In a dynamic group key exchange protocol, besides the basic group setup protocol, there are also a join protocol and a leave protocol, which allow the membership of an existing group to be changed more efficiently than rerunning the group setup protocol. The join and leave protocols should ensure that the session key is updated upon every membership change so that the subsequent sessions are protected from leaving members (
) and the previous sessions are protected from joining members (
). In this paper, we present a new security model for dynamic group key exchange. Comparing to existing models, we do a special treatment to the state information that a user may use in a sequence of setup/join/leave sessions. Our treatment gives a clear and more concise definition of session freshness for group key exchange in the dynamic setting. We also construct a new dynamic group key exchange protocol that achieves strong security and high efficiency in the standard model.