# Pas: Privacy-Preserving Authentication Scheme Based on SDN for VANETs
## 2 Relate Works
### Blockchain-Based Authentication Schemes  
Due to the characteristics of decentralization, immunity, auditability and fault tolerance, blockchain technology is being tried by researchers to support VANETs authentication. BUA [24] realizes the maintenance of the blockchain storing vehicle information by adding multiple service managers. On this basis, the vehicles are able to independently generate legal pseudonyms to participate in anonymous authentication. However, due to the complete anonymization, BUA cannot meet the conditional privacy. It is difficult to track the malfunctioning vehicle. In order to solve the issue, BPPA [25] integrates certificate with blockchain, where all activities of issuing certificate for the vehicle are recorded in blockchain. Furthermore, in order to verify the legitimacy of vehicles, BPPA requires blockchain nodes to synchronize the issued certificate to RSUs and vehicles through the consensus mechanism. BPPA hides the real identity of the vehicle in the certificate, realizes the anonymity of the vehicle and ensures the traceability to the malicious vehicle. However, the proposed scheme does not solve the problem of high computational cost caused by verifying the certificates. EPAM [26] supports the distributed authentication by extending the blockchain with an asynchronous accumulator, and reduces the computational cost through a one-way hash function instead of certificates. In order to improve the authentication efficiency, the RSU can query the hash value of the vehicle certificate from the regional service manager who maintains the blockchain. However, the above schemes ignore the cost of vehicle information maintenance. Due to the high computational cost of blockchain and communication delay caused by the consensus mechanism, blockchain-based authentication schemes usually require blockchain nodes to have high computation and storage capacity.  
A summary and comparison of the above schemes and PAS are shown in Table 1.  
\begin{table}
\begin{tabular}{l l l l l l l l} \hline \hline
**Schemes** & **Identity** & **Message** & **Awnymity** & **Key** & **Distributed** & **Conditional** & **Unlinkablity** \\  & **Authentication** & **Authentication** & **Awreement** & **Resolution** & **Tracking** & **Unlinkablity** \\ \hline [PERSON] et al. [8] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & \(\times\) \\ [PERSON] et al. [9] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & \(\times\) \\ [PERSON] et al. [10] & ✓ & ✓ & \(\times\) & \(\times\) & \(\times\) & \(\times\) & \(\times\) \\ [PERSON] et al. [11] & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & \(\times\) & \(\times\) & ✓ \\ [PERSON] et al. [13] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & ✓ & ✓ \\ [PERSON] et al. [14] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & ✓ & ✓ \\ [PERSON] et al. [16] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & \(\times\) & ✓ \\ [PERSON] et al. [17] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & ✓ & ✓ \\ [PERSON] et al. [18] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & \(\times\) \\ [PERSON] et al. [19] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & ✓ & \(\times\) \\ [PERSON] et al. [20] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & ✓ & ✓ \\ [PERSON] et al. [21] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & \(\times\) \\ [PERSON] et al. [22] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & ✓ \\ [PERSON] et al. [23] & ✓ & ✓ & ✓ & \(\times\) & \(\times\) & \(\times\) & \(\times\) \\ [PERSON] et al. [24] & ✓ & ✓ & ✓ & \(\times\) & ✓ & \(\times\) & ✓ \\ [PERSON] et al. [25] & ✓ & ✓ & ✓ & \(\times\) & ✓ & ✓ & ✓ \\ [PERSON] et al. [26] & ✓ & ✓ & ✓ & \(\times\) & ✓ & ✓ & ✓ \\ PAS & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table}
Table 1: Comparison of the related schemes.