RIS-Aided Covert Communication in Two-Way DF Relays under Adversarial Joint-Warden Detection

Abstract

This paper investigates adversarially constrained covert communication in a two-way decode-and-forward (DF) relay system enhanced by a reconfigurable intelligent surface (RIS). Unlike existing studies that assume passive or single-warden monitoring, we consider a cooperative joint-warden scenario where multiple detectors minimize their probability of detection error (PDE). To guarantee covertness, the relay power allocation and RIS phase shifts are jointly optimized under a minimum adversarial PDE constraint. The resulting nonconvex problem is decomposed into two tractable subproblems: a bisection-based power allocation and a projected-gradient ascent (PGA) phase optimization. Analytical derivations are validated through Monte-Carlo simulations, showing that the proposed design achieves up to 70% higher covert throughput compared to random and no-RIS benchmarks. These findings confirm that programmable propagation control offers an effective physical layer mechanism for secure and stealthy wireless communication.