Robust beamforming and RIS phase shift design for secure ISAC against a multi-antenna active eavesdropper

In this paper, we design a secure reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) system in the presence of a multi-antenna active eavesdropper. Unlike conventional passive eavesdroppers, the active eavesdropper can intercept confidential information and transmit jamming signals toward legitimate users and the sensing target. To counter this dual threat, the RIS is employed to reconfigure the wireless propagation environment and enhance the effective links from the base station to both communication users and the sensing target. Considering imperfect channel state information of the eavesdropper and target angular uncertainty, we formulate a robust sum secrecy rate maximization problem by jointly optimizing the communication and sensing beamforming, artificial noise covariance matrix, and RIS phase shifts, subject to transmit power, unit-modulus, and sensing echo signal-to-interference-plus-noise ratio constraints. To solve the resulting highly non-convex problem, we develop an alternating optimization framework that decomposes it into tractable subproblems, which are addressed using semidefinite relaxation, a penalty-based method, and successive convex approximation. Simulation results show that the proposed scheme outperforms the baseline schemes, achieving approximately 55% higher average sum secrecy rate than the random RIS baseline, while maintaining robust secure communication and reliable sensing under channel and angular uncertainties.