## 3 The Proposed Protocol
### System Initialization
#### 3.2.1 Vehicle Registration  
Vehicles in each zone have a unique pair of secret and public keys to communicate securely with vehicles in the same zone or another zone. A vehicle can send a message to vehicles using a single key pair or a set of key pairs, depending on the selected authentication method. Each vehicle requests the security parameters in advance, including the key pairs for multiple zones that are near the vehicle or are in its travel direction. BSs in each zone provides a security parameters list (secret keys, public keys, zone generators, and pseudo-identities) to the vehicles entering the zone. When a vehicle enters a new zone, it connects directly with the nearest base station and requests the authentication parameters for the current and neighboring zones. The process of authorizing the vehicles to access a 5G cell or zone is conducted using the AKA protocol of 5G standards. The details of the AKA protocol is out of scope for this study, but can be found in [29]. For example, vehicle \(v_{i}\) sends an authorization request to the nearest BS, which might include a list of neighboring zone IDs \(L_{ZID}\). The BS forwards the authorization request message to the 5G core network, and sends back a response to the vehicle, once the authorization is successful. For each vehicle \(v_{i}\), BS generates a list of pseudo identities \(Lpid_{i}=(pid_{i1},\;pid_{i2},\;pid_{i3}\ldots,pid_{in})\), list of secret keys \(Lsk_{i}=(sk_{i1},\;sk_{i2},\;sk_{i3},\ldots,sk_{in})\), and a list of the corresponding public keys \(Lpk_{i}=(pk_{i1},pk_{i2},\;pk_{i3},\ldots,pk_{in})\), for every zone in the zone ID list \(L_{ZID}\), using the zone security parameters. Pseudo identities are used to preserve privacy and allow vehicles to communicate anonymously without exposing their real identities. These pseudo-identities can hide the real identity of the vehicle from other vehicles and prevent tracking attacks. BSs generate a list of pseudo-identities per vehicle, to reduce the vehicles' frequent communication with the BSs. Pseudo-identities are short IDs that represent the anonymous identity of each vehicle and can replace the full certificate in PKI traditional solutions. We target a certificateless security solution that provides identity authentication using short size IDs. In authentication-based certificate solutions, each vehicle transmits a long certificate with each message that introduce long delays and a high communication cost. In contrast, our protocol supports anonymity and identity authentication, using a short size pseudo-identity. Pseudo-identities are generated by the BSs to allow CA to track vehicles under misbehaving conditions. Each vehicle stores a list of pseudo identities that are valid for a short time to support unlinkability and prevent traceability. Linking of pseudo-identities can disclose some information about the vehicle. We recommend changing pseudo-identity every 10 min, to enhance the security level while protecting the real vehicle information.  
We also introduce a dynamic key generation by delegating the key generation to the BSs in each zone. In previous certificate-based methods, vehicles use a pair of the secret key and public key for a long time, which exposes the system key attacks. In contrast, our protocol supports updated key generation in each zone, which allows vehicles to use a different pair of secret and public keys for a short time. We provide the key generation based on the bilinear pairing cryptography over the Elliptic Curve. The BSs configured by the CA with commonly shared security parameters (elliptic curve \(E\), two groups of points \(\{G_{1},G_{2}\}\) over the \(E\), one-way hash function \(H\), and a bilinear pairing function \(e\)).  
The pseudo-identities, the secret, and public keys are used to support identity authentication and message authentication, respectively. \(v_{i}\) stores \((Lpid_{i},Lsk_{i},Lpk_{i})\) until the next security parameters update, as shown in Figure 4.