Secure Integrated Sensing and Communication Systems in the Presence of Illegal Reconfigurable Intelligent Surface

Reconfigurable intelligent surface (RIS)-enhanced secure wireless communication systems have attracted growing attention. However, RIS can also be misused to assist eavesdropping, introducing new security challenges. This paper investigates the secrecy rate degradation caused by illegal RIS (IRIS) in a typical RIS-assisted integrated sensing and communication (ISAC) system, where a dual-functional base station performs multiuser communication and radar sensing simultaneously with the help of an RIS, while an eavesdropper (Eve) leverages an IRIS for malicious interception. Specifically, two types of sensing targets are considered, including point target and extended target. The detection probability is used as the sensing performance metric for point target, while the the Cramér–Rao bound (CRB) of the complete target response matrix is used for extended target. Based on these metrics, two different secrecy rate maximization problems are formulated under either a detection probability or a CRB threshold constraint. To solve the first non-convex problem, we adopt a combination of closed-form fractional programming, minorization–maximization, successive convex approximation, and the alternating direction method of multipliers. For the second problem, semidefinite relaxation and alternating optimization are employed to obtain suboptimal solutions. Simulation results demonstrate the great potential of ISAC systems and highlight the critical role of joint beamforming and reflection design, as well as strategic RIS deployment, in mitigating potential security threats posed by the IRIS.