Nonreciprocal RIS-Aided Covert Channel Reciprocity Attacks and Countermeasures

Reconfigurable intelligent surface (RIS) technology enhances wireless communication performance, but it also introduces new vulnerabilities that can be exploited by adversaries. This paper investigates channel reciprocity attack (CRACK) threats in multi-antenna wireless systems operating in time-division duplexing mode using a physically consistent non-reciprocal RIS (NR-RIS) model. CRACK can degrade communication rate and facilitate passive eavesdropping behavior by distorting the downlink precoding, without requiring any additional signal transmission or channel state information (CSI). Unlike conventional RIS jamming strategies, the NR-RIS does not need synchronization with the legitimate system and thus can operate with slow or fixed configurations to implement CRACK, obscuring the distinction between the direct and RIS-induced channels and thereby complicating corresponding defensive precoding designs. To counter the CRACK threat posed by NR-RIS, we develop ``SecureCoder,'' a deep reinforcement learning-based framework that can mitigate CRACK and determine an improved downlink precoder matrix using the estimated uplink CSI and rate feedback from the users. Simulation results demonstrate the severe performance degradation caused by NR-RIS CRACK and validate the effectiveness of SecureCoder in improving both throughput and reducing security threats, thereby enhancing system robustness.