# A Secure and Efficient Group Key Management Scheme for Clusters of String Inverters
## 7 Conclusions  
Currently, in the so-called Smart Grid's context, most PV plants consist of clusters of smart string inverters whereby data exchange takes place according to a multicast paradigm typically over IP. However, IP multicast does not provide secure communication. Indeed, any multicast-enabled host can decide to join, at any time, whatever multicast group without being involved neither in an authentication nor the access control mechanism. Once a host has joined a multicast group, it can immediately send/receive messages to/from such a multicast group. Multicast data is sent in clear. In this regard, a security violation may lead to both exposure of sensitive data and cluster misbehavior. Particularly, in this second case, an attacker can either launch a zero-energy attack or an energy overflow attack. In the former case, the adversary can lead the cluster into exporting no energy into the grid. In the latter case, he/she can cause the cluster into exporting more energy than the limit set with the DNOs. Whatever the attack considered, the environmental and social consequences could be devastating. Issues are even more relevant bearing in mind the investments and the wide-spreading of PV systems until 2050. On the one hand, there are estimates imply a 68% increase in average annual PV investments [4]. On the other hand, there are predictions of an exponential increase in global installed PV capacity, with a peak of 4500 (GW) [3]. Therefore, to effectively deploy PV plants the need of secure multicast communication is of primary importance. In this work, we addressed this issue. To the best of authors' knowledge, we are the first that focused on the security of the group communication in clusters of smart string inverters. We proposed a point-to-point public key-based approach to group key management. Specifically, one authorized inverter in the cluster acts as KDC for group key generation and distribution. The proposed scheme provides forward secrecy and key secrecy over time. The KDC accomplishes the latter task through periodic group key refresh and secure multicast distribution. Finally, we also suggested an ad-hoc election algorithm for clever KDC election. In the (informal) security analysis we showed that the KMS was secure with respect to both an external attacker and an evicted group member. As to the external attacker, we showed that the execution of the join, leave and key refresh operations were secure because they took place through a secure channel. Specifically, in the case of join and leave operations, the secure channel consisted of a TLS connection. The latter guaranteed inverters mutual authentication and data confidentiality, integrity and authentication. The same guarantees were also provided in the case of a key refresh operation as it exploits a scheme of authenticated encryption known as Encrypt-then-MAC. Finally, the presence of a timestamp guarantees the freshness of messages and thus prevents replay attacks. Regarding an evicted inverter, we demonstrated that it could neither eavesdrop nor inject fake messages. The reason was twofold. On the one side, it could not eavesdrop the new group key \(K_{\text{G}}\) during the execution of the leave operation because the key was always transmitted through point-to-point secure channels, namely TLS connections. On the other side, as the new group key \(K_{\text{G}}\) was randomly generated, the evicted inverter could not guess it even though it held several past keys. To demonstrate the feasibility of the KMS we built a prototype and implemented it on a cluster of Power-One Italy S.p.A. smart string inverters. Finally, we evaluated the performance of the scheme, namely latency for the group key distribution and memory overhead. As far as latency is concerned, we showed that when inverters were equipped with RSA-keys providing 128-bit security level, the scheme only required 5.7 s to distribute a new group key to 50 inverters.