# Blockchain-Based Authentication in Internet of Vehicles: A Survey
## 5 Discussion  
In Tables 4-6, we provide a summary of the blockchain-based authentication papers surveyed in Section 5. The table highlights the main properties of each scheme which are represented in the blockchain and cryptographic techniques used, the network model designed, the tools used for security verification and performance evaluation, the features, and limitations (if any), and whether each scheme supports a privacy preservation option for user's identity during the authentication process or not. From the table, we can conclude the following:  
1. The choice of the consensus mechanism used by the blockchain depends on many factors such as the amount of power consumption the system can afford, the percentage of faulty nodes the network can tolerate, and others. However, the size of the IoV network is also an important factor to be considered. For example, although the PBFT consensus is lightweight in power consumption and has a good fault-tolerance of approximately 33%, it does not support high network scalability; thus, it is chosen only by the small-scale IoV platforms such as in [128], or by fog-based IoV networks where the whole network can be large but the consensus is performed in fog-area-basis such as in [123; 130; 131; 133]. On the other hand, PoW has been chosen by the large-scale IoV environments [119; 122; 125; 126] due to its high scalability and high fault tolerance, despite its large power consumption. In general, we can say that these two consensus mechanisms are the most commonly used by the blockchains when applied for IoV and VANETs authentication.
2. Some of the papers only considered a V2V model to investigate their authentication schemes such as [120], while most of them have considered an integrated network model (V2V and V2I). Due to the ad-hoc and mobile nature of V2V communication, the authentication is more challenging in it.
3. Most of the papers support anonymous authentication schemes, while some of them provide pure authentication without supporting such privacy preservation option as in [111; 120; 126; 128].
4. In terms of performance analysis, the use of blockchain for authentication in IoV and VANETS networks introduces the need to include additional evaluation measures that reflect the efficiency of the blockchain itself such as the throughput which refers here to the number of transactions that can be verified and added to the blockchain per second [117; 119; 127; 139], the transactions latency [119; 122; 127], and the blockchain storage overhead [133; 126; 133; 136; 137], beside other measures that are used commonly to evaluate the conventional cryptographic-based authentication schemes including the average packet delay, the average packet loss ratio, the communication cost, and the computational cost.
5. Regarding the security analysis, the different authentication schemes have shown uneven levels of resistance against the various security attacks introduced in the previous sections.  
To conclude, a comprehensive view of the recent blockchain-assisted authentication schemes in IoV and VANETs was provided in this survey while highlighting the main differences between them in terms of security performance and operational performance. This helps in identifying the research gaps and creates a map that guides the future researchers in this area of IoV blockchain-based authentication. Although the adoption of blockchain for IoV authentication has brought many benefits such as the increased time-efficiency due to the decentralization of the authentication scheme among the distributed blockchain servers, and the more secure authentication reflected by the high resistance against different security attacks, a lot of work still must be done. This area needs more research and development to design new IoV-specific consensus mechanisms that can provide a more efficient balance between scalability and power consumption by consuming less power, supporting highly scalable IoV networks while maintaining the same good level of security represented by the high fault tolerance.