Vehicular Ad Hoc Network (VANET) is a technology derived from a wireless ad hoc network technology called Mobile Ad Hoc Networks (MANET). Both technologies are using the same networking concept where each of mobile nodes is connected to create a mobile network. In VANET, moving cars are used as networking nodes to create and expand VANET networks. Each node integrates mobile connectivity protocol to establish a communication platform between other nodes and all roadside equipment available within the network coverage.
Thus, every car which is connected to VANET is able to communicate with others while moving freely within the network coverage. Important messages or notification alerts such as traffic alert can be sent to nearby receivers. This information can be used to improve road safety and optimize transportation system capabilities. This concept of mobility networking which exists in VANET draws a lot of reaction and feedback from both academia and industry with the effort to effectively develop and improve current transportation system to a brand new level which is often referred as Intelligent Transportation System (ITS) .
Although VANET offers a great and promising potential in contributing to the development of ITS, there are still crucial issues and challenges that need to be taken into consideration. One of the most important issues in VANET is security.
Security in VANET
The security of VANETs is one of the most critical issues because their information transmission is propagated in open access environments . Since VANET networks are mostly depending on mobile nodes, the security environment in the networks was not fixed. Owing to the nature of these kinds of mobile nodes, VANETs can be challenged with frequent topology changes as well as physical threats (that create potential vulnerabilities for potential attackers) .
Due to most of Ad hoc protocols do not propose special security mechanisms due to the nature of Ad hoc which is focused on reducing the consumption of energy and resource , the VANET network will remain as an open network and can be accessible from everywhere in the VANET radio range. Thus, it is expected to be an easy target for security attacks and malicious users .
In order for VANET to be the core networking platform for ITS development and realization, applying appropriate security architecture in VANET is a must and need to be on the top list of priority before being able to become a technology which can offers and guarantee public safety on the roads. A successful attack on VANETs can have catastrophic results (such as the loss of lives) or may lead to financial losses (for payment services) .
Therefore security in VANET is vital to ensure a smooth and successful network operation in any ITS development or project. Maxim Raya et.al  suggested that a security system for safety messaging in a VANET should satisfy several requirements to be able to thwart any generic attack on vehicular networks such as authentication, verification of data consistency, availability, non-repudiation, privacy, and real-time constraints.
In following chapters, we will talk about few major security attacks and threats which may affect ITS future development. Proposed security requirement and solution will also be discussed.
VANET Security Attacks
As mentioned above, VANET networks are an easy target for security attacks and threat due to its networking principle and fundamental. In this chapter, we will discuss about the capacities of an attackers by referring to the attacker’s model  and the classes of attacks in VANET .
In order to understand various types of security attacks, we must first define and understand the classes of attackers. Maxim Raya et.al  has described an attacker’s model to classify the capacities of an attacker in four dimensions which can be referred below:
|1) Insider vs. Outsider||An insider is an attacker with local authorization and authenticated access in the network while an outsider is an attacker which is not posse any access within the network and can be considered as an intruder.|
|2) Malicious vs. Rational||A malicious attacker only aims to attack and harm the network and system functionality without any personal attention or benefit. Malicious attackers also tend to attack randomly based on current exploits. A rational attacker is on the opposite site where everything done are specifically on purpose and with a benefit.|
|3) Active vs. Passive||An active attacker can generate packets or signals, whereas a passive attacker contents himself with eavesdropping on the wireless channel.|
|4) Local vs. Extended||Local attacker control several entities which is local and limited such as the vehicle or base stations while extended attacker controls several entities across the network.|
B. Classes of Attacks
VANET face many security attacks and these attacks and threat can be categorized in few classes. Sumra et al  have proposed five different types of classes for the attacks with the aim to provide an easy identification with association to respective class. Each of the classes will represent different types of attack level and priority.
Below are the proposed classes of attack:
|1) Network Attack||Network attacks are always on the top of the list and are classified as a top priority since it can be dangerous to the entire network. A single successful network attack may easily affect the whole network. Few example of network attack are such as Denial of service (DOS) Attack, Distributed Denial of service (DDOS) Attack and Syibil Attack. DOS and DDOS attack as its name suggest, tend to attack and denied access to the resources machine or network with attention to make it unavailable to its intended users. While in Sybil attack, the attacker sends multiple messages to other vehicles and each message contains different fabricated source identity to provide an illusion to other vehicle by sending some wrong messages like traffic jam message .|
|2) Application Attack||In application attack class, the attacker attention is no other than to manipulate application content for its own benefit. These attackers will tend to suppress or alter the actual message and change it with a false content which may cause harm to other vehicle. This type of attack might be done by either malicious or rational attacker for fun or to serve their own benefits. Few examples of application attack are such as message suppression attack, fabrication attack, alteration attack and replay attack .|
|3) Timing Attack||In this kind of attack, the attacker’s main concern and its objective is to create a delay in message delivery by adding in some time slot in the original message while leaving the content untouched. A successful timing attack in any road safety ITS application would create serious and dangerous effects including life accident if an important message not delivered in time.|
|4) Social Attack||Social attack contains all unmoral and emotional messages . The main objective in most of social attack is to indirectly create problem in the network by bad and unwanted messages which can affect the behavior of others road users.|
|5) Monitoring Attack||Monitoring attack is a serious threat for the road safety officials. In this kind of attack, the attackers which can be considered both local and outsider would silently monitor and track important messages which are not supposed to be release in public network. Attackers would use the valuable information gathered from eavesdropping Police line for example to serve their own benefit.|
VANET Security Requirement and Solution
Road safety is an issue that cannot be seen as a frivolous in any ITS applications. Security requirement and solution is a must to ensure the safety of road users.
In this chapter, we will discuss about few set of security requirement and solutions which are possible and practical to be deployed and used in securing VANET networks , , , , , and .
Authentication need to be made as a part of compulsory element in message communication in VANET. Every message must be authenticated to ensure its origin and to control the level of vehicle authorization  while every message reactions to event must be based from legitimate members .
B. Messaging Protocol
Stability in communication in VANET is difficult to achieve due to rapid network changes  and since the research and finding on VANET network and ITS applications is still in the preliminary stages , it is crucial for us to define and come out with a strong messaging protocol in effort to secure VANET messaging communication in the first place.
C. Batch Messaging Legitimacy
Road safety batch messages should be filtered and legitimated before public broadcast. It is essentials to secure the network from any DOS or message volume attack. The batch size must be fixed .
D. Message Encryption
Any sensitive information which was send as a communication message should not be sent in plaintext. Encryption and data compression are important to strengthen cryptographic security because it can reduce the patterns found in languages . However, public safety messages in VANET which are meant to be a broadcast message can be excluded in message encryption.
E. Digital Signatures
The communications messages in VANET are interchangeable between legitimate ITS application’s users. Message legitimacy is mandatory to protect VANETs from outsiders, as well as misbehaving insiders . A digital signature is practically suitable to control message legitimacy in the exchange of safety message in VANET.
F. Information Privacy
Information of drivers or vehicles such as car number, local identity number etc. must be treated as privacy information. This is important to ensure that this information is not breach from any unauthorized access. To achieve information privacy, an anonymous key can be used to act as a temporary key which will be changed frequently and limited with an expired time to preserve the information privacy .
Non-repudiation is a capability to identify the attackers even after the attack happens  which are important to offense the convicted criminals and to prevent them from denying their crimes  by storing all related information in the a device which should be made as compulsory for all road users.
H. Secure Routing
To secure ad hoc routing protocols, a proactive approach need be started to enhance the existing ad hoc routing protocols, such as DSR and AODV with security extensions . Securing ad hoc routing could also mean restricted ad hoc nature of networking which are known to be open for all network protocol. However, author believed that there should be a security extension to secure routing protocol for ad hoc in VANET in the effort to provide a safe and smooth networking and communication platform.
Although the capability of mobility networking with geographical extension is a promising potential for VANET networks to be the core networking platform in ITS, its security aspect is low under the par and too far behind the expectation in providing a stability and usability of road safety application in ITS. Based on analysis and results obtained by studies of Maxim Raya et al ,  it was clearly that the existing network security solutions cannot be readily applied to VANETs.
However, there is a huge opportunities if the security aspect of VANET is revised and improved. A lot of effort, studies and research are needed to be contributed to insure the security of VANET would work safe and sound.
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