Contents I. Introduction
Wireless communication has increasingly become the standard. However, the wireless propagation environment presents considerably more challenges compared to wired channels, leading to a notable degradation in wireless communication performance. In recent years, attention has been drawn to reconfigurable intelligent surfaces (RISs) due to their ability to smartly adapt the wireless propagation environment [1]–[3]. To provide further context, an RIS is a two-dimensional (2-D) metasurface that incorporates a software-based controller and an array of low-cost passive reflecting elements. These elements can reflect incident signals with adjustable phase shifts without consuming transmission power. As a result, RISs can effectively control incident electromagnetic waves by applying desired phase shifts in a manageable way. However, due to the inherent broadcast nature of wireless links, signals transmitted from the source to legitimate users remain vulnerable to malicious eavesdropping. Physical layer security (PLS), which aims to safeguard confidential information transmission by capitalizing on the unique properties of wireless channels, has been extensively studied [4], [5]. One common PLS strategy is to employ the assistance of active relays to amplify the signals received at the intended destination [6]–[8].
