Improving the trade-off between storage and communication in broadcast encryption schemes

The most important point in the design of broadcast encryption schemes (BESs) is obtain a good trade-o between the amount of secret information that must be stored by every user and the length of the broadcast message, which are measured, respectively, by the information rate and the broadcast information rate B. In this paper we present a simple method to combine two given BESs in order to improve the trade-o between and B by nding BESs with good information rate for arbitrarily many di erent values of the broadcast information rate B. We apply this technique to threshold (R;T )-BESs and we present a method to obtain, for every rational value 1=R B 1, a (R; T )-BES with optimal information rate among all (R; T )-BESs that can be obtained by combining two of the (R;T )-BESs proposed by Blundo et al. [7].