A secure physical layer transport scheme with illegal reconfigurable intelligent surface

As a passive programmable reflecting device, Reconfigurable Intelligent Surface (RIS) can change signal transmission conditions very well and is now widely used in physical layer security (PLS) communications. However, due to the low cost of RIS, eavesdroppers (Eves) may equip themselves with illegal RIS in the future to reduce the secrecy rate (SR) of the system. In this correspondence, we design a maximum–minimum problem for the interplay between legal and illegal RISs on the SR, and propose two schemes to improve the SR. Scheme I designs an alternating optimization algorithm based on Riemannian manifolds to determine the degree of the illegal RIS on the system SR and tries to offset its impact by increasing the number of reflected elements of the legal RIS. Scheme II replaces the legal passive RIS with the legal active RIS and designs an alternating optimization scheme based on semi-positive definite relaxation (SDR) and Lemma I for the non-convex problem caused by the legal active RIS. The final simulation results show that both proposed schemes can effectively ensure PLS in the presence of illegal RIS.