  This paper presents an efficient group key management protocol, CKCS (Code
for Key Calculation in Simultaneous join/leave) for simultaneous join/leave in
secure multicast. This protocol is based on logical key hierarchy. In this
protocol, when new members join the group simultaneously, server sends only the
group key for those new members. Then, current members and new members
calculate the necessary keys by node codes and one-way hash function. A node
code is a random number which is assigned to each key to help users calculate
the necessary keys. Again, at leave, the server just sends the new group key to
remaining members. The results show that CKCS reduces computational and
communication overhead, and also message size in simultaneous join/leave.
