# A Survey on Satellite Communication System Security
## 5 Defense in Satellite Communication Systems
### Defense Methods for Confidentiality
This process is depicted in Figure 12.  
[PERSON] et al. [64] propose a line-of-sight, non-diffused link setup for inter-satellite optical wireless communication systems, employing coherent optical quadrature phase-shift keying modulation. This system is tailored for high bit-rates of up to 400 Gbps and is capable of significant coverage across various orbits, including LEO, MEO, and GEO. Also, a high-speed inter-satellite optical wireless communication system was designed in [35]. The research on free-space optical communication make it inherently more secure against eavesdropping compared to traditional RF communication. The narrow, direct path of the laser beam and its inability to penetrate walls create a communication channel that is hard to intercept without being noticed, thereby enhancing security and privacy in  
Figure 12: Anti-eavesdropping methods’ scheme. optical communications, especially in space, where line-of-sight communication paths are common [65]. * Anti-Eavesdropping--Cryptographic Methods  
**Quantum Key Distribution:** QKD is a technology that transmits secret keys using quantum principles over a communication network. Recently, with the rapid advancement of quantum computer research and the development in various countries, the security of contemporary cryptography based on mathematical complexity has started to be significantly threatened. As a response, QKD technology, which derives its security not from computational difficulty but from physical laws, has emerged. Recently, there has been development in the key distribution technology based on SCSs [66]. The process of QKD is depicted in Figure 13 and involves the following steps. First, [PERSON] sends photons to [PERSON], encoding bits (0 or 1) onto each photon's quantum properties, such as the polarization direction. The photons travel from [PERSON] to [PERSON] through a quantum channel, which could be fiber optic cables or free space. Second, [PERSON] measures the incoming photons using a basis that he chooses randomly. This basis may or may not match the basis used by [PERSON] for encoding. After the transmission, [PERSON] and [PERSON] publicly share which basis they used for each photon. They discard any bits where their bases do not match. Also, they compare a subset of their matching bits to check for errors or eavesdropping. A high error rate indicates a potential eavesdropper. Lastly, the remaining matching bits, where the bases align and no errors are found, form the secret key. QKD has the following features: (1) Immunity to mathematical decryption. Quantum cryptography is based on irreversible physical natural phenomena, making it immune to decryption through mathematical approaches. (2) Resistance to replay attacks. Quantum cryptography is immune to replay attacks. Any slight mistake by an attacker in measuring the signal distorts it, leading to the receiver detecting that the signal has been intercepted. (3) Principle of non-clonability. In quantum cryptography, a photon cannot be perfectly copied due to the principle of non-clonability, a fundamental phenomenon in quantum mechanics. Without perfect measurement of the original signal, distortion occurs, making it impossible to obtain information [67]. Because of these features, QKD is considered the most perfect solution against eavesdropping and is one of the ideal methods for generating one-time pads. The process is depicted in Figure 14. Like Figure 14, numerous studies have been conducted on using this technology in SCSs. Early research [68] discovered the ability to create entanglement in the Earth's orbit. Subsequently, we progressed rapidly to stages where actual data could be sent. Initially, satellite-based QKD was close to the experimental stage, resulting in a trade-off between key delivery speed and distance [69; 70; 71; 72]. Later, satellite-based QKD evolved to capture both distances and key transmission speeds [73; 74], and research also emerged  
Figure 13: Quantum key distribution scheme.