# A Secure and Efficient Group Key Management Scheme for Clusters of String Inverters
## 2 Related Works  
According to the National Institute of Standard and Technology (NIST), the architecture of the Smart Grid (SG) consists of seven domains each of which offers specific Smart Grid's services [63]. The seven domains are:  
1. Generation: the electricity generators, including both traditional energy sources and distributed RES;
2. Transmission: the mediums of bulk electricity over long distances;
3. Distribution: the distributors of electricity to and from customers;
4. Customer: the energy consumers. However, they can also generate electricity. We differentiate between three types of customers: residential, commercial and industrial (C&I);
5. Markets: the operators and the players in electricity markets;
6. Service Provider: the entities offering services to customers;
7. Operations: the handlers of the energy flow. To offer SG's intendent services, in addition to energy generation, transportation and consumption, communication is required both within devices of the same domain, so-called intradomain communication, and between devices of different domains, i.e., interdomain communication. Communication consists of data packets exchanged via a network of communication links and routers, namely the Internet. In Figure 2 we present the SG architecture together with the supporting Internet network infrastructure. We differentiated between energy flow (red lines) and data flow (blue lines). In this SG's context, IP multicast, i.e., the means of sending IP data packets from one sender to a group of intended recipients, has been recognized to have a variety of applications, including but not limited to monitoring, protection and information dissemination [64; 65]. However, existing schemes for secure multicast communication in the Smart Grid only focus on the interactions between grid's control centers and smart devices installed at customers' premises. Figure 2: Smart Grid architecture and supporting Internet network infrastructure. [PERSON] et al. present an IPSECurity-based (IPSEC) protocol to protect the multicast traffic between energy utilities control centers and a group of customer's smart meters [22]. The protocol consists of a group key management scheme and allows authentication of the smart meters. The IPSECurity-based protocol avoids eavesdropping, data injection and replay attacks. In [23], both a mutual authentication scheme between customers' smart meters and a Smart Grid's security associate server and a key management protocol is discussed. In this respect, a key-pair, similar to the private-public key pair, is proposed. One key is for the group source, i.e., the Smart Grid's security associate server. This is called Multicast-Source-Key (MSK). The second key is known as Multicast-Receiver-Key (MRK) and is for group receivers, i.e., smart meters. Whenever the Smart Grid's security associate server wants to send information to the smart meters, it uses the MSK to encrypt multicast packets. On the other side, group receivers use MRK to decrypt the packets. The scheme also foresees refresh of the group key. [PERSON] et al. [24] describe a multicast scheme that mutually authenticates a customer's smart meter and an authentication server in the Smart Grid. Authentication takes place through an initial password. Then, the authors describe a key management protocol based on enhanced identity-based cryptography. The scheme exploits the public key infrastructure. The scheme also embeds a key update process to provide both backward and forward secrecy and to preserve the group key secrecy over time. In [25], a secure multicast routing protocol is proposed. During an initial authentication phase, the protocol registers a group of customers' smart meters with an access point acting as certificate authority. After that, a communication phase follows. This consists of individual SMs securely sending data to the grid's operation centers and the latter securely sending, occasionally, control messages to the multicast group. [PERSON] et al.