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From Partial Consistency to Global Broadcast

Matthias Fitzi and Ueli Maurer

This paper considers unconditionally secure protocols for reliable broadcast among a set of $n$ players, some of which may be corrupted by an active (Byzantine) adversary. In the standard model with a complete, synchronous network of pairwise authentic communication channels among the players, broadcast is achievable if and only if the number of corrupted players is less than $n/3$. We show that, by extending this model only by the existence of a broadcast channel among three players, global broadcast is achievable if and only if the number of corrupted players is less than $n/2$. Moreover, for this an even weaker primitive than broadcast among three players is sufficient. All protocols are efficient.