## 1 Introduction  
In wireless communications, relaying expands communication coverage and offers spatial and temporal diversity to mitigate the effects of fading [1, 2]. This capability makes relaying critical in enhancing signal reliability, particularly in challenging environments with obstructed direct paths between transmitters and receivers. As networks evolve towards 5G and beyond, the role of relays is becoming increasingly significant, not only for improving coverage but also for supporting the high data rates and low latency required by advanced applications such as the internet of things (IoT) and autonomous systems. Recent studies highlight the growing importance of relaying in heterogeneous networks, where devices with varying capabilities and security levels must interact seamlessly [3]. Traditionally, network security has been addressed at higher layers using cryptographic techniques. However, these methods often fall short of fully addressing the security demands of emerging applications like smart transportation, the Vehicle to X, and industrial automation networks [4] due to challenges such as reliance on infrastructure, inefficient spectrum usage, significant resource demands, and the complexity of signal processing. In contrast, physical layer security (PLS), grounded in information theory, leverages the inherent physical characteristics of the communication medium, including shadowing, fading, and interference [5]. This approach offers a low-complexity security solution that remains effective regardless of the computational power of the devices involved. The critical advantage of PLS is that its security remains unaffected by the computational capabilities of eavesdroppers. Furthermore, PLS provides greater flexibility in deployment compared to bit-level cryptographic methods [6], making it a promising alternative for securing these advanced applications. Interest in physical layer security for untrusted relay mechanisms has been increasing. While many studies have focused on protocols like amplify-and-forward and compress-and-forward, the decode-and-forward (DF) protocol presents unique challenges for untrusted relaying due to the conflicting needs for reliability and security [7]. In next-generation networks, 5G and beyond 5G and future mobile IoT networks with densely distributed mobile nodes, including sensor networks for traffic data collection and smart drone systems, will face heightened security concerns. This is particularly true in heterogeneous networks where nodes possess varying levels of security clearance. In such environments, a node that behaves unexpectedly could act as a potential eavesdropper and might pose significant security risks, rendering the relay untrusted [8]. Specifically, relays with lower trust levels or insufficient security clearance could act as potential unauthorized eavesdroppers [9]. These untrusted relays are not inherently malicious; rather, their lack of trust or inadequate security clearance makes them less reliable [9]. A non-zero achievable secrecy rate was indicated in [10] in a two-way untrusted relay system where the two sources only communicated through an untrusted intermediate relay. The ergodic secrecy capacity in an untrusted relay two-hop network was analyzed, and the compact expressions for the ergodic secrecy capacity were present in [11] under the assumption that a direct connection between the source and destination is absent. In the foundational work on untrusted relay networks, [12] established conditions to achieve a positive secrecy capacity, where the information is securely sent such that the confidentiality of the information surpasses a certain threshold. The security of this confidential information was measured through conditional entropy, focusing on coding/decoding challenges within relay channels. Subsequently, in a different study [13], [PERSON] assessed the secrecy-keeping level for confidential information from the relay by analyzing the private information's entropy rate, given by the relay's channel outputs.