## 2 Authentication in the Smart Grid (SG) Network
### Authentication Protocols  
In this subsection, we discuss the challenges and desired objectives of authentication protocols in the SG network, and the existing solutions with their pros and cons towards meeting these objectives. There are some standardized protocols exist in the literature for the SG, which support authentication process, such as device language message specification/companion specification for energy metering (DLMS/COSEM) for the AMI network and OpenADR for the demand response program. The DLMS is an application layer communication protocol and COSEM is a data model. Both together provide aninterface model for metering applications belonging to IEC 62056 standards, such as electricity [27]. Basically, three authentication procedures are used by DLMS/COSEM, _i.e._, no security (public access with no identity verification), low level security authentication (server identifies client by password), and high level security authentication (mutual identification) with exchange challenges. DLMS/COSEM specifies its own security services (authentication and confidentiality), which may not necessarily be an advantage as it is restricted to symmetric key encryption. If smart meters combine their measured data with digital signatures, the meters would need asymmetric keys that can be used in secure sockets layer/transport layer security (SSL/TLS). However, DLMS/COSEM does not allow the use of TLS/SSL. The support for asymmetric encryption is done by the European committee for electro-technical standardization CENELEC TC-13 [28]. Further, DLMS/COSEM protocol is suitable only for the AMI network of the smart grid in which a large number of smart meters are involved. Similarly in demand response, OpenADR, a standard development effort supports authentication based on public key cryptography with exchange of certificates [29]. This standard maintains a hierarchy of certified authorities and requires a PKI to use three-tier PKI technology, which ultimately results in high cost. In addition, some standardized authentication protocols also exist, such as remote authentication dial-in user service (RADIUS) and diameter protocols for the 2G, 3G, and 4G cellular networks. RADIUS is used to provide remote user authentication and accounting in 2G, 3G, and 4G networks, and WLAN interworking and Wi-Fi offload situations [30]. It provides centralized services and maintains a central database. However, the smart grid requires decentralized solutions as a single-point-of-failure can massively affect the centralized system. Furthermore, RADIUS has poor scalability and uses the user datagram protocol (UDP), which does not provide reliable data transfer. Therefore, it is not suitable for the smart grid where the availability of information is extremely important. Further, diameter protocol is an authentication, authorization, and accounting protocol used in networking, which supports transmission control protocol (TCP) instead of UDP. However, it does not provide transition support and application level congestion control [31]. Additionally, its supported capabilities are sometimes more expansive. Diameter implementation supports peer authentication between communication endpoints using a pre-shared key. Hence, it brings some key management issues and is not suitable for large systems, such as the smart grid. Further, RADIUS and diameter protocols do not directly protect against denial-of-service (DoS) attacks carried out by flooding the target equipment with bogus traffic. In the case of V2G network having mobile vehicles, a modified diameter with scalable asymmetric cryptographic support may be used to prevent against well-known attacks. Unlike the other communication systems, the SG system requires timely authentication. The proposed authentication protocol in the SG network should be cost effective and efficient in terms of overhead as the authentication process happens to be frequently executed among billions of the entities (users, devices, _etc._).