## 2 Principles and Technical Architecture of the Satellite Navigation Signal Authentication
### Principles
#### 2.1.2 Satellite Navigation Message Authentication Type  
The navigation message authentication protocol includes Digital Signatures (DS) and the Timed Efficient Stream Loss-Tolerant Authentication (TESLA).  
Digital signatures are implemented based on asymmetric cryptography (also known as public key cryptography). The sender uses the private key to sign the message, and the receiver uses the public key to verify the signature of the message [27]. Digital signatures commonly use the Elliptic Curve Digital Signature Algorithm (ECDSA), which has the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Type** & **Indictors** & **Receiver Processing** & **Feature** \\ \hline NMA [25] & Galileo-OSNMA & Message bit authentication & The project implementation is less difficult, the security level is not as good as SCA, and it is not as good as SCA, and it can be processed in real time \\  & Time Between Authentication: 10 s & Code (MAC) & can be processed in real time \\  & & & at the terminal. \\ \hline \multirow{4}{*}{SCA [26]} & NTS3-CHIMERA & \multirow{4}{*}{Power Domain Authentication Using Sampled Data for Spreading Cache, and the project implementation is costly.} \\  & Time Between Authentication for fast channel: 1.5 s or 6 s & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of NMA and SCA.  
Figure 3: CHIMERA spreading code [26].  
Figure 2: GALILEO NMA message structure [25].  
characteristics of high security and complex algorithm strength. In addition, European scholars proposed [PERSON]'s digital signature algorithm [18]. The digital signature schematic is shown in Figure 4 below.  
The TESLA protocol is a broadcast authentication protocol that can be applied to satellite navigation broadcast signals with limited bandwidth [28,29]. The TESLA protocol, designed by [PERSON] et al., is an MAC-based broadcast authentication protocol [30,31]. The protocol uses a symmetric cryptography method, and the key is to use the delayed key release to ensure the security of the broadcast key.  
The TESLA protocol generates a set of keychains through the hash function. The generation order of the keychain is \(Key_{i}\), \(Key_{i-1}\),..., \(Key_{1}\), \(Key_{0}\), while the keychain system uses \(Key_{0}\), \(Key_{1}\),..., \(Key_{i-1}\), \(Key_{i}\). The advantage is that when the key is not received or not received at a certain moment, the key can be obtained via the key hash of the subsequent epoch. Then, according to the key \(Key_{i}\) and the navigation message \(M_{i}\) at the current moment, the Hash-based Message Authentication Code (HMAC) algorithm is used to generate the message authentication code \(MAC_{i}\). The GNSS system broadcasts the navigation message \(M_{i}\), the message authentication code \(MAC_{i}\), and the \(Key_{i-1}\) of the previous epoch to the user; that is, the symmetric key used to generate the MAC is sent after the broadcast MAC is delayed by \(\delta\) time. The user receives the GNSS message \(M_{i}\) for storage and the delayed symmetric key \(Key_{i}\), then generates delay \(MAC_{i}^{\prime}\), and compares it with the \(MAC_{i}\) of the GNSS broadcast. If the two are consistent, the authentication is passed. Key chain generation and the key usage of TESLA are shown in Figure 5 below.  
Compared with the ECDSA algorithm, TESLA has a lower computational load and communication load, and is suitable for satellite navigation systems with limited message bandwidth. TESLA's one-way keychain generation and transmission improve the stability of authentication services. ECDSA has a variety of international standards, and the imple  
Figure 4: Digital Signature Schematic.  
Figure 5: Key chain generation and key usage of TESLA.  
mentation process is simple, but ECDSA occupies more data bits. The comparison between TESLA and the digital signature is shown in Table 2.