# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 2 Background
PLS techniques primarily rely on the inherent characteristics of the physical communication channel rather than complex mathematical computations. Therefore, quantum algorithms like [PERSON]'s algorithm, which excels at factoring large numbers and solving discrete logarithms, are not directly applicable to breaking PLS schemes. This makes PLS a promising approach for ensuring secure communication in a post-quantum era, complementing traditional cryptographic methods. Secure information is embedded into the transmitted signal in a way that is imperceptible to an eavesdropper but can be recovered by the legitimate receiver. Physical layer coding can be employed to protect the embedded information from being extracted by unauthorized parties. PLS techniques need to be adapted to the unique characteristics of satellite channels, such as high path loss, Doppler shift, and shadowing. PLS could be combined with upper-layer security mechanisms to provide a comprehensive security solution. The security functions of PLS can be summarized as follows:  
* Confidentiality: PLS safeguards information from unauthorized access through various methods. It can establish a secure key exchange by leveraging channel characteristics as a shared secret. Moreover, sensitive data can be embedded subtly within the transmitted signal, rendering it imperceptible to eavesdroppers. Additionally, PLS complements traditional encryption by providing an extra layer of protection rooted in the physical layer's properties. * Authentication: PLS contributes to verifying the authenticity of communication participants. By utilizing unique channel characteristics, physical layer fingerprinting can authenticate the transmitter. This technology also aids in confirming the identity of devices through their distinct physical layer parameters. Furthermore, PLS enhances message integrity by detecting and mitigating tampering attempts at the physical layer. * Integrity: To preserve data integrity, PLS incorporates error correction codes to rectify transmission errors. It also functions as an anomaly detection tool, identifying irregularities that may signal malicious activities. Additionally, PLS contributes to data origin authentication by incorporating physical layer information into the verification process. By harnessing the physical properties of the communication channel, PLS offers an approach that complements traditional cryptographic methods, enhancing overall security. PLS, enabled by information-theoretic models, entails lower computational complexity compared to cryptographic techniques. It lowers the burden of additional code space required and saves the energy required to perform complex mathematical computations for cryptographic techniques [37]. PLS utilizes the randomness of the wireless channel, such as fading and noise, to secure information [38]. Leveraging the inherent characteristics of the physical layer can significantly reduce the energy overhead for security, crucial for resource-constrained systems where additional computational burdens are impractical. By exploiting the physical properties of the channel, PLS can lead to a significant reduction in the required energy overhead for security, which is critical for resource-constrained devices where additional computational burden may be needed. It becomes an efficient way to ensure security in novel cases of 6G, such as mMTC, URLLC, and cyber-physical systems. Newly emerging services like IoT, tactile internet, remote surgery, and mMTC are required to have lower latency and are power-limited applications. Encryption methods would be less suitable to scale for such applications [39]. PLS can complement upper-layer authentication methods to provide robust security to ISTNs [40]. In summary, PLS can be useful in the following scenarios:  
* Latency requirement: For low-latency requirements, delays incurred by communication and computation overhead should be minimized.