## 3 Development History of Navigation Signal Authentication Technology
### Design and Analysis of a Public Key Infrastructure for BDS Data Authentication Key Management  
Key management involves the management process of the key life cycle, such as key generation, distribution, update and revocation. It is also related to the administrative management system of keys. The functions of key management are as follows: Firstly, when using authentication services, a chain of trust for keys needs to be built. Secondly, keys are regularly replaced to prevent them from being intercepted and exploited by malicious attackers. Thirdly, when keys are leaked, they can be changed in time. Considering one-way communication and the small bandwidth of satellite broadcasting, the key management scheme includes three-level key management based on a Merkle tree, two-level key management based on ECDSA, and three-level key management based on the TESLA protocol. The details are shown in Table 8. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**System** & **Key Length** & **MAC Length** & \
\begin{tabular}{c} **Keychain** \\ **Update Cycle** \\ \end{tabular} &
\begin{tabular}{c} **Time Synchronization** \\ **Requirements** \\ \end{tabular} \\ \hline Galileo [13] & 128 bits & 40 bits & 168 h (1 week) & \(\leq\)30 s \\ \hline SBAS-BigMAC [28] & 30 bits & 115 bits & - & need \\ \hline SBAS-LittleMAC [28] & 30 bits & 15 bit & - & need \\ \hline NavIC [15] & 116 bits & 30 bits & - & \(\leq\)48 s \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Security design of TESLA protocol for typical satellite navigation system. 2. Second-level key management based on ECDSA. The ECDSA scheme is an alternative scheme for SBAS authentication, and its key management scheme adopts second-level key management. The second-level keys are the public and private keys for message authentication, and the first-level key is the system CA public key [72]. The scheme is as shown in Figure 10.
3. Three-level key management based on TESLA. The TESLA protocol is an alternative scheme for SBAS authentication, and its key management scheme adopts three-level key management. The third-level key is the TESLA key, the second-level key is the TESLA public key to authenticate the root key, and the first-level key is the CA public key [72]. The scheme is as shown in Figure 11.
4. Three-level key management based on TESLA. The TESLA protocol is an alternative scheme for SBAS authentication, and its key management scheme adopts three-level key management. The third-level key is the TESLA key, the second-level key is the TESLA public key to authenticate the root key, and the first-level key is the CA public key [72]. The scheme is as shown in Figure 11.  
Figure 10: Second-level scheme based on ECDSA [72]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**System** & **Key Management** & **First-Level** & **Second-Level** & **Third-Level** & **Receiver Built-In Key** \\ \hline Galileo [25; 26] & Three-level scheme based on the Merkle tree & Merkle tree root key & TESLA public key & TESLA shared key & Built-in Merkle tree root key \\ \hline SBAS [72] & Second-level scheme based on ECDSA & CA public key & Message authentication public and private key & – & Built-in CA public key \\ \hline SBAS [72] & Three-level scheme based on TESLA protocol & CA public key & TESLA public key & TESLA shared key & Built-in CA public key \\ \hline \hline \end{tabular} (1) Three-level key management based on a Merkle tree.  
\end{table} 
Table 8: Key management scheme. Figure 9: Three-level scheme based on the Merkle tree.