BAR primer

Byzantine and rational behaviors are increasingly rec- ognized as unavoidable realities in today's cooperative ser- vices. Yet, how to design BAR-tolerant protocols and rig- orously prove them strategy proof remains somewhat of a mystery: existing examples tend either to focus on unrealis- tically simple problems or to want in rigor. The goal of this paper is to demystify the process by presenting the full al- gorithmic development cycle that, starting from the classic synchronous Repeated Terminating Reliable Broadcast (R- TRB) problem statement, leads to a provably BAR-tolerant solution. We i) express R-TRB as a game; ii) why the strat- egy corresponding to the optimal Byzantine Fault Tolerant algorithm of Dolev and Strong does not guarantee safety when non-Byzantine players behave selfishly; iii) how to derive a BAR-tolerant R-TRB protocol: iv) how to prove rigorously that the protocol ensures safety in the presence of non-Byzantine selfish players.