Efficient Unconditional Asynchronous Byzantine Agreement with Optimal Resilience

We present an efficient and optimally resilient Asynchronous Byzantine Agreement (ABA) protocol involving n = 3t+1 parties over a completely asynchronous network, tolerating a computationally unbounded Byzantine adversary, who can control at most t parties out of the n parties. The amortized communication complexity of our ABA protocol is O(n log 1 ) bits for attaining agreement on a single bit, where ( > 0) denotes the probability of nontermination. We compare our protocol with the best known optimally resilient ABA protocols of Canetti et al. (STOC 1993) and Abraham et al. (PODC 2008) and show that our protocol gains by a factor of O(n(log 1 ) ) over the ABA protocol of Canetti et al. and by a factor of O(n logn log 1 ) over the ABA protocol of Abraham et al. in terms of the communication complexity. To design our protocol, we first present a new, optimally resilient statistical asynchronous verifiable secret sharing (AVSS) protocol with n = 3t + 1, which significantly improves the communication complexity of the only known optimally resilient statistical AVSS protocol of Canetti et al. Our AVSS protocol shares multiple secrets simultaneously and incurs lower communication complexity than executing Few of the results in this paper appeared in PODC 2009 and PODC 2012. Arpita Patra Department of Computer Science ETH Zurich, Switzerland E-mail: [email protected], [email protected] Ashish Choudhury Department of Computer Science University of Bristol, United Kingdom E-mail: [email protected], [email protected] C. Pandu Rangan Department of Computer Science and Engineering IIT Madras, Chennai India E-mail: [email protected], [email protected] multiple instances of an AVSS protocol sharing a single secret. To design our AVSS protocol, we further present a new asynchronous primitive called asynchronous weak commitment (AWC), which acts as a substitute for asynchronous weak secret sharing (AWSS), which was used as a primitive for designing AVSS by Canetti et al. We observe that AWC has weaker requirements than the AWSS and hence can be designed more efficiently. The common coin primitive is one of the most important building blocks for the construction of an ABA protocol. The best known common coin protocol of Feldman et al. requires multiple instances of an AVSS protocol sharing a single secret as a black-box. Unfortunately, this common coin protocol does not achieve its goal when the multiple invocations of AVSS sharing a single secret is replaced by a single invocation of an AVSS protocol sharing multiple secrets simultaneously. Therefore in this paper, we extend the existing common coin protocol to make it compatible with our new AVSS protocol (sharing multiple secrets). As a byproduct, our new common coin protocol is much more communication efficient than the existing common coin protocol.