# Secure Trust-Based Blockchain Architecture to Prevent Attacks in VANET
## 1 Introduction  
Vehicle ad hoc networks (VANET) emerged as a subset of a mobile ad hoc network (MANET) [1] application. VANET is considered a substantial approach for intelligent transportation systems (ITS) [2]. VANET has recently been the focus of various researchers in the wireless mobile communication field. The aim of VANET is to provide inter-vehicle communication and roadside units to vehicle communication to increase road safety and improve local traffic flow and the efficiency of road traffic by providing accurate and timely information to road users [3]. In VANET, vehicles are used as network nodes, as seen in Figure 1.  
There are two types of communications in VANET, which are vehicle to vehicle (V2V) and vehicle to infrastructure (V2I) communications [4]. The on-board units (OBUs) and road side units (RSUs) in VANET establish a connection among themselves with the help of dedicated short-range communication (DSRC) in a single or multi-hop communication [5; 6; 7]. VANET offers various services and applications to the users, most of which are concerned with the safety of the drivers, infotainment, and navigational aid [8]. There are two types of information shared in VANET: safety (vehicle speed warning, curve warning) and non-safety information (value-added comfort application) [9]. By default, safety information is given a higher priority in VANET as compared to non-safety information, since safety information notifies drivers of expected dangers to allow anearly response [10; 11]. Despite the benefits offered by VANET, it comes with challenges, especially in terms of the security and privacy of users and transmitted messages [12]. As vehicles enter and exit highways, they require certain safety information, such as traffic congestion and road conditions, to make decisions on which route to take to their destination. It is essential that this information be delivered in a timely manner; otherwise, it could lead to delays in reaching the destination safely [13]. In certain scenarios, some malicious nodes refuse to relay or even intentionally modify the required safety messages before transmitting to the requesting user, which could result in longer delay or fatalities. Besides this, characteristics of VANET (e.g., high mobility, volatility) which are distinct from other wireless communication networks have caused VANET to be susceptible to numerous internal and external attacks [14]. Due to the decentralized structure and dynamic topology of VANET, the security of the vehicles, users, and data has become essential, since the identification of malicious or faulty nodes or users has become difficult [7]. In VANET, vehicles exchange sensitive information and traffic changes with each other [15]. However, a lack of authentication of this information can result in malevolent attacks which present harm to drivers [16]. However, for messages to be authenticated, the vehicles in the network are tracked for their identification and whereabouts at any given time [17], compromising the privacy of the users. Hence, there must be a perfect balance between authentication and preserving the privacy of the users [18]. Several researchers have developed various techniques for preserving privacy, such as pseudonyms [19] and anonymous authentication [20], which could achieve the goal of preserving the privacy of the users, as long as the pseudonyms cannot be linked to the user. Nevertheless, these schemes may not be very secure, because reported traffic information can be utilized to link the pseudonyms to the users, as vehicles do not change their pseudonyms during information exchange [21]. Not only that, the availability of abundant valid pseudonyms for each vehicle makes VANET vulnerable to attacks, such as Sybil attacks, as the pseudonyms could be used to correctly authenticate non-existing vehicles [22]. Although solutions that can provide secure communication channels against external attacks are available, trust management and privacy protection for vehicles are still open issues for VANETs [23].