## 2 Authentication in the Smart Grid (SG) Network
### Authentication Protocols
#### 2.2.1 Obj-1: Low Execution and Protocol Delay  
The authentication protocol in the SG must be fast enough and should use the system resources as less as possible. In certain cases (such as IEC 61850 Protection Class 2/3), multicast messages in the transmission substation network should be delivered within 4 ms as mentioned by [PERSON] _et al._[34] and [PERSON] _et al._[35]. Since several mutual authentications must be performed among various SG entities in a distributed manner, the protocol execution time and the end-to-end delay are critical in the SG system. Fast authentication with low delay improves the overall efficiency of the system and enables the computations under the threshold. [PERSON] _et al._[36] made a change in the message authentication protocol by [PERSON] _et al._[37] and used a variable MAC between the AMI devices and the controller nodes instead of HMAC to reduce the time for verification. However, it involves only HAN-GW and BAN (building area network)-GW communication. [PERSON] _et al._[38] analyzed the security requirements in the SG environments and proposed a certification structure and a chip design for an efficient message authentication between a SM and the HAs. Further, [PERSON] and [PERSON] [39] presented a bump-in-the-wire (BITW) solution, which maintains security during time-critical communications over bandwidth-limited serial links between the devices in the legacy SCADA system. Their solution guarantees data authenticity and freshness, and achieves low latency. A time-valid one-time-signature (TV-OTS) as a data authentication protocol for potential use in the SG applications is proposed to enable secure multicast, low latency, and sufficient lifespan even at high data rates [40]. .2.2 Obj-2: Low Computational and Storage Cost and Obj-3: Low Communication and Computation Overhead  
Since there are numerous entities involved in the SG system, computation and storage costs are big issue. As well, communication and computation overhead are also a major concern, as it will affect the bandwidth requirement and the overall efficiency of the SG system. In order to accomplish these objectives, multicast authentication can be used in various applications of SG, such as wide area network (WAN) security, demand-response protection, substation security management, _etc._ However, the existing multicast authentication solutions are not suitable for the SG due to the requirements of fast authentication (low delay) and small computation and storage costs. The schemes relying on per-packet signature and verification introduce substantial computation cost. In this direction, [PERSON] and [PERSON] [41] have introduced a one-time signature scheme for multicast authentication in the SG, which reduces the storage cost by eight-times and lowers signature size by 40% as compared to HORS by [PERSON] and [PERSON] [42], and prevents message forgery attacks. After that, a batch verification and signature authorization protocol that generates low overhead and provides fault tolerance was proposed by [PERSON] _et al._[43]. However, the protocol does not provide authentication among SM, HAN, and HAs. [PERSON] _et al._[44] proposed a scheme that provides the mutual authentication between the SM and the AS of the SG. This scheme is based on an identity-based PKI and reduces the exchanged packets during authentication, but, generates a large overhead. Zigbee authentication protocol also has some security weaknesses that were overcome by [PERSON] and [PERSON] [45]. This scheme reduces the number of key distribution messages per node. A matrix based homomorphic hash is used by [PERSON] and [PERSON] [46] to decreases the amount of computations at SM as compared to the exponential operation based hash. Further, a scheme presented by [PERSON] _et al._[47] reduces the pairing operations in aggregation and verification to improve the computational efficiency in the SG network. Many schemes are not well suited in the SG due to their large key sizes.