To provide secure media delivery in pay-TV systems, a large number of messages are exchanged for key updates in the conventional key distribution schemes. This is inefficient and costly when the client side (set-top box) uses a smart card with limited computing power. In this paper, we present three key distribution schemes for channel protection and secure media delivery in pay-TV systems. With the proposed schemes, encryption keys of the subscribed programs can be efficiently and securely distributed to the authorized subscribers. Only one message is needed to renew key in the key distribution schemes for subscription channel protection. In addition, we use simpler computation functions, including one-way hash function and exclusive-OR operation, for key updates to reduce the computation cost. With our key distribution schemes, only authorized subscribers can watch the subscribed programs correctly. Unauthorized subscribers have no information to retrieve the correct programs over the networks. Thus, service providers can charge their subscribers according to their subscriptions, and the illegal access of the media and video programs from networks can be prevented, based on the proposed schemes.

In this paper, we reevaluate the message cost of X. Jia's multicast mechanism (see ibid., vol. 6, no. 6, p. 617-27 (1995)). Our analysis shows that its message cost should be (D+k+1) instead of (D+2), where D is the depth of the subtree and k is the number of metagroups containing the destination multicast group.