## 1 Introduction
Institute of Electrical and Electronics Engineers (IEEE) and ETSI segmented DSRC band into seven different channels each of 10 MHz. Among the seven channels, there is one control channel and six are service channels. The service channels are used for data transmission while the control channel is used for setting the services and applications strived on service channels. In order to provide the services for resource management, security, networking, multichannel operations and single channel operations, IEEE has advised the standards for WAVE. WAVE IEEE standard adds the functionalities of IEEE 802.11P and IEEE 1609.x protocol stack [15; 16; 17; 18; 19]. As shown in Figure 4, in order to use the wireless medium in ITS, IEEE advised, IEEE WAVE also called IEEE 1609.x protocol stack [19]. IEEE 1609.1, IEEE 1609.2, IEEE 1609.3, and IEEE 1609.4 (1609.x) define the architecture, transmission framework, management, security and access in ITS. In Europe, IEEE WAVE is called ITS-G5. IEEE 1609.1 defines beacon format and storage of beacons by facilities layer. IEEE 1609.2 standard defines secure beacons format for DSRC. IEEE 1609.2 determines techniques to secure messages. It specifies the processes of how ITS-Ss (vehicles) performed security assistance, such as confidentiality, authentication, integrity, access control, and non-repudiation. IEEE 1609.3  
Figure 3: Intelligent transport system dissemination formation. specifies WAVE Short Message (WSM) and its related protocol WAVE Short Message Protocol (WSMP) to ensure the services of the networking and transport layer related to safety applications. It also specifies WAVE Service Advertisement (WSA) message. WSA message is used in a given area to advertise the accessibility of DSRC services like a WSA can be broadcasted by an ITS-S (RSU) to advertise the presence of media downloading service. IEEE 1609.4 standard discusses the management and usage of DSRC channels. ITS can be differentiated from mobile ad hoc network (MANET) [20] in terms of its unique characteristics. The unique characteristics are: (a) dynamic topology, (b) high speed, (c) vehicles mobility is restricted to fixed roads/maps, (d) sparse and dense scenarios, (e) vehicle privacy, and (f) unlimited storage and power. In ITS, every vehicle generates beacons regarding its current position, heading, speed, and road condition. However, there are also malicious vehicles and their aim is to damage the network. Malicious vehicles can misguide the honest vehicles. Checking the reliability of beacons is a challenge in ITS. There should be trustworthy techniques to verify the originality of beacons. Reliable security frameworks should be designed and developed to achieve the ITS objectives. If there is no proper security and privacy approaches, attackers may misguide or track vehicles to get malicious benefits. In order to provide a safe and efficient environment for ITS, first, the security and privacy challenges must be addressed. In ITS, the main issues are due to the following reasons:  
* Dynamic speed and topology: As the nature of ITS is ad hoc. There is no fixed topology. The speed of vehicles is changing, with respect to time. The beacon generation and verification should be done in minimum time, otherwise, there may be congestion and accidents [21]. * Sparse and dense scenarios: In the sparse scenario, inter-vehicle distance is large. While in the dense scenario, the numbers of vehicles are more with reduced distances. Thus, verification of more beacons in the dense scenario is difficult as compared to the sparse scenario. There  
Figure 4: ITS-S architecture with 802.11P/WAVE and IEEE 1609.x. should be smart security and privacy approaches that work efficiently in both sparse and dense scenarios [22]. * Bandwidth limitation: The problem of bandwidth limitation arises if there are more vehicles (dense scenarios) [23]. This problem may cause communication interference, delay, and affects the delivery ratio. Well-established security approaches need to be developed in order to address this issue.