our analytical results show that the proposed defense strategies
can mitigate the impact of the attack on the physical estimation and guarantee
the stability of the CPSs. Finally, we use an illustrative application to
evaluate the performance of the proposed framework.
