## 4 ITS Threats Analysis and Classification
### Classification of ITS Attacks and Cryptographic Countermeasures
#### 4.4.1 Attacks on Availability and Countermeasures  
The availability of ITS systems is mandatory to ensure the safety of the involved drivers and vehicles. In this context, denial of service (DoS) attacks are currently recognized as the most dangerous threat to the availability of ITS systems, due to their major impact on the network resources. Indeed, the main objective of these attacks is to prevent legitimate users from using the network services and resources [18]. This kind of attack can be realized in the network by internal or external malicious nodes [51]. Moreover, an important variant of DoS attacks is the distributed denial of service (DDoS) attack [52], which is a distributed attack ordered by an attack manager with other agents who may be also victims unknowingly. In the following, several examples of intentional DoS and DDoS attacks are described, along with their corresponding countermeasures. * Jamming attacks: This kind of attack is realized at the physical layer, and its goal is to disrupt the communication channel [53] by transmitting noisy signals with high frequency, so as to increase the interference level. This leads to attaining a lower signal-to-noise ratio (SNR) and makes the vehicles unable to communicate with other vehicles and RSU stations [54]. The effects of jamming can be detected using specific techniques [53,55] and can be mitigated, for example, by randomizing the frequency hopping spread spectrum (FHSS) mechanism of the orthogonal frequency-division multiplexing (OFDM) standard [56], using efficient pseudo-random generator algorithms;
* Flooding attacks: This type of attack consists of flooding the network with a huge volume of dummy messages that are intentionally generated by malicious nodes [57], making thus the OBUs and RSUs unable to communicate over the wireless channel. This can lead, for example, to accidents if the basic safety messages are not received in time by the legitimate vehicles;
* Sybil attacks: The main goal of Sybil attacks is to jam the network by employing false nodes identities [16,58]. Therefore, legitimate vehicles conclude that the malicious messages are coming from legitimate vehicles and cannot detect the real identities of the attackers. To overcome this attack, a central validation authority (CVA) can be deployed to validate the entities in real time. The process of validation can be direct or indirect: for the direct one, any incoming node should authenticate itself with the CVA by establishing a direct connection. On the other hand, indirect validation enables an already authenticated entity to accept an incoming entity. The certificates that are used by the CVA are generally temporary [17]. Moreover, the authentication process should be further strengthened by using distance bounding protocols, such as the bit commitment and zero-knowledge methods [59]. Furthermore, other solutions to Sybil attacks consist of validating unknown nodes with the method of secure location verification [60];
* Malware attacks: Malware attacks consist of virus, worms and Trojan horses that can affect the vehicular network [51], as well as the software components of the OBU and RSUs [17, 18]. Such attacks can lead to dangerous consequences for ITS systems and can be mitigated by using anti-virus and anti-malware software. However, modern polymorphic and/or metamorphic malware can transform their structures and properties during the replication phase, thus making their detection much more difficult [61]. The typical cryptographic countermeasure consists of signing software updates and to verify them prior to their installation by the ECUs and OBUs;
* Spamming attacks: The main goal of spamming attacks is to consume the network bandwidth and to introduce a high latency in the network, by sending spamming messages (e.g., advertisement messages) to a group of users.