Physical Layer Authentication With Simultaneous Reflecting and Sensing RIS

Hybrid reconfigurable intelligent surfaces (H-RISs) are panels of metamaterial elements that can be electrically reconfigured to reflect radio signals in the desired direction. At the same time, they can also act as receiving antennas. We propose to use an H-RIS to authenticate radio signals, i.e., to enable a receiver Bob to authenticate signals received by a legitimate transmitter Alice rather than an impersonating attacker Eve. Such a decision is made on the basis of the channel response estimated from the received signal, which operates as a signature in this physical-layer authentication (PLA) mechanism. We consider two attacks, where Eve can transmit signals either through the H-RIS to Bob (as she is close to Alice) or directly to Bob (as she is close to Bob). As the H-RIS can receive signals, the authentication test is performed jointly at Bob and at the H-RIS, significantly improving the distinguishability of the legitimate and attack scenarios. In particular, the optimal attack strategy and a generalized likelihood ratio test for authentication are derived.