# A Survey on Satellite Communication System Security
## 5 Defense in Satellite Communication Systems
### Solutions for Integrity
#### 5.3.3 Cryptographic Methods  
(i) Navigation Message Authentication: NMA is an anti-spoofing method for GNSSs. It involves verifying the integrity and authenticity of the messages sent by GNSS satellites to receivers. NMA is a method to ensure that the navigation data received by a GNSS receiver are genuine and have not been tampered with or fabricated by a spoofer. Ref. [115] discusses an anti-spoofing methodology using QZSS L1C/A and L1-SAIF signals, highlighting how signature data generated from a part of the L1C/A navigation message are broadcasted for authentication. This study emphasizes the vulnerability of civilian GPS signals, traditionally not intended for security-critical applications, to various forms of interference and attacks. It also reviews different methodologies to combat spoofing and meaconing problems, detailing the use of QZSS L1-SAIF signals for broadcasting signature data for signal authentication. This paper explains the process of spoofing GPS/QZSS receivers and the manipulation of receiver output data. Also, ref. [54] presents a practical technique for authenticating civil GPS signals, combining the cryptographic authentication of the GPS navigation message with signal timing authentication. This approach aims to secure civil GPS receivers against spoofing attacks and discusses NMA as a basis for civil GPS signal authentication, where the navigation message is encrypted or digitally signed for verification by the receiver. It provides a generalized model for security-enhanced GNSS signals and describes various spoofing attacks, including meaconing and SCER attacks. The authors of [116] discuss an anti-spoofing scheme for BeiDou-II NMA and spread spectrum information (BD-II NMA&SSI). This scheme is designed to protect BeiDou II D2 navigation messages from spoofing attacks using cryptographic algorithms and spread spectrum information. Unlike the previous studies, ref. [117] presents a new type of method that uses the timed efficient stream loss-tolerant authentication (TESLA) protocol for NMA in GNSSs. TESLA, originally designed for internet applications, has been adapted for GNSSs to address their unique requirements. The key features of TESLA in this context include the use of symmetric cryptography, which lowers the receiver's computational burden, and the generation of a one-way key chain by the sender, with keys disclosed in reverse order. Users verify each key using a previously authenticated key or the root key, which is typically authenticated using asymmetric encryption techniques like digital signatures. To suit GNSSs, TESLA underwent modifications, such as reducing the key size and bandwidth through truncating key chain outputs and introducing deterministic padding, which may incorporate elements like the Galileo system time. (ii) Spreading Code Authentication: SCA is a technique in anti-spoofing that focuses on securing the GNSSs used in satellite signals, which are essential for accurate positioning and timing information. SCA works by ensuring that the spreading codes received by a GNSS receiver are authentic and have not been manipulated or imitated by a spoofer. For example, the authors of [118] discuss the generation and application of supersonic codes for GNSSs. The key idea is multiplexing supersonic codes with an open code, achieved through block-cipher encryption of the open code. This process generates an encrypted code, referred to as the \"fundamental code\", valid for a predetermined cryptoperiod. Supersonic codes, transmitted alongside open codes like GPS C/ A or Galileo OS, are based on symmetric cryptographic schemes and block ciphers. This setup enables direct authentication without time dependency and allows interoperability with open services. Also, [PERSON] et al. [119] propose a method named Chimera to enhance the security of GPS civilian signals against spoofing attacks. The Chimera concept involves authenticating both the navigation data and the spreading code of a GPS signal through 'time binding'.