This paper describes the design, implementation, and evaluation of Mykil, which is a new key distribution protocol for secure group multicast. Mykil has been designed to be scalable to large group sizes. It is based on a combination of group-based hierarchy and key-based hierarchy systems for group key management. Important advantages of Mykil include a fast rekeying operation for large group sizes, continuous availability of the key management service in a disconnected network environment, an ability to map the group structure to the underlying network infrastructure, robustness, support for user mobility, and support for smaller hand-held devices.