  Secure group communications are a mechanism facilitating protected
transmission of messages from a sender to multiple receivers, and many emerging
applications in both wired and wireless networks need the support of such a
mechanism. There have been many secure group communication schemes in wired
networks, which can be directly adopted in, or appropriately adapted to,
wireless networks such as mobile ad hoc networks (MANETs) and sensor networks.
In this paper we show that the popular group communication schemes that we have
examined are vulnerable to the following attack: An outside adversary who
compromises a certain legitimate group member could obtain {\em all} past and
present group keys (and thus all the messages protected by them); this is in
sharp contrast to the widely-accepted belief that a such adversary can only
obtain the present group key (and thus the messages protected by it). In order
to understand and deal with the attack, we formalize two security models for
stateful and stateless group communication schemes. We show that some practical
methods can make a {\em subclass} of existing group communication schemes
immune to the attack.
