2. The Composition of Authenticated Byzantine Agreement

Abstract

In this chapter we show that the basic problem of achieving secure broadcast in a distributed network is strictly harder when composition is required. In particular, assuming a public-key infrastructure for digital signatures, it is known that secure broadcast can be achieved for any number of corrupted parties in the stand-alone model. In this chapter, we show that an analogous result that composes cannot be achieved. That is, when a third or more of the parties may be corrupt, it is impossible to obtain secure broadcast that remains secure under parallel self composition, (even given a publickey infrastructure). Notice that the type of composition considered here is rather weak. That is, impossibility occurs even in the case of parallel self composition. Wealso present lower bounds for the sequential self composition of deterministic protocols (the impossibility result for the parallel case holds even for randomized protocols).