The Contest Between Simplicity and Efficiency in Asynchronous Byzantine Agreement

In the wake of the decisive impossibility result of Fischer, Lynch, and Paterson for deterministic consensus protocols in the aynchronous model with just one failure, Ben-Or and Bracha demonstrated that the problem could be solved with randomness, even for Byzantine failures. Both protocols are natural and intuitive to verify, and Bracha's achieves optimal resilience. However, the expected running time of these protocols is exponential in general. Recently, Kapron, Kempe, King, Saia, and Sanwalani presented the first efficient Byzantine agreement algorithm in the asynchronous, full information model, running in polylogarithmic time. Their algorithm is Monte Carlo and drastically departs from the simple structure of Ben-Or and Bracha's Las Vegas algorithms. In this paper, we begin an investigation of the question: to what extent is this departure necessary? Might there be a much simpler and intuitive Las Vegas protocol that runs in expected polynomial time? We will show that the exponential running time of Ben-Or and Bracha's algorithms is no mere accident of their specific details, but rather an unavoidable consequence of their general symmetry and round structure. We define a natural class of "fully symmetric round protocols" for solving Byzantine agreement in an asynchronous setting and show that any such protocol can be forced to run in expected exponential time by an adversary in the full information model. We assume the adversary controls $t$ Byzantine processors for $t = cn$, where $c$ is an arbitrary positive constant $< 1/3$. We view our result as a step toward identifying the level of complexity required for a polynomial-time algorithm in this setting, and also as a guide in the search for new efficient algorithms.Comment: 21 page

Towards Practical Sleepy BFT

Bitcoin\u27s longest-chain protocol pioneered consensus under dynamic participation, also known as sleepy consensus, where nodes do not need to be permanently active. However, existing solutions for sleepy consensus still face two major issues, which we address in this work. First, existing sleepy consensus protocols have high latency (either asymptotically or concretely). We tackle this problem and achieve $4\Delta$ latency ($\Delta$ is the bound on network delay) in the best case, which is comparable to classic BFT protocols without dynamic participation support. Second, existing protocols have to assume that the set of corrupt participants remains fixed throughout the lifetime of the protocol due to a problem we call \emph{costless simulation}. We resolve this problem and support growing participation of corrupt nodes. Our new protocol also offers several other important advantages, including support for arbitrary fluctuation of honest participation as well as an efficient recovery mechanism for new active nodes