## 4 ITS Threats Analysis and Classification
### Classification of ITS Attacks and Cryptographic Countermeasures
#### 4.4.2 Attacks on Authenticity/Identification and Countermeasures  
Authenticity is a key requirement in ITS systems to ensure the protection of the legitimate nodes against several attacks, including black holes, spoofing and replay attacks. The digital signature represents the most commonly-used cryptographic countermeasure for ensuring the authentication of the ITS entities. It allows the receivers to verify the origin of the data. Only the authenticated nodes can access the ITS resources and services. Any weakness in the process of identification and/or authentication can expose the entire network to serious consequences. Indeed, external or internal attacks can be achieved using falsified identities [51]. In the following, several examples of attacks on authentication are described along with their corresponding cryptographic countermeasures.  
* Falsified entities attacks: In the falsified entities attack, the attacker obtains a valid identifier and passes for another legitimate node. This constitutes a violation of the authentication process. Every ITS entity has a network identifier, which allows distinguishing it from the other nodes of the ITS system [17]. For example, rogue access points (AP) can be deployed along the roads to mimic legitimate RSUs and to launch attacks on the associated users and vehicles [67]. This attack can be prevented by implementing proper authentication mechanisms, for example by using a public key infrastructure, where each ITS entity is associated with a valid certificate, which is signed by the ITS authority;
* Cryptographic replication attacks: In this kind of attack, keys and/or certificates are duplicated to create ambiguity. This can prevent the authorities from identifying a vehicle, especially in the case of a dispute. The first countermeasure consists of the use of certified and disposable keys to resist these attacks. Another solution is the real-time verification of the certificate validity through a certificate revocation list (CRL) [47,68]. However, this solution is challenging in the context of ITS, since it requires cross-certification trusts between the different certification authorities involved in the ITS security scheme [68];
* GNSS spoofing and injection attacks: In ITS, position information is of crucial importance and must be accurate and authentic [18]. Such information is generally obtained from Global Navigation Satellite Systems (GNSS). In this context, the GNSS spoofing and injection attack is considered as the most dangerous threat on cooperative ITS [15]. It consists of providing the neighboring vehicles with false location information. The exact location information is generally obtained from a GPS system. Each vehicle is equipped with a GPS receiver, then the attack can be achieved using a transmitter generating localization signals stronger than those generated by the real GPS satellites [17,69]. The successful GPS spoofing attack can facilitate other attacks, such as attacks against location-based identification methods. This attack can be prevented by using bit commitment and signature schemes with positioning systems that accept only authentic location data [59,70];
* Timing attacks: In ITS safety applications, the timely delivery/reception of safety messages is of prime importance to ensure the safety of drivers and passengers. In this context, the timing attack consists of delaying the transmission of latency-sensitive messages so that the safety requirements are not achieved [71]. This attack can be enabled by forcing legitimate ITS entities to transmit their messages through a malicious node or tunnel, which will delay the reception of these messagesby the other legitimate entities. The most common countermeasure consists of time-stamping the delay-sensitive packets at the cost, however, of more complex time synchronization mechanisms.