Improving the trade-off between storage and communication in broadcast encryption schemes*1

Abstract

The most important point in the design of broadcast encryption schemes (BESs) is to obtain a good trade-off 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 ρ and the broadcast information rate ρ B . In this paper, we present a simple method to combine two given BESs in order to improve the trade-off between ρ and ρ B by finding BESs with good information rate ρ for arbitrarily many different values of the broadcast information rate ρ B . We apply this technique to threshold ( R , T )-BESs and we present a method to obtain, for every rational value 1/ R ⩽ ρ B ⩽1, a ( R , T )-BES with optimal information rate ρ among all ( R , T )-BESs that can be obtained by combining two of the ( R , T )-BESs proposed by Blundo et al. (Lecture Notes in Comput. Sci. 1190 (1996) 387–400).