  A reconfigurable intelligent surface (RIS) is commonly made of low-cost
passive and reflective meta-materials with excellent beam steering
capabilities. It is applied to enhance wireless communication systems as a
customizable signal reflector. However, RIS can also be adversely employed to
disrupt the existing communication systems by introducing new types of
vulnerability to the physical layer. We consider the \emph{RIS-In-The-Middle
(RITM) attack}, in which an adversary uses RIS to jeopardize the direct channel
between two transceivers by providing an alternative one with higher signal
quality. This adversary can eavesdrop on all exchanged data by the legitimate
users, but also perform a false data injection to the receiver. This work
devises anti-attack techniques based on a non-reciprocal channel produced by a
defensive RIS (D-RIS). The proposed precoding and combining methods and the
channel estimation procedure for a non-reciprocal link are effective against
potential adversaries while keeping the existing advantages of the RIS. We
analyse the robustness of the system against attacks in terms of achievable
secrecy rate and probability of detecting fake data. We believe that this
defensive role of RIS can be a basis for new protocols and algorithms in the
area.
