Secure Two-Way Transmission via Autonomous Reconfigurable Intelligent Surface

In this letter, we propose a secure two-way communication scheme using an autonomous reconfigurable intelligent surface (RIS) to protect the information exchange between two legitimate nodes, both equipped with full-duplex radios, against passive eavesdropping. The proposed scheme consists of two stages, i.e., the secret channel estimation stage and the secure two-way data transmission stage. In stage 1, we use the Takagi decomposition to obtain an estimation of the secret channel state information (CSI). In stage 2, the secrecy of the two-way transmission is protected by the randomness of the estimated secret CSI. The reflection patterns are determined at the RIS autonomously and are carefully designed such that they become transparent at the receiver side. Moreover, the RIS does not need to inform the legitimate nodes of the reflection patterns. Without any pre-shared information, the proposed scheme prohibits eavesdropping even if the eavesdropper is close to the transmitter, the receiver, or the RIS. The secret CSI is updated in each of the transmission, which reduces the overhead and enables the proposed scheme to apply to both stable and varying channels. Two transceivers are designed to achieve different balances between efficiency and complexity. Numerical results verify the effectiveness of the proposed scheme.