## 4 Group/Ring Signature Based Privacy Schemes  
In order to provide anonymity and secure communication between V2X, as debated in the previous sections of this paper, there is a need to develop effective security and privacy mechanisms that address the issues of security and privacy efficiently. In ITS, a GSB approach is proposed by [PERSON] et al. [74] that suggests the use of group signature to provide unlinkability of messages generated by the same user. Only the group manager can reveal the privacy because the group manager has all the ITS  
Figure 6: ITS security and privacy schemes categorization. users information and thus a single point of security risk. [PERSON] el al. [PERSON] et al. (2018) proposed that the privacy of an ITS user can be achieved through group signature. The asymmetric cryptography can be used in group signature. The group members have their private keys and the group asymmetric key. The private key is used to produce signatures and send messages. To verify the signatures with messages at the receiving vehicle, the group asymmetric key is used. The real identity of an ITS user can be exposed only to the group manager. A modified form of short group signature that uses batch verification proposed in [PERSON] et al. (2018). However, the approach produces high computational and communication overheads. A decentralized approach of group signature is proposed by [PERSON] et al. [PERSON] et al. (2018), in which RSU acts as a manager of the small group. However, it is prone to side-channel attack. [PERSON] et al. [PERSON] et al. (2018) discussed a technique based on the group signature and identity-based signature. The technique combined short group signature and identity-based signature to provide anonymity. The main disadvantage of this technique is RSU participation in signature generation and verification, as there are side-channel attacks. [PERSON] et al. [PERSON] et al. (2018), discussed a technique of group verification by using elliptic curve cryptography (ECC) to minimize signature verification and communication overheads. However, the technique is prone to DoS attack. In order to minimize the computational time in verification of group signatures through RSUs, [PERSON] et al. [PERSON] et al. (2018) presented a technique which uses pseudonyms to achieve anonymity of ITS user. However, RSUs are located in open infrastructure and can easily be targeted by the adversaries. Similarly, the scheme suggested by [PERSON] et al. [PERSON] et al. (2018) is prone to DoS attacks, because bogus messages can easily be injected. Similarly, the mechanism presented in [PERSON] et al. (2018), improve the shortcomings of anonymity through pseudonyms. However, the mechanism is prone to Sybil and replay attacks. [PERSON] et al. [PERSON] et al. (2018) suggested Cooperative Message Authentication Protocol (CMAP) by using a short group signature approach. CMAP reduced computational and transmission costs, but the property of non-repudiation is not achieved. Furthermore, a group of vehicles can only verify the authenticity of the messages, while other vehicles just accept the messages on behalf of the verifier group. However, if any member of the verifier group is impersonated, the overall security of ITS is put at risk. To reduce the computational and communication overheads in GSB schemes, [PERSON] et al. [PERSON] et al. (2018) presented an approach based on group authentication. The technique presented in [PERSON] et al. (2018), reduced computational and transmission costs, but is prone to DoS attacks and is also not scalable. In summary, some group signature schemes incur high computational overhead and medium security, while in some schemes computational overhead is low but security is also low. The performance, security, and privacy analysis of GSB/RSB approaches are shown in Tables 1-3, respectively.