In spite of the efficiency of scalable schemes for multicast
applications, changing the key has two major weak points:
synchronization and inefficiency [6]. Periodic batch rekeying
provides a good trade-off between security improvement and
computation complexity. Usually, a new user has to wait
longer to join the service group, and the period of batch
rekeying is thus a design parameter that can be adjusted
according to different security requirements and current network
conditions. To accommodate different multimedia appliapplication
needs, three modes of operation are
suggested [9]: