A key feature of the non-orthogonal multiple access (NOMA) technique is that users with better channel conditions have prior knowledge about the information of other weak users. Given this prior knowledge, the idea that a strong user can serve as a relay node for other weak users in order to improve their performance, is known as cooperative NOMA. In this paper, we study the physical layer security of such a cooperative NOMA system. In order to reduce the complexity of the analytical process, the considered system in this paper has three users, in which the performance of the weaker users are enhanced by the stronger users. Given that there is an eavesdropper in the system that can hear all the transmissions, we study the secrecy performance of all the users. More specifically, we make an attempt to derive the ergodic secrecy capacity (ESC) and secrecy outage probability (SOP) of all the users. Due to the intractable nature of the exact analysis for the weak users, we provide the closed form expressions of the ESC and SOP for these users at the high SNR regime, while providing the exact analysis for the strongest user. Targeting on the optimality, we further reveal that better secrecy performance of the system is achievable through an appropriate power control mechanism. Finally, based on the analytical methodology of the three-user cooperative system, we provide insightful observations on the performance (in terms of ESC and SOP) of a multi-phase cooperative NOMA system with $N$ users at the high SNR regime. Through rigorous numerical simulations, we verify the correctness of our analytical derivations under different practical scenarios while providing evidence of achieving optimal secrecy performance with the proposed power control scheme.

非正交多路访问（NOMA）技术的关键特征是，具有较好信道条件的用户具有关于其他弱用户信息的先验知识。有了这些先验知识，强用户可以充当其他弱用户的中继节点以提高其性能的想法被称为合作NOMA。在本文中，我们研究了这种协作式NOMA系统的物理层安全性。为了减少分析过程的复杂性，本文考虑的系统具有三个用户，其中，较强的用户可以提高较弱用户的性能。假设系统中存在一个可以监听所有传输的窃听者，我们将研究所有用户的保密性能。进一步来说，我们尝试得出所有用户的遍历保密能力（ESC）和保密中断概率（SOP）。由于对弱用户进行精确分析的顽固性，我们在高SNR情况下为这些用户提供了ESC和SOP的闭式表达式，同时为最强用户提供了精确分析。针对最优性，我们进一步揭示了通过适当的电源控制机制可以实现更好的系统保密性能。最后，基于三用户协作系统的分析方法，我们对具有以下特征的多阶段协作NOMA系统的性能（在ESC和SOP方面）提供了深刻见解。我们在高SNR情况下为这些用户提供ESC和SOP的闭式表达式，同时为最强大的用户提供精确的分析。针对最优性，我们进一步揭示了通过适当的电源控制机制可以实现更好的系统保密性能。最后，基于三用户协作系统的分析方法，我们对具有以下特征的多阶段协作NOMA系统的性能（在ESC和SOP方面）提供了深刻见解。我们在高SNR情况下为这些用户提供ESC和SOP的闭式表达式，同时为最强大的用户提供精确的分析。针对最优性，我们进一步揭示了通过适当的电源控制机制可以实现更好的系统保密性能。最后，基于三用户协作系统的分析方法，我们对具有以下特征的多阶段协作NOMA系统的性能（在ESC和SOP方面）提供了深刻见解。$ñ$SNR高的用户。通过严格的数值模拟，我们在不同的实际情况下验证了分析推导的正确性，同时提供了通过提出的功率控制方案实现最佳保密性能的证据。