# Blockchain-Based Authentication in Internet of Vehicles: A Survey
## 6 Future Research Directions and Open Challenges
### Efficient Design of Blockchain-Assisted IoV Authentication
In the asynchronous mode, the sending entity does not wait for acknowledgement from the receiving node on a previousrequest, instead, it directly proceeds with the following communication steps. The consensus protocols that use this mode of communication such as Proof of Work (PoW) and Proof of Time (PoT) could be thought of as perfectly suitable to be used for authentication in the different IoV applications which are mostly time-critical applications in which even a small delay of milli seconds is not forgivable. However, when considering some availability issues scenarios where the receiving entity can be temporarily out of network, waiting for an acknowledgement from the receiving entity before proceeding on with further interaction would save a lot of time and bandwidth that were used for such useless communication. Therefore, developing novel consensus making protocols that combine synchronous and asynchronous modes into one hybrid mode that incorporates the advantages of both communication modes, for example, by setting some thresholds for the number of acknowledgments to be received before proceeding the rest of communication asynchronously, or by using timeouts. However, such proposed solution is in turn full of challenges, since this hybrid mode of operation needs to be repeated periodically and regulated with other thresholds, that is, the synchronous mode should be injected once after every consecutive asynchronous interaction to check periodically that the receiving entity can be reached. This arrangement imposes an extra overhead associated with designing the optimal thresholds, monitoring, and managing the different time windows and timeouts. Thus, the feasibility of developing such complicated hybrid consensus protocols to be used for IoV authentication while maintaining a relatively low operational cost is another future challenge to be investigated. Thus, to design an efficient authentication protocol for IoV based on blockchain that maintains a good performance balance while highly considering all the above-mentioned factors is a great challenge that should be addressed by future researchers. In the way to achieve this, serious efforts should be dedicated to developing all-in-one IoV-specific consensus mechanisms that can meet all the requirements of IoV applications including high scalability, high fault tolerance, real-time response with low energy consumption. Beside these factors, another important challenge in this area to consider is how to achieve the optimal assignment of the various blockchain-related tasks such as blocks creation, validation, and consensus making to the different IoV nodes based on their computing capabilities and the energy consumption requirements of the adopted consensus protocols in order to achieve an efficient authentication process. In [119] for instance, blocks creation is assigned to the infrastructure units through PoW due to their high computing power, whereas blocks verification is assigned to the vehicles through PBFT consensus due to their relatively low computing capabilities. However, more technical details and conditions need to be fully explored in upcoming research studies.