## 5 Pseudonym-Based Privacy Schemes  
In pseudonym-based approaches, fictitious names are assigned to provide anonymity of ITS-Ss (vehicles). Pseudonym-based schemes are further categorized into asymmetric cryptographic based schemes and symmetric cryptographic-based schemes. To provide a trustworthy and reliable relationship between the vehicles and ITS servers, there should be proper verification and integrity checks for ITS communication. There are various cryptographic protocols that can be used in ITS to achieve reliable verification and integrity of ITS communication. In summary, asymmetric/public key cryptographic techniques and symmetric/secret key cryptographic techniques can be used;  
(1) Public key cryptographic techniques: In public key cryptographic techniques [84] as shown in Figure 7, two keys are used namely public key and private key. Public key is used for encryption. It is publicly known in the system. While private key is used for decryption, it is only known to the receiver/authorized party of the messages. A certificate authority (CA) generates the key pairs in ITS. CA provides the public key certificates to ITS-Ss (vehicles) for reliable communication. CA can generate certificate revocation list (CRL) if any vehicle is violating the rules of CA (e.g., certificate expires, the participation of vehicle in malicious activities, keys compromised etc.). The CRL contains information of revoked certificates [80; 85]. However, standalone usages of public key techniques produce substantial overheads in terms of a large number of certificates and CRLs.  
(2) Secret key cryptographic techniques: Secret key cryptography uses a single key for both encryption and decryption [4], as shown in Figure 8. The key is shared between the two communicating nodes (ITS-Ss). However, secret key techniques alone can provide confidentiality of data. There are no proper guidelines for privacy and revocation of malicious users in ITS.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Research Papers** & **Node Capture** & **Malicious Code** & **False Data** & **Replay** & **Side-Channel** & **Eavesdropping** \\ \hline
[74] & No & No & Yes & Yes & No & Yes \\
[75] & No & No & Yes & No & No & Yes \\
[76] & No & Yes & Yes & Yes & No & Yes \\
[77] & No & Yes & Yes & Yes & Yes & Yes \\
[78] & No & No & Yes & No & Yes & Yes \\
[79] & No & No & Yes & Yes & No & Yes \\
[80] & No & Yes & Yes & No & Yes & Yes \\
[81] & No & Yes & Yes & Yes & No & Yes \\
[82] & No & Yes & Yes & Yes & No & Yes \\
[83] & No & No & No & No & No & No \\ \hline \hline \end{tabular}
\end{table}
Table 2: Attacks on GSB/RSB schemes.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Research Papers** & **DoS** & **Spoofing** & **Sinkhole** & **Wormhole** & **MIMA** & **Sybil** & **Phishing** & **Malicious Virus** & **Malicious** \\ \hline
[74] & No & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[75] & No & Yes & No & Yes & Yes & Yes & Yes & Yes & Yes \\
[76] & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[77] & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[78] & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[79] & Yes & Yes & No & No & Yes & Yes & Yes & Yes & Yes \\
[80] & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[81] & No & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[82] & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\
[83] & Yes & No & Yes & Yes & Yes & Yes & Yes & Yes & Yes \\ \hline \hline \end{tabular}
\end{table}
Table 3: Attacks on GSB/RSB schemes.