# A Secure and Efficient Group Key Management Scheme for Clusters of String Inverters
###### Abstract
As a prudent cryptographic engineering practice, the group key should be periodically refreshed to prevent a passive attacker from collecting enough encrypted material to mount an analytical attack [16; 17; 18]. When a new inverter joins the group, e.g., to increase the PV plant's capacity, the inverter must be allowed to access the group communication. To achieve this, the joining inverter must be provided with the current group key. In contrast, when an inverter leaves the group because its mission is over, or it is forced to leave because it is suspected to be compromised or faulty, the inverter must be prevented from accessing any further group communication (forward security) [19]. This requirement can be fulfilled by rekeying the group, that is by generating and securely distributing a new group key to group inverters but the leaving ones. It follows that key revocation has the same level of importance as key distribution. In fact, compromised string inverters must be logically removed from the network communication and, usually, the ability to logically remove them translates into the ability to revoke keys [20; 21]. If cryptographic algorithms do not expose the secret keys, then they can only be compromised by compromising the device itself. It follows that by revoking all keys of a compromised device, it is possible to remove the logical presence of that device from the network. In this paper we present a secure and efficient group Key Management System (KMS) to protect the group communication in a PV plant. The novelty of this work is twofold. First, research works do not consider the issue of secure multicast in clusters of solar inverters [22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33]. To the best of our knowledge, we are the first tackling the topic of secure group communication, and the related group key management, in the context of PV plants. Secondly, many group key management systems have been proposed [34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58; 59; 60]. In this paper we prove that a point-to-point public key-based approach to group key management is a simple yet effective solution to guarantee performance and scalability requirements of a real cluster setup. The KMS supports group key generation, rekeying, refreshing and distribution. During an initialization phase, the KMS initializes inverters in the cluster with the group key and forms the initial group membership. Upon an inverter's leaving, the KMS generates a new group key and securely distributes to group members but the leaving one (i.e., rekeying). Furthermore, upon an inverter joining, the KMS securely transmits the current group key to the joining inverter. Secure distribution of the group key takes place through point-to-point secure Transport Layer Security channels [61] wherein mutual authentication is enabled. Finally, the KMS periodically refreshes the group key by securely multicasting a newly generated group key (i.e., refreshing). The multicast message containing the refreshed group key is protected by means of an Encrypt-then-Message Authentication Code (MAC; EtM) authenticated encrypted scheme [62]. The resulting multicast communication system is secure as only authorized inverters can take part in the cluster communications. Upon a leave event, the leaving inverter's keys are revoked, so logically evicting the inverter from the cluster. The Key Management System is architecturally organized in two main components: Key Distribution Supervisor (KDS) and Key Distribution Centre (KDC). The KDS resides in the cloud, in the case of Internet-connected clusters, or runs on a mobile device as a mobile application, in the case of Internet-disconnected clusters, which is operated by a trustworthy operator who physically goes to the cluster. An instance of the KDC is resident on every inverter in the cluster.