# Blockchain-Based Authentication in Internet of Vehicles: A Survey
## 4 Blockchain-Based Authentication Schemes in Vehicular Networks
### Private Blockchain-Based Authentication Protocols
The authors also addressed the authentication of registered vehicles by adopting a Public Key Infrastructure (PKI) technology based on cryptographic accumulator to enhance the authentication efficiency. The authentication process is performed in two phases; the first phase is to verify the vehicle's identity to the roadside unit, and the subsequent phase is the mutual verification between the RSU and the cloud server. In the first phase, the vehicle sends its ID and public key to the RSU which verifies the vehicle identity and adds its own public key to the vehicle information and sends them to the corresponding cloud service provider authority (CA) after encryption with the public key of this CA. Upon receiving the message from the RSU, the CA starts the second phase by first decrypting the message with its own private key, and then searches the vehicle ID and the public key of the RSU in the blockchain to check whether the provided information is correct. If successfully found, a new session key is generated by the CA and sent to the RSU along with the corresponding vehicle ID after encryption with the RSU public key. The RSU decrypts the packet with its private key, stores the session key for securing further communication with the CA, and sends the issued digital certificate to the vehicle encrypted with its public key, upon which the authentication process is culminated. The proposed solution is evaluated using Veins open-source framework through a small-scale network of 13 nodes in terms of the time overhead and communication cost needed for the encryption process related to the blockchain technology and for the whole authentication process. Their scheme proves to provide high authentication efficiency in preventing malicious attacks with low time and communication costs. However, large packet loss is encountered during the registration of the vehicles and the key distribution process. The authors in [112] discussed the issue of computing and communication bottlenecks faced by the centralized authentication protocols and single Trusted Authority (TA) schemes that could fail to authenticate the large number of simultaneous vehicle requests within a limited time during high mobility. Thus, they suggested a blockchain-based RSU-assisted authentication and key agreement protocol for a multi-TA network model by offloading part of the authentication process to the RSUs to achieve more decentralization. The aim of offloading is to reduce the resource bottlenecks of the TAs, which results in improving the authentication efficiency. The blockchain technology is used to address the cross-TA authentication issue which is caused by the high mobility of the vehicles in a multi-TA environment. Instead of restarting the whole authentication process when a vehicle exits the coverage area of one TA to the coverage area of another TA, the blockchain adoption allows the new TA to continue the authentication process that was started by the old TA since they manage the same ledger that keeps all vehicle-related information. The network model consists of four types of nodes: vehicle nodes (VNs) and RSUs which are both considered as untrusted nodes in their threat model, TAs which are assumed to be semi-trusted nodes, and a data center (DC) which is assumed as a fully trusted network entity that stores all vehicle-related information. A smart contract is used to automate part of the authentication process and the delegated proof-of-stack [113] is adopted as a consensus mechanism for more efficient resource utilization and power consumption. The whole scheme is composed of three phases, namely the initialization phase, the registration phase, and the authentication phase. In the initialization phase, the system administrator generates a master key and sends it to all RSUs and TAs. Each VN must be registered with the nearest TA during the registration phase in which a unique ID is granted to it and kept as a record with a unique pointer P in the ledger as well as in the vehicle's memory.