# Group Authentication Scheme for Neighbourhood Area Networks (NANs) in Smart Grids
## 1 Introduction  
The Smart Grid (SG) can be defined as an electric system that uses information, two-way, cyber-secure communication technologies, and computational intelligence in an integrated fashion [1]. The system is used across the SG functional units, such as electricity generation, transmission, substations, distribution, and consumption to achieve a system that is clean, safe, secure, reliable, resilient, efficient, and sustainable. This definition covers the energy system from the generation to the end points of consumption of the electricity. Security is a critical issue because millions of electronic devices are inter-connected via communication networks throughout critical power facilities, which has an immediate impact on reliability of such a widespread infrastructure [2]. The Electric Power Research Institute (EPRI) identifies cyber security as one of the greatest challenges facing smart grid deployment [3]. The security of the grid will strongly depend on authentication, authorization, and privacy technologies. Privacy technologies are well matured. Federal Information Processing Standard (FIPS) approved Advanced Encryption Standard (AES) [4] and Triple Data Encryption Standard (3 DES) [5] implementations are readily available. However, the available schemes and tools for a generic Internet infrastructure cannot be directly used on a SG infrastructure. Apart from the fact that the devices used in the various domains of the SG are very different in terms of function, capabilities as well as form factor, the security objectives of the SG are different from those of a generic Internet infrastructure [6; 7; 8]. The broad security objectives for the SG are mentioned as Availability, Integrity, and Confidentiality in the report of the Smart Grid Interoperability Panel (SGIP), National Institute of Standards & Technology (NIST), USA [7]. It mentions the need for additional security relating to cyberspace and the physical security of the devices. The scope of our discussion is limited to the former. In particular, our focus is on key management that enables the security schemes. The SG is a heterogeneous network with multiple devices and technologies interconnecting them. As a data network, Figure 1 illustrates, it comprises three parts--the Home Area Networks (HANs), the Neighbourhood Area Networks (NAN), and the Wide Area Network [7]. The HAN and NAN are functional groups of the smart grid infrastructure that are interconnected in a functional hierarchy. The HAN performs the basic data collection, and interfaces with the smart grid via the smart meters. The SMs, in turn, interconnect and as part of the Advanced Metering Infrastructure (AMI), termed as the NAN, interface to a substation which houses the control gear for electricity supply. Several substations interconnect via a WAN to the core network at the provider's premises. The devices in the HANs and NANs communicate with the core in the backhaul, accessible via the WAN. Interconnected wireless sensors/sensor enabled appliances constitute the HAN. The WAN and the HAN are interconnected via the NAN. The interconnection of wireless SMs providing a path from the HAN to resources in the WAN constitutes the AMI. The NAN segment of the smart grid is our area of interest in this paper. SMs require communicating with a data sink and a control centre for purposes of monitoring and management. The interconnectivity between the SMs includes an interface to the upstream network, typically another SM, playing the role of such an interface and termed as a gateway SM. The topology depends upon the wireless coverage, reachability, potential redundant paths, and similar design parameters. A single hop to the gateway SM from all devices in a locality is not practical and multiple hops to the gateway SM become necessary. It requires the intermediate nodes in the path to the gateway to forward data from the downstream nodes. Security mechanisms deployed must ensure that each node is authenticated centrally, as well as by the group.