## 2 Literature Study
This means the gateways are responsible for the authentication of vehicles and ZKRP is implemented within gateways. Although the proposed system claims to achieve security and privacy, it does not illustrate the mechanism for vehicles' registration and authentication and how ZKRP is generated. The privacy-preserving fair exchange scheme (Vehicle to Grid Exchange) V2 GEx has been proposed for Vehicle to Grid (V2G) and is based on a blockchain, utilizing zero-knowledge funds for fair funds deposit and claims [28]. The proposed system makes use of multiple zk-SNARKS for fund deposits and claims, causing the system slow processing since proof generation is an extensive task. A distributed firmware update scheme is proposed for autonomous vehicles using blockchain and zero-knowledge proof to generate proof of distribution [29]. Smart contracts are used for firmware updates. The updates are added to the smart contract as a transaction by the manufacturers, defining the access policy for vehicles in such a way that a vehicle meeting the criteria, defined in the access policy, can obtain updates. For vehicle authentication, attribute-based encryption (ABE) is used in the proposed system. The proposed system relies on ABE for the justification of access policy but since ABE is quite time-consuming, it can make the overall transmission process slow albeit secure. A blockchain-based security and privacy system named \"BPAS\" has been proposed for VANET to ensure the trustworthiness and accuracy of messages within VANET in addition to protecting their privacy [30]. The proposed system provides single registration for vehicles through Trusted Authority (TA) and the registration information is saved in the onboard unit (OBU) of the vehicles. If a vehicle wants to be part of the proposed network, it has to be authenticated from its OBU. The proposed system also relies on attribute-based encryptions (ABE) and a biometric extraction Fuzzy extractor. It works on the assumption that OBU is temper-proof, which can be risky. A blockchain framework is proposed for the security and transparency of the connected vehicles and users by recording every activity of all the entities within the blockchain network [31]. It is to ensure secrecy and transparency between customers and cab drivers. The registration and authentication of the vehicles is done using IoT device numbers within the vehicles and their identifiers. The proposed system is quite obscure and relies on the IoT devices' identifiers installed within the vehicles. A multi-agent AIM (MA-AIM) system is proposed based on Vehicle-to-Vehicle/Vehicle-to-Infrastructure (V2V/V2I) communication using blockchain for the security of vehicles crossing through an intersection [32]. The proposed system uses the concepts of gateways (Intersection Management-IM) to authenticate the vehicles' information by allowing them to cross the intersections. The proposed system is quite open in its working which means any vehicle can send messages/requests to intersection management (IM) by using its registration number. Hence any malicious vehicle can easily mislead the IM, resulting in traffic congestion near the IM. A summary of the literature study is presented in Table A1 in Appendix A. The work more closely related to ours is [33; 34]. Both are different from one another in terms of the system being introduced and the design goals. The system proposed in [33] is mainly for the fair dissemination of ads within a vehicular network using blockchain. It uses ZKPoK, a variant of ZKP to hide the identity of vehicles, but it does not take into consideration the RSUs. The proposed system incurs heavy computational costs during registration, ad dissemination, and reward payment. The major computational cost relates to ZKP, which is quite a heavy variant of ZKP. The overhead is also observed for on-chain computation as compared to off-chain computation. The proposed system is by large based on honest assumptions such as the honest working of RSUs to prove the correctness of the system.