  In this paper, we introduce the concept of two-way coding, which originates
in communication theory characterizing coding schemes for two-way channels,
into control theory, particularly to facilitate the analysis and design of
feedback control systems under injection attacks. Moreover, we propose the
notion of attack decoupling, and show how the controller and the two-way coding
can be co-designed to nullify the transfer function from attack to plant,
rendering the attack effect zero both in transient phase and in steady state.
