An almost-surely terminating polynomial protocol for asynchronous Byzantine agreement with optimal resilience

Consider an asynchronous system with private channels and n processes, up to t of which may be faulty. We settle a longstanding open question by providing a Byzantine agreement protocol that simultaneously achieves three properties: 1. (optimal) resilience: it works as long as n > 3t; 2. (almost-sure) termination: with probability one, all nonfaulty processes terminate; 3. (polynomial) efficiency: the expected computation time, memory consumption, message size, and number of messages sent are all polynomial in n. Earlier protocols have achieved only two of these three properties. In particular, the protocol of Bracha is not polynomially efficient, the protocol of Feldman and Micali is not optimally resilient, and the protocol of Canetti and Rabin does not have almost-sure termination. Our protocol utilizes a new primitive called shunning (asynchronous) verifiable secret sharing (SVSS), which ensures, roughly speaking, that either a secret is successfully shared or a new faulty process is ignored from this point onwards by some nonfaulty process.