In this Letter, the physical layer security for a novel reconfigurable intelligent surface (RIS)-assisted non-orthogonal multiple access (NOMA) system in a multiuser scenario is investigated, where the authors consider the worst case that the eavesdropper also utilises the advantage of the RISs. More specifically, they derive analytical results for the secrecy outage probability (SOP). From the numerical results, they observe that the use of RISs can improve the secrecy performance compared to traditional NOMA systems. However, for the worst case that the received signals at the eavesdropper come from the RISs and source, increasing the number of intelligent elements on the RIS has a negative impact on the secrecy performance. At high signal-to-noise ratios, the system's SOP tends to a constant. Finally, the secrecy performance can be improved through group selection. Introduction: 5G has been commercialised in 2020, and nonorthogonal multiple access (NOMA) plays a key role in this. NOMA has been used for many scenarios to solve the problems caused by the explosive growth of the number of mobile terminals [1]. Unlike the traditional orthogonal multiple access (OMA) system structure, powerdomain NOMA serves more users at the same time and frequency based on the power allocation of the transmitted signals. Consequently, NOMA can enhance the communication quality of users in poor channel conditions [2]. In wireless communication systems, the signals are broadcast so that physical layer security (PLS) has become a hot issue. In [3], PLS for cooperative NOMA systems was investigated, where both decode-and-forward and amplify-and-forward were considered. Recently, a new material called reconfigurable intelligent surfaces (RIS) has been proposed. RISs have a large number of application scenarios in wireless communication and even change the traditional communication structure [4]. So far, there are many works based on RISs that have been reported in [5-11]. For example, the authors in [5] proposed highly accurate closed-form approximations to channel distributions of RIS-based wireless systems. A RIS-assisted dual-hop unmanned aerial vehicle (UAV) communication system was proposed in [6]. The authors in [7] quantitatively analysed the coverage for a RIS-aided communication system. A RIS-aided downlink multiuser communication system was investigated in [8]. Moreover, an important application of RISs is to combine with NOMA to further improve communication quality. For instance, in [9], the authors proposed a theoretical performance comparison between NOMA and OMA in RIS-assisted downlink communication. The authors in [10] derived the BER performance of the RIS-assisted power domain NOMA system. In [11], the authors studied both downlink and uplink RIS-aided NOMA and OMA networks. However, considering the PLS for the RIS-aided NOMA system is still not reported in the literature. Therefore, this is the main innovation of this work. In this Letter, we propose a RIS-assisted multiuser NOMA system. In particular, we assume th...

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In this paper, the physical layer security (PLS) for a novel reconfigurable intelligent surface (RIS)-assisted nonorthogonal multiple access (NOMA) system in a multi-user scenario is investigated, where we consider the worst case that the eavesdropper also utilizes the advantage of the RISs. More specifically, we derive analytical results for the secrecy outage probability (SOP). From the numerical results, we observe that the use of RISs can improve the secrecy performance compared to traditional NOMA systems. However, for the worst case that the received signals at the eavesdropper comes from the RISs and source, increasing the number of intelligent elements on the RIS has a negative impact on the secrecy performance. At high SNRs, the system's SOP tends to a constant. Finally, the secrecy performance can be improved through the group selection.