## 4 ITS Threats Analysis and Classification
### ITS Security Requirements  
To ensure a practical deployment of ITS systems, diverse security requirements must be attained to ensure secure V2X communications and ultimately safe driving. In particular, the design of ITS applications requires special attention and is characterized by specific challenges and requirements, as listed in Table 3 and discussed below in more detail:  
* Authentication: This is one of the most important ITS security requirement, which can be classified into three sub-requirements: (i) user authentication to prevent Sybil attacks and dismiss malicious entities; (ii) source authentication to ensure that messages were generated by legitimate ITS stations; and (iii) location authentication to ensure the integrity and relevance of the received information;* Data integrity: ITS entities (e.g., OBUs, RSUs, _etc._) should be able to verify and validate the integrity of the received messages in order to prevent any unauthorized or malicious modification, manipulation or deletion during transmission;
* Privacy and anonymity: The identities of drivers and vehicles should not be easily identifiable from the exchanged messages, and the right of the driver to control the access and use of her/his personal data should be enforced;
* Availability: Exchanged information should be processed and made available in real time, requiring thus the implementation of low-overhead and lightweight cryptographic algorithms;
* Traceability and revocation: ITS authorities should be able to track malicious ITS entities that are misusing the ITS system, in order to revoke them in a timely manner. The trust authority (TA) should be able to trace the vehicle and reveal its true identity. Furthermore, in case of a dispute or when a malicious vehicle is detected, the TA must revoke it and add its identity to the revocation list;
* Authorization: It is necessary to define the access control and authorization for the different entities. Specific rules should be enforced for accessing or denying specific ITS entities access and/or use of certain functions or data;
* Non-repudiation: Each ITS entity should be uniquely associated with its information and actions in order to achieve data authenticity and origination;
* Robustness against external attacks: ITS entities should be robust against external attacks, such as availability attacks, and ITS software should be almost free of vulnerabilities (e.g., buffer overflow) and logic flaws. * Data confidentiality: Exchanged messages should be properly encrypted and protected in order to prevent the disclosure of sensitive information to malicious nodes or unauthorized parties; \begin{table}
\begin{tabular}{l|c|c|l} \hline \hline Security Requirement & Broadcast & Unicast & Security Mechanisms \\ \hline Confidentiality & ✗ & ✓ & Encryption on sensitive messages; randomizing traffic patterns \\ \hline Authenticity & ✓ & ✓ & Message signature \\  & & & Trusted hardware module; active detection systems \\ \hline Integrity & ✓ & ✓ & Message signature and other integrity metrics for content delivery \\ \hline Authorization & ✓ & ✓ & Certificate accompanying message signature \\ \hline Non-repudiation of origin & ✓ & ✓ & Message signature \\ \hline Non-repudiation of receipt & ✗ & ✗ & Not necessary \\ \hline Anti-replay & ✓ & ✓ & Message signature containing verifiable time variant data \\ \hline Plausibility verification & ✓ & ✓ & Check mechanisms ensured by IEEE P1609.2. \\ \hline Availability & ✓ & ✓ & Pseudo-random frequency hopping \\  & & & Access control and signature-based authentication \\ \hline Privacy & ✓ & ✓ & Pseudonymity, unlinkability \\  & & & ID-based system for user privacy \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Security requirements _versus_ V2X communication types. As shown in Table 3, the above ITS security requirements depend on the considered V2X communications types. For instance, vehicle-originating broadcast (VOB) and infrastructure-originating broadcast (IOB) involve the broadcast of cooperative awareness messages to enable road safety applications.