  In this paper, we introduce the method of two-way coding, a concept
originating in communication theory characterizing coding schemes for two-way
channels, into (networked) feedback control systems under injection attacks. We
first show that the presence of two-way coding can distort the perspective of
the attacker on the control system. In general, the distorted viewpoint on the
attacker side as a consequence of two-way coding will facilitate detecting the
attacks, or restricting what the attacker can do, or even correcting the attack
effect. In the particular case of zero-dynamics attacks, if the attacks are to
be designed according to the original plant, then they will be easily detected;
while if the attacks are designed with respect to the equivalent plant as
viewed by the attacker, then under the additional assumption that the plant is
stabilizable by static output feedback, the attack effect may be corrected in
steady state.
