## 5 Pseudonym-Based Privacy Schemes
### Symmetric Cryptographic-Based Pseudonym Schemes/Symmetric Encryption
An advanced version of TESLA, to avoid the problem of storage stationed DoS attack, TESLA++ is presented by [PERSON] et al. [102]. TESLA++ is a symmetric cryptographic based scheme, in which the source reveals the secret key subsequently after some wait. However, TESLA++ does not furnish the property of non-repudiation. TESLA++ is the combination of TESLA and elliptical curve digital signature algorithm (ECDSA) [92]. TESLA++ is an advanced variant of TESLA, but TESLA++ incurs substantial computational overheads by calculating message authentication codes (MACs) of messages in addition to performing the activities of TESLA and ECDSA. In ITS, a vehicle has to generate and verify alarge number of messages within a second to properly achieve the benefits of ITS. However, by using TESLA++ the generation and verification of beacons status at a vehicle are affected. Another TESLA based approach presented in [103] for security checks in ad hoc networks. However, TESLA is prone to DoS attack [103]. In TESLA, the delay keys concept provides an opportunity for the adversary to attack. TESLA is also prone to pollution attack [104; 105]. In pollution attack, the attackers can deluge the receiving node storage with enormous bogus messages with wrong keys and strip to a pollution attack state. However, TESLA is also prone to storage stationed DoS attack [106]. In summary the performance, security and privacy analysis of symmetric encryption-based pseudonym approaches are shown in Tables 7-9, respectively. technique by combining standard pseudonym and group signature-based approach to induce self pseudonyms. The technique presented in [107], minimizes the computational overhead but introduces the problem of maintaining a large CRL and timely sharing of CRL to vehicles. Furthermore, self-generated pseudonyms introduce the problem of Sybil attacks. Another hybrid security approach is authentication with multiple levels of anonymity (AMLA) discussed by [PERSON] et al. [108]. The AMLA reduces computational and transmission costs but is prone to identity leaks, replay, and DoS attacks. Hence the security of AMLA is low and does not provide strong anonymity. [PERSON] et al. [109] presented a hybrid approach, in which a group captain is selected on the basis of the same direction traveling. However, how a group captain is selected has not been properly addressed. Similarly, there are chances of malicious activities as no proper revocation mechanism for malicious vehicles are discussed. The approach [109] is time consuming because of the processes of computing keys, random numbers, and hashes are performed at the same time besides verification of hashes. The scheme is not scalable and provides low security. [PERSON] et al. [110] presented a hybrid scheme called RHyTHM. The scheme [110] suggested that the group manager allows a vehicle to sign a safety beacon in lieu of the class. If a vehicle runs out of pseudonyms, the vehicle executes a RHyTHM protocol, in which RHyTHM flag is sent in the message to the class. The outdated vehicle then uses self-generated pseudonyms for communication. However, there is the possibility of malicious attacks, in the case of self-generated pseudonyms. The approach has a high computational overhead in case of ITS. Similarly, the average latency and communication overhead is also high. In order to ensure that only trustworthy vehicles are tabbed for the relay is still a great challenge. [PERSON] et al. [111] proposed a hybrid approach called privacy preserving trust basedrelay scheme (PTRS). The scheme suggested that there should be a trust/reputation value that ensures the trustworthiness of a relayed node. Trust authority (TA) is the sole entity of all information on vehicles and is a single point of attack. TA decides the trust level. However, the scheme is prone to replay attack. Furthermore, the computational and communication overheads are high. The vehicle can choose another vehicle for a relay that is to transmit messages.