OptRand: Optimistically responsive distributed random beacons.

Distributed random beacons publish random numbers at regular intervals, which anyone can obtain and verify. The design of public distributed random beacons has been an exciting research direction with significant implication to blockchains, voting and beyond. Random beacons, in addition to being bias-resistant and unpredictable, also need to have low communication cost, low latency, and ease of reconfigurability. Existing works on synchronous random beacons sacrifice one or more of these properties. In this work, we design an efficient unpredictable synchronous random beacon protocol, OptRand, with quadratic (in the number n of system nodes) communication complexity per beacon output. First, we innovate by employing a novel combination of bilinear pairing based publicly verifiable secret sharing and non-interactive zero-knowledge proofs to build a linear (in n) sized publicly verifiable random sharing. Second, we develop a state machine replication protocol with linear-sized inputs that is also optimistically responsive, i.e., it can progress responsively at actual network speed during optimistic conditions, despite the synchrony assumption, and thus incur low latency. In addition, we present an efficient reconfiguration mechanism for OptRand that allows nodes to leave and join the system.