Hybrid Channel- and Coding-Based Challenge-Response Physical-Layer Authentication with IRS

This paper proposes a new physical-layer authentication (PLA) mechanism that operates in environments where the receiver partially controls the channel conditions. We focus in particular on a channel controlled by an intelligent reflecting surface (IRS). It combines a channel-based challenge-response PLA (CH-CRPLA), which uses instantaneous channel state information (CSI), and a coding-based PLA (CD-PLA), which uses a shared key. The combination allows for reaping the benefits of CD-PLA in a high signal-to-noise ratio (SNR) of the legitimate channel and CH-CRPLA, imposing a high variation of channels even at low SNRs of the legitimate channel, thanks to the increased secrecy (wiretap) capacity. We investigate the trade-off between the pilot symbols for CSI estimation (for CH-CRPLA) and the key bits (for CD-PLA) and evaluate security in terms of the number of secret bits needed by an attacker to succeed. Numerical results confirm the advantage of the proposed hybrid approach over standalone methods.