Secure mmWave MIMO Communication against Signal Leakage When Meeting Illegal Reconfigurable Intelligent Surface

Reconfigurable intelligent surface (RIS)-enhanced secure wireless system has captured widespread concern recently. However, there is still a lack of research on the evaluation of security threats from the illegal RIS (IRIS) deployed by malicious eavesdroppers. In this paper, we investigate the signal leakage brought by IRIS in typical RIS-enhanced millimeter wave (mmWave) multiple-input multiple-output (MIMO) wiretap system. We firstly illustrate the detriment and challenges of IRIS with respect to system secrecy rate (SR) competition. The existence of IRIS aggravates the difficulty in detecting behaviors of the eavesdropper, making it infeasible to globally solve the SR competition problem. Therefore, we propose an artificial noise (AN)-based interference scheme to moderate the security degradation caused by IRIS. Specifically, the transmit precoder, receiver combiner and RIS discrete phase shifts are jointly designed by exploiting the interpolation search method and sparsity of mmWave channels for the sake of generating more AN. Simulation results demonstrate the non-negligible SR degradation caused by IRIS and validate the effective security enhancement of the proposed scheme. Besides, we emphatically analyze specific system factors that should be weighed to better combat IRIS especially when IRIS is powerful.