Efficient Asynchronous Byzantine Agreement without Private Setups

Efficient asynchronous Byzantine agreement (BA) protocols were mostly studied with private setups, e.g., pre-setup threshold cryptosystem. Challenges remain to reduce the large communication in the absence of such setups. Recently, Abraham et al. (PODC’21) presented the first asynchronous validated BA (VBA) with expected $\mathcal{O}$(n
³
) messages and $\mathcal{O}$ (1) rounds, relying on only public key infrastructure (PKI) setup, but the design still costs $\mathcal{O}$ (λn
³
logn) bits. Here n is the number of parties, and λ is a cryptographic security parameter.In this paper, we reduce the communication of private-setup free asynchronous BA to expected $\mathcal{O}$(λn
³
) bits. At the core of our design, we give a systematic treatment of common randomness protocols in the asynchronous network, and proceed as:•We give an efficient reasonably fair common coin protocol in the asynchronous setting with only PKI setup. It costs only $\mathcal{O}$ (λn
³
) bit and $\mathcal{O}$(1) rounds, and ensures that with at least 1/3 probability, all honest parties can output a common bit that is as if randomly flipped. This directly renders more efficient private-setup free asynchronous binary agreement (ABA) with expected $\mathcal{O}$(λn
³
) bits and $\mathcal{O}$(1) rounds.•Then, we lift our common coin to attain perfect agreement by using a single ABA. This gives us a reasonably fair random leader election protocol with expected $\mathcal{O}$(λn
³
) communication and expected constant rounds. It is pluggable in all existing VBA protocols (e.g., Cachin et al., CRYPTO’01; Abraham et al., PODC’19; Lu et al., PODC’20) to remove the needed private setup or distributed key generation (DKG). As such, the communication of private-setup free VBA is reduced to expected $\mathcal{O}$(λn
³
) bits while preserving fast termination in expected $\mathcal{O}$(1) rounds. Moreover, our result paves a generic path to private-setup free asynchronous BA protocols, as it is not restricted to merely improve Abraham et al.’s specific VBA protocol (PODC’21).Our results and techniques could be found useful and interesting for a broad array of applications such as asynchronous DKG and DKG-free asynchronous random beacon that is friendly for dynamic participation and reconfiguration.