Byzantine Agreement in Expected Polynomial Time

Abstract

We address the problem of Byzantine agreement, to bring processors to agreement on a bit in the presence of a strong adversary. This adversary has full information of the state of all processors, the ability to control message scheduling in an asynchronous model, and the ability to control the behavior of a constant fraction of processors that it may choose to corrupt adaptively. In 1983, Ben-Or proposed an algorithm for solving this problem with expected exponential communication time. In this article, we improve that result to require expected polynomial communication time and computation time. Like Ben-Or’s algorithm, our algorithm uses coinflips from individual processors to repeatedly try to generate a fair global coin. We introduce a method that uses spectral analysis to identify processors that have thwarted this goal by flipping biased coins.