## 1 Introduction  
Vehicular ad hoc network (VANET) is one of the most significant current discussion in the intelligent transport [1]. VANET provides a platform for vehicles to share road-related messages with other peers. This ability has greatly improved the management of urban traffic and road accidents. The point is event messages that dramatically increase in this network. Moreover, due to the numerous and diversity of vehicles in this network, vehicles are unable to fully trust each other. As a consequence, a trust management system and a method to coordinate the vehicle's interactions efficiently are required. The primary option, for the defined purpose, is a centralized system. In this kind of trust management system, all messages are stored and processed in a central server. Centralized systems can carry risks such as bottlenecks caused by increased layers of approvals and slower decision-making and single-point of failure. This challenge faces the system with efficiency and communication between node's problems. On the other hand, decentralized system is one of the most well-known approaches to overcome centralized issues. In decentralized systems, every node makes its own decision. One also should not overlook the fact that the final behavior of the system is an aggregation of the individual node's decisions. With this in mind, Blockchain is a one of the solutions to solve above problems. However, regarding the decentralized approach, it is a practicable tool for VANET. Blockchain enables distributed nodes to cooperate with each other and maintain a consist and tamper-proof ledger. Furthermore, there is no requires trusted third party (TTP) to establish trust, the Blockchain technology can be seen as a feasible distributed platform. It is a distributed peer-to-peer (P2P) system for communicating and transferring transactions between nodes without TTP. Some Blockchain technology applications include digital currencies [2], health insurance system [3], health care system [4], Internet of things [5], social networks [6], intelligent contracts [7], voting system [8], authentication [9], notary [10], data storage [11], energy supply [12], supply chain management [13] and protection of intellectual property[14]. In all of these applications, Blockchain tries to eliminate the focus on a single entity. This has great potential for implementing centralized systems in a distributed way without the mentioned problems. A combination of Blockchain and IoT technology can greatly improve the field of security, reliability, storage of data, and immutability. However, due to the mining process, most of the Blockchain technologies have some shortcomings such as low transaction throughput and poor scalability [15]. In VANET the number of vehicles and their exchanged messages increase continuously, therefore the scalability is another important issue. Sharding as a consensus algorithm can make Blockchain's ledger more efficient, scalable, and sustainable by dividing large amounts of data into chunks. In this paper, a sharding consensus algorithm is employed to provide scalability. Sharding algorithm distributed mining tasks into committees, each of the committees processes a set of transactions [16]. Based on the distributed nature of Blockchain, trust management can be applied in distributed way among road side units (RSU), which can forbear the centralized issues. Moreover, Blockchain enables RSUs to work simultaneously together and patronize a consistent database. The main contribution of this paper can be summarized as follows:  
1. We propose a scalable and tamper-proof distributed trust management system for VANET. In this system, RSUs response to vehicle's queries and calculate the trust by employing sharding consensus algorithm in several committees. 2. We propose a new Bayesian Inference-based validation procedure for vehicles to neutralize the effect of deceptive messages. 3. We simulate the proposed model to show that the proposed scalable Blockchain trust management system is efficient in VANET environment.