The Ensemble project at Cornell is an outgrowth of an older project called Horus . It provides a framework for reliable, secure group communications. It uses a layered approach to provide applications with an interface group communications mechanisms. Its security layer uses shared keys, and is based on Fortezza and Kerberos. It is currently based on a PC oriented LAN architecture.
Software is available.
The University of California Santa Barbara is currently working on a number of secure multicast protocols for group communications. The Secure Group Communication Service Protocols provide Byzantine fault tolerance, total ordering of messages, maitenance of group membership, and a mechanism to reduce the number of acknowledgements sent in reply to a broadcast. The Secure Ring protocols provide reliable, Byzantine fault tolerant ordered message delivery and group membership maintenance. Performance is improved during fault free processing by passing message digests in a token, rather than in each message. My recomendation would be to read this research after you get a good background on the basics.