## 1 Introduction  
In the era of global 5G deployment, research attention is now turning toward B5G and 6G networks. These next-generation networks necessitate enhanced spectral and energy efficiencies, ultra-low latency in the microsecond range, and extensive network coverage [1]. Furthermore, 6G networks are anticipated to facilitate superior wireless communications and simultaneous high-precision radio sensing, which are critical for emerging applications such as smart cities and the Internet of Vehicles (IoV). Consequently, integrated sensing and communication (ISAC) has emerged as a mainstream trend and a focal point of research in recent years within the context of 6G technology [2]. Nevertheless, private data are exposed to the communication environment when electromagnetic waves are transmitted over open wireless channels, making them highly susceptible to security threats such as jamming and eavesdropping [3]. However, 6G networks provide more security challenges across various communication scenarios. First, in multi-user environments characteristic of the 6G era, the proximity of communication nodes and users increases the likelihood of inter-user interference and potential eavesdroppers, significantly heightening security risks in multi-user communication scenarios. This is due to the increase in network connection density. Physical layer security (PLS) has emerged as a critical driver for the development of 6G networks. PLS enables safe communication by using wireless channel natural qualities and characteristics [4]. Unlike traditional encryption methods, PLS avoids the high complexity of encryption procedures while requiring minimal hardware resources. [PERSON]. [5] utilized some of the power available from the transmitter to generate artificial noise (AN) to interfere with the eavesdropper's channel. This technique enhances the confidentiality of communications by diminishing the quality of the signal received by the eavesdropper. [PERSON]. [6] investigated two different eavesdropping channel models and proposed cooperative jamming to improve the system's secrecy rate. However, many physical security methods, such as traditional beamforming and directional modulation, are inadequate for ensuring complete communication security [7]. To address current limitations, it is crucial to develop cost-effective approaches for wireless systems. By leveraging advancements in metasurfaces and fabrication technologies, Reconfigurable Intelligent Surfaces (RISs) have emerged as promising innovations [8; 9; 10]. RISs are two-dimensional planar arrays composed of multiple sub-wavelength electromagnetic metamaterial components [11]. The electromagnetic response of these RIS units can be precisely controlled by adjusting their microstructure, which includes the amplitude, phase, polarization, and frequency of incident electromagnetic waves. With the assistance of RISs, signals from various links can be coherently combined with legitimate users (Bobs) to enhance the signal constructively and suppress information leakage to eavesdroppers (Eves). This capability positions RISs as a key technology in advancing physical layer security [12]. Nevertheless, RISs are designed solely to reflect incident signals, providing a 180-degree semi-plane reflection. Consequently, users must be located on the same side as both the RISs and the base station (BS), resulting in relatively limited coverage. For instance, consider a challenging scenario with multiple Bobs and Eves distributed on both sides of the RISs. In this situation, there will always be some Bobs and Eves in the communication dead zone that are not supported by the RISs. This leads to undesirable security threats and energy loss [13]. In response to this limitation, the concept of Simultaneously Transmitting and Reflecting Reconfigurable Intelligent Surface (STAR-RIS) has been introduced [14; 15; 16].