## 5 Pseudonym-Based Privacy Schemes
### Symmetric Cryptographic-Based Pseudonym Schemes/Symmetric Encryption  
Symmetric encryption is more efficient than asymmetric encryption in terms of computation. However, symmetric cryptographic based schemes alone do not provide non-repudiation property. [PERSON] et al. [93] presented a symmetric cryptographic based scheme that does not depend on identity mapping to be performed by any party and the identity mapping information is directly incorporated in pseudonyms. The pseudonyms can be resolved only when multiple parties cooperate. This scheme suggests that fictitious perseverance is performed by decrypting the V-token. In this scheme, the CA and pseudonym provider (PP) can encrypt pseudonym linking information. However, the incorporation of pseudonyms resolution information directly in pseudonyms certificates can jeopardize the privacy of vehicles. In addition, there exists replay, Sybil and side-channel attacks. The scheme proposed in [94] uses secret key cryptography. The scheme computes hashes (message digest) and sends along with safety messages for integrity. At the receiving end hashes are calculated and then compare with the received hashes to ensure integrity. However, the approach is prone to DoS, Sybil, and side-channel attacks. [PERSON] et al. [95] discussed the strategy to improve the privacy of vehicle location in mix-zones called anonymizing area using cryptographic mix-zone (CMAX) protocol. The assigned private key by RSU is used to encode the status information (heading, position, and velocity). As per CMAX protocol, the RSU assigns private key only to authorize vehicle that enters the mix-zone. The private key is then used to encrypt the messages/beacons. The sender private key will be used in a mix-zone to encrypt the status information. The RSU decrypts the messages using the private key of the receiving vehicle and forwards them to the receiving vehicles that are within the source vehicle proximity also called neighbor vehicle. However, the approach is prone to replay and side-channel attacks. The approach presented in [96] minimizes computational cost in signature verification but does not properly address the revocation process of malicious vehicles. In [97], the researchers tried to protect the privacy of vehicles by using three entities in the model i.e., Key Management Center (KMC), RSU, and vehicle. The KMC is responsible for the registration of vehicles, road side units, and traceability of vehicles. KMC knows all the information of vehicles i.e., vehicle owner, date of registration, engine number. RSU will be responsible for the forwarding of messages and key updating distribution within the transmission range of 1 to 3 km. While the vehicle is incorporated with an OBU and tamper-proof device (TPD). TPD is designed to keep cryptographic stuff and to process the operations related to cryptography. While the OBU is used for communication of messages. For preserving the privacy and incidental derivative, self-generated pseudonyms are used. However, KMC is a single threat model, having all the information. The scheme is prone to false data injection, malicious code injection, side-channel, and Sybil attacks. The secret key pseudonym-based scheme presented by [PERSON] et al. [98] uses RSU for integrity check, but is prone to side-channel attacks. Similarly, the approaches presented in [99; 100] employed symmetric encryption along with message digest to address the DoS attacks, but in these approaches the computational overhead is high and they are prone to impersonation and Sybil attacks. [PERSON] et al. [101] presented a time efficient stream authentication (TESLA) based scheme. TESLA is a secret key cryptographic based approach in which a message authentication code is calculated through the secret shared key. The message authentication code is then affixed with the safety message. The message authentication code is verified through the secret shared key and is prone to replay and side-channel attacks.