Byzantine Fault Tolerant Protocols with Near-Constant Work per Node without Signatures

Numerous distributed tasks have to be handled in a setting where a fraction of nodes behaves Byzantine, that is, deviates arbitrarily from the intended protocol. Resilient, deterministic protocols rely on the detection of majorities to avoid inconsistencies if there is a Byzantine minority, which requires individual nodes to handle a communication load that is proportional to the size of the network -- an intolerable disadvantage in large networks. Randomized protocols circumvent this by probing only small parts of the network, thus allowing for consistent decisions quickly and with a high level of confidence with communication that is near-constant in the network size. However, such protocols usually come with the drawback of limiting the fault tolerance of the protocol. For instance, by severely restricting the number or type of failures that the protocol can tolerate. We present randomized protocols to reliably aggregate and broadcast information, form consensus and compute common coins that tolerate a constant fraction of Byzantine failures, do not require cryptographic methods and have a near-constant time and message complexity per node. Our main technique is to compute a system of witness committees as a pre-computation step almost optimally. This pre-computation step allows to solve the aforementioned distributed tasks repeatedly and efficiently, but may have far reaching further applications, e.g., for sharding of distributed data structures.