## Cryptology ePrint Archive: Report 2001/088

Ignacio Gracia and Sebastià Martín and Carles Padró

Abstract: The most important point in the design of broadcast encryption schemes (BESs) is 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 $\rho$ and the broadcast information rate $\rho_B$. In this paper we present a simple method to combine two given BESs in order to improve the trade-off between $\rho$ and $\rho_B$ by finding BESs with good information rate $\rho$ for arbitrarily many different values of the broadcast information rate $\rho_B$. We apply this technique to threshold $(R,T)$-BESs and we present a method to obtain, for every rational value $1/R \le \rho_B \le 1$, a $(R,T)$-BES with optimal information rate $\rho$ among all $(R,T)$-BESs that can be obtained by combining two of the $(R,T)$-BESs proposed by Blundo et al.

Category / Keywords: cryptographic protocols / broadcast encryption