Signature-Free Asynchronous Byzantine Consensus with t < n/3 and O(n2) Messages

This paper presents a new round-based asynchronous consensus algorithm that copes with up to t < n/3 Byzantine processes, where n is the total number of processes. In addition of not using signature, not assuming a computationally-limited adversary, while being optimal with respect to the value of t, this algorithm has several noteworthy properties: the expected number of rounds to decide is four, each round is composed of two or three communication steps and involves O(n(2)) messages, and a message is composed of a round number plus a single bit. To attain this goal, the consensus algorithm relies on a common coin as defined by Rabin, and a new extremely simple and powerful broadcast abstraction suited to binary values. The main target when designing this algorithm was to obtain a cheap and simple algorithm. This was motivated by the fact that, among the first-class properties, simplicity -albeit sometimes under-estimated or even ignored- is a major one.