What is FBFT?

FBFT (Fast Byzantine Fault Tolerance) is the consensus protocol used by the Harmony blockchain, designed to optimize the communication complexity of traditional PBFT for large validator sets. FBFT modifies the classical PBFT protocol by replacing the O(n²) communication pattern in the prepare and commit phases with an aggregated multi-signature scheme based on BLS (Boneh-Lynn-Shacham) signatures. Instead of each validator broadcasting its vote to every other validator, validators send their votes to the leader, who aggregates them into a single multi-signature that can be verified by anyone. This reduces communication complexity from O(n²) to O(n), making it feasible to run BFT consensus with hundreds of validators.

FBFT enables Harmony to achieve block finality in approximately 2 seconds while supporting a large validator set distributed across multiple shards. The protocol maintains the standard BFT safety guarantee of tolerating up to one-third Byzantine validators within each shard. The use of BLS signature aggregation is key to FBFT's efficiency — a multi-signature from hundreds of validators is the same size as a single signature, dramatically reducing both network bandwidth and verification costs. Combined with Harmony's sharding architecture, FBFT allows the network to scale throughput by adding more shards, each running its own instance of FBFT consensus, while a beacon chain coordinates cross-shard communication and validator assignment.

FBFT has been running on Harmony's mainnet since June 2019. Harmony is a sharded proof-of-stake blockchain that aims to provide both scalability and decentralization, using Effective Proof-of-Stake (EPoS) for validator election to encourage stake distribution across validators and prevent centralization. The network has operated with multiple shards, each secured by a subset of validators running FBFT. Harmony was founded by Stephen Tse and a team with backgrounds from Google, Apple, and Amazon, and was one of the early blockchain projects to implement BLS-based signature aggregation in a production BFT consensus protocol.