# From Partial Consistency to Global Broadcast

## Matthias Fitzi and Ueli Maurer

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```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.