# Group Authentication Scheme for Neighbourhood Area Networks (NANs) in Smart Grids
## 2 Group Authentication
### Security Requirements  
For a group of SMs to authenticate, there are two potential scenarios to consider  
1. All the devices being able to authenticate in one go (single message) rather than one-to-one with the authentication server and each device with a unique verified identity, (Centralised Authentication) and
2. All devices being able to authenticate between themselves in the absence of an upstream authentication server (Distributed Autonomous Authentication)The case we consider is in-between. Each node authenticates with the NOC, a central authority. There is no local autonomy to the group leader (or the gateway, as termed in this paper) to perform authentication, unless a group member has been previously authenticated by the NOC. Once a group member has been authenticated by a NOC, the group leader exercises control over the group. In the context of the SG, there is no apparent need to economise on communication overheads and the resulting delays, for two reasons. Firstly, smart meters have sustained power availability and rarely have to use an on-board power source, which is limited. Secondly, the performance requirements for the SM on the AMI are of the order of a few seconds and can, therefore, accommodate multiple message exchanges for authentication. The topology determines three critical factors for any network--performance, availability, and cost--each one of them comprising several factors by themselves. In our context of threats and attacks, availability (redundancy, robustness, scalability) is critical. Performance, in terms of end-to-end delays (network latency, complexity of data routing, and processing) is sufficiently lenient. Costs are not considered as part of our discussion. The topology of the smart meter network can be different, including star, tree, mesh, or a cluster tree network. The topology formation depends upon the wireless range of the devices and, therefore, dependent on the location of the SMs. The most practical topology operationally feasible, we believe, is a partial mesh or a cluster tree topology. Therefore, the authentication scheme we propose should attempt to be topology-neutral and address a multi-hop scenario. With a multi-hop scenario, it is essential to ensure that any data being forwarded indeed belongs to the group and no one else. This evolves the need for validated forwarding of packets, both upstream and downstream. Each packet of data received is checked for its integrity as well as source. Therefore, validated forwarding at intermediate nodes where every forwarder on the routing path should be able to verify the authenticity and integrity of the messages is a necessary feature in the authentication scheme. The data from the end devices transits multiple nodes before reaching the central server. It could require multiple encryption/decryption tasks per packet at the intermediate nodes. This impacts the processing, memory requirement (packet length), and energy requirements per SM. Using symmetric cryptography contains these three factors when compared to asymmetric cryptography, for a given key length, in addition to a higher crypto-strength. The security scheme should use symmetric cryptography and provide a secure means for key management. Time synchronization of all devices on the network is another feature that several security schemes deploy. Using a time stamp in all packet data provides a sufficient time tracking for data freshness validation. It requires the time stamps to be monitored continually. Given that the data flow is between the devices and the NOC, the NOC provides a centralized repository for tracking the time stamps. We avoid adding the communication overhead of time synchronization across the network to the routing overhead we may incur due to the choice of topology.