# Privacy-Preserving Solutions in Blockchain-Enabled Internet of Vehicles
## 6 Privacy-Preserving Blockchain-Based Solutions
### Data Exchange  
There have been several research works which aimed to provide efficient data exchange, while ensuring privacy between users and between users and infrastructure in IoV, and they are presented below. [PERSON] et al. [49] proposed a scheme that focused in multimedia data sharing and threat management in vehicular system using blockchain. In this model, the participants involved are electric vehicles (EV) and their users, Roadside Units (RSUs), Trust Authority (TA) and a permissionless blockchain based on Ethereum. At the time of system initialisation, the Trust Authority identifies the entities involved and distributes their keys. The registration takes place as described: Vehicles' users or RSUs send their personal identities to the TA which generates a unique identity for each of them by performing a pseudo-random function and also a key pair by using a key generation algorithm. Furthermore, a hash chain is generated for each participant in order to be used in multimedia sharing authentication phase. Afterwards, the association relationship between each participant's (users and RSUs) identity and his public key is published to blockchain by the TA which also saves the connection between the users and their vehicles locally. Afterwards, TA stays offline, thus it does not have any influence to the decentralisation of blockchain. RSUs are considered as the full nodes which record on the blockchain registration and communication data among the vehicular system. Each entity that wants to share data firstly connects with the nearest RSU through his hash chain and afterwards calculates the hash value of the multimedia data, records the timestamp and signs those data which are then sent to the RSU. The RSU is responsible for verifying those data and updating the blockchain with the new transactions. Finally, a connection between the RSU and the receiver of the data is established, in order to send the shared data to him. During multimedia data sharing, users use pseudonyms to communicate with each other. The association between each entity's pseudonym (ID) and its personal data is known only by TA. The real identity of the participants and their sharing information is hidden through cryptographical primitives, so attackers cannot gain personal information and communication data. Furthermore, because every entity must be registered by the TA, unauthorised users who do not own keys and pseudonyms cannot communicate with others. In addition, malicious users trying to share illegal data are tracked and punished by TA. Therefore, the anonymous environment prevents the privacy of involved users, helps tracing malicious behaviour and also ensures the integrity and reliability of shared data. The blockchain adopted to test system's performance is Ethereum and according to the evaluation results the integrity of data is efficiently ensured while also protecting the privacy of the users involved. A cognitive engine can be added to improve the intelligence of traditional IoV. In addition, technologies like artificial intelligence and cloud/edge computing can be used to create an intelligent vehicular system. Therefore, Cognitive Internet of Vehicles (CIoV) have been developed to upgrade intelligence, security and safety of vehicular system by mining significant information from the network and from the environment [51, 64]. Experiments have been conducted on Ethereum chain to evaluate the system's performance and prove that the proposed scheme achieves privacy protection. [PERSON] et al. [50] proposed a scenario of using a hybrid blockchain architecture named PermiDAG, which is based on federated learning, in order to ensure data transmission in IoV. In the asynchronous learning proposed, the aggregation slot is divided into local and global aggregations. Vehicles run the local Directed Acyclic Graph (DAG) for local training and shared update models in federated learning while RSUs maintain the main permissioned blockchain for global aggregation.