# Group Authentication Scheme for Neighbourhood Area Networks (NANs) in Smart Grids
## 3 The Smart Grid Network Model
### Threat Model and Assumptions  
There are two basic types of threats that need to be countered--attacks that originate due to malicious users eavesdropping to monitor the wireless communications between the nodes in the network, and attacks that originate due to the capture of a node, physically, or causing it to fail. Figure 2: Smart Grid Network Model. Eavesdropping: unauthorized users may try to eavesdrop on exchanged data and control messages within the HAN and NAN. The eavesdroppers can use the information exchanged and the exchange patterns to launch man-in-the-middle (MITM) attacks or replay attacks to impersonate a node. Therefore, all nodes should be authenticated and all messages should be encrypted. The keys used for privacy should not be easily guessable. Node Capture: physical node captures or forced failure of nodes, such as in a DoS attacks, amount to a node capture attack. In such an event, if the keys on the node are captured, the attacker should not be able to gain access to the network. The solution should minimize the impact of such attack on the remaining nodes and ensure the rest of the network functions normally. Authentication Scheme for NAN: in a Neighbourhood Area Network, authentication is required to secure routing in the network. Smart meters have to be registered with the group controller to obtain permission to communicate in the network. For our authentication process, we make the following assumptions:  
1. Smart meters are grouped together based on a policy and are aware of the group members. The events and functionalities of the policy are not in the scope of this paper. This work does not address the policy on which smart meter groups are constituted. 2. Every smart meter in a group has a unique identity, which is a serial number and each group has a unique group identity, which are used in the authentication process. All network devices involved in the group authentication process know these details. 3. The link layer between the smart meters and gateway are protected at the link layer, which makes communication encrypted at the link layer. 4. Every smart meter in a group maintains a wireless connection with its gateway and the network topology between the home smart meter and the gateway node is a tree. The topology between the gateway and the utility could be a mesh. They form a cluster-tree topology between the SM and the gateway. 5. The smart meters have pre-distributed shared symmetric keys, which are used for initiating the authentication process and keys during authentication. 6. Symmetric cryptography yields a better cryptographic strength for a given key length compared to asymmetric cryptography. The resulting data length is close to the size of the input. 7. Smart meters cooperate with one another to forward packets on multi-hop paths to the NOC. A routing protocol to handle the mesh topology is active and provides the shortest route from a given end device to the GW, within the group. 8. GW nodes have sufficient power (more than the end devices) to be able to perform the forwarding from the group to the NOC and vice versa. 9. In the event of the failure of a gateway node, all nodes in the group will be unable to access the NOC, until the GW is reinstated/active. There is no fall-back node that will take on the role of a gateway. The failure rates of the GW are low. 10. The groups and the group gateways are pre-identified and formed. These formations are not _ad hoc_ and, therefore, there is no need for a node to play the role of a gateway. 11. The nodes on the network are not time synchronized. 12. The value of the clock ticks of a node cannot be retrieved to set the same clock value on another node. Such an operation is possible only with a reset of the node, which essentially implies that the clock tick value is lost since the clock is reset. It can be argued that such is the exact function of a time protocol such as Network Time Protocol (NTP), but sufficient care is taken to ensure that this value is not accessed by any network function. 13.