## 2 ITS Security and Privacy Challenges
* Networking and Transport Layer: The primary function of the networking and transport layer in ITS, is to transmit data between ITS-Ss. In the network and transport layer security threats target on the smashup of the accessibility of the network stocks. Due to the wireless nature of ITS, security threats in this layer are crucial. Network and transport layer attacks in ITS are as follows:
* Denial of service (DoS) attacks: Due to DoS attacks [53], all the resources of ITS are exhausted with desperate flux. As a result, honest ITS-S cannot receive the service. In order to relieve from DoS attacks, effective ITS schemes should be designed to address DoS attacks [36]. * Spoofing attacks: The main objective of spoofing attack [25; 54] is to gain complete access to ITS. After keeping the full access to the ITS, adversary sends bogus beacons. To defend against spoofing attacks, effective security techniques are needed to focus on proper authentication [37; 55]. * Sinkhole attacks: In sinkhole attacks [56], the attacked ITS-S, arrogates prodigious capabilities of key generation and communication. This will attract other vehicles to communicate through the particular ITS-S (RSU), thus disrupting the ITS. To relieve the ITS from sinkhole attacks [57], proper security routing approaches are needed. * Wormhole attacks: Wormhole attacks [58] are launched by two petty ITS-Ss, in ITS to transfer beacons with secure links in order to claim a fake single node communication interpolate them [58]. Due to wormhole attacks, forwarding nodes (ITS-Ss) are decreased. Now maximum messages are delivering through malicious ITS-Ss. As a result, honest vehicles can be misguided. To defend the ITS against wormhole attacks, proper authentication approaches are needed. * Man in the middle attack (MIMA): In MIMA [59], a malicious ITS-S (vehicle) is controlled by an opponent is placed virtually among other ITS-Ss (vehicles and RSUs). Malicious
ITS-S is a middle node between honest ITS-Ss, can record all the communication of the honest vehicles. This type of attack breaks the privacy, confidentiality, and integrity of vehicles by eavesdropping, modification, and full access to the communication between honest ITS-Ss. Secure and reliable communication approaches which can guarantee the authentication of uncompromised ITS-Ss can be an effective mechanism to MIMA [34; 36]. * Sybil Attacks: In Sybil attacks [25; 60], a malicious vehicle can arrogate many real identities and imitate them in the ITS. Due to these attacks, a malicious vehicle can have many real identities. An honest ITS user cannot distinguish fake messages transmitted by the malicious vehicle. As a result, honest vehicles are misguided. In order to protect an ITS from Sybil attacks, proper authentication and identification secure approaches are needed to be developed [37]. * Facilities layer: User-requested services are provided by the facilities layer. Thus, security threats at the facilities layer are a center on services attacks. Here in ITS, many applicants inquiring in the facilities layer are discussed as follows: *  
encryption using encryption techniques (e.g., zero-knowledge proof, secret sharing, homomorphism encryption etc.) can be used to guarantee that actual information related to the vehicle cannot be leaked by eavesdroppers [66]. However, encryption-based approaches only ensure confidentiality of data, not the privacy of the vehicles. Similarly, privacy preservation based on perturbation approaches--like information sharing, information customization, etc.--changes the order of information being sent to achieve privacy preservation [67]. However, by using perturbation techniques, revocation in ITS cannot be achieved efficiently with respect to time. Therefore, the application of privacy preservation based on perturbation cannot be considered in ITS. Thus privacy preservation techniques along with confidentiality in ITS are still a great challenge.