  Many Internet of Things (IoT) scenarios require communication to and data
acquisition from multiple devices with similar functionalities. For such
scenarios, group communication in the form of multicasting and broadcasting has
proven to be effective. Group Key Management (GKM) involves the handling,
revocation, updation and distribution of cryptographic keys to members of
various groups. Classical GKM schemes perform inefficiently in dynamic IoT
environments, which are those wherein nodes frequently leave or join a network
or migrate from one group to another over time. Recently, the `GroupIt' scheme
has been proposed for GKM in dynamic IoT environments. However, this scheme has
several limitations such as vulnerability to collusion attacks, the use of
computationally expensive asymmetric encryption and threats to the backward
secrecy of the system. In this paper, we present a highly efficient and secure
GKM protocol for dynamic IoT settings, which maintains forward and backward
secrecy at all times. Our proposed protocol uses only symmetric encryption, and
is completely resistant to collusion attacks. Also, our protocol is highly
flexible and can handle several new scenarios in which device or user dynamics
may take place, e.g., allowing a device group to join or leave the network or
creation or dissolution of a user group, which are not handled by schemes
proposed in prior literature. We evaluate the performance of the proposed
protocol via extensive mathematical analysis and numerical computations, and
show that it outperforms the GroupIt scheme in terms of the communication and
computation costs incurred by users and devices.
