Pay-per-view is one of several applications that require efficient key management in the context of encrypting broadcast transmissions in order to implement selectively restricted multicast operations sent over an insecure channel. Since information-theoretical arguments establish a lower bound on the number of required keys, which grows as klogk , where k is the number of targeted recipients (or nonrecipients, whichever is smaller), the authors’ approach relaxes the assumption that exactly the intended recipients receive the restricted multicast; instead, they assume that no more than a fixed percentage of freeloaders will receive the restricted transmission as well.

In the theoretical portion of this well-written paper, the authors show that this relaxation permits a significant reduction in the number of keys required to implement the scheme. They discuss practical aspects of the problem and propose a dynamic key management infrastructure whose cost is logarithmic in the size of the total user population. This infrastructure can be used to provide a variety of services, which makes it more efficient than competing approaches.