## 3 Batch Authentication in the Vehicle-to-Grid (V2G) Network
### Security and Privacy Challenges in the Vehicle-to-Grid (V2G) Network  
The specific protocols are required in the V2G network as the vehicles are allowed to charge, as well as discharge, their battery to the grid many times in a real-time environment. The authentication scheme must enable joining and disjoining of the vehicle dynamically for charging/discharging purposes. Further, the mobility of the vehicle is another issue. The vehicle can perform these operations within HAN as well as in the visiting area network (outside of the home network). The design of authentication protocol also varies when we consider distributed, as well as centralized, V2G network for vehicle charging and discharging.  
The ISO 15118 standard specifies the scope of the automotive as well as the infrastructure (load-balancing) side. For the automotive side, it discusses the human-machine interface (HMI) interactions and the integration with the standardized V2G message flow. On the infrastructure side, the communication with the head-end systems is enabled using the DLMS/COSEM AMI [76].  
Figure 2: V2G communication in the SG network.  
The available security in DLMS/COSEM is security suite zero defined by AES-GCM-128 for authenticated encryption and AES128 for key wrapping. In order to enable the communication interoperability between the different deployed systems, ISO and IEC are working together for developing standards for the V2G communication interface, which is the protocol family ISO/IEC 15118. It specifies messages to be exchanged between the electric vehicle communication controller (EVCC) and the supply equipment communication controller (SECC) based on PLC. These protocols depend on the charging mode and scenario, identification, and authentication methods.  
Security requirements of the V2G network include authentication, message integrity, and data confidentiality. According to IEC 15118-2 only unilateral authentication (only server side authentication) is mandatory and mutual authentication (both server and EV authentication) is optional [77]. However, mutual authentication is one of the mandatory requirements for an authentication scheme in order to defeat redirection and impersonation attacks, where the vehicle and the local AG verify each other's identity. The secret information should be sent over the network in such a way that only the respective recipient can extract the information. For example, the vehicles input their personal preferences at the charging station and are further sent to the AG/server. Additionally, the integrity of each message sent over the network must be maintained. There are some privacy issues that exist in the V2G network as well, such as vehicle's identity, vehicle charging/discharging location, battery status of the vehicle, selection of operation (charging or discharging) to be performed, and time duration for such an operation. An adversary or any untrusted entity in the SG network can misuse this information and can also trace the users' patterns and behavior. It may lead to some serious privacy issues. Therefore, the security and privacy issues are more challenging in the V2G network [24, 35]. In the following subsections, we will examine the existing authentication protocols for the V2G system.