What is Threshold BFT?

Threshold BFT is the consensus protocol used by the Internet Computer (ICP), developed by DFINITY. The protocol relies heavily on threshold cryptography — specifically BLS threshold signatures — where no single validator holds a complete signing key. Instead, each validator holds a share of the key, and a threshold number of shares (typically two-thirds) must be combined to produce a valid signature. Consensus proceeds through rounds where a randomly selected leader proposes a block, and validators contribute their signature shares. When enough shares are collected and combined into a threshold signature, the block is considered finalized. The random beacon, itself produced through threshold signatures from the previous round, determines the leader selection, creating an unbiasable and unpredictable randomness source.

Threshold BFT achieves fast finality (approximately 1-2 seconds) while providing strong BFT guarantees tolerating up to one-third Byzantine validators. The threshold signature scheme ensures that the consensus output is compact — regardless of the number of validators, the finalization proof is a single BLS signature rather than a collection of individual signatures. This is particularly important for the Internet Computer's architecture, where consensus runs independently within each subnet (a group of validators running a specific set of canisters/smart contracts). The protocol also provides an unbiasable random beacon that is used not only for leader election but also as a source of on-chain randomness for smart contracts. The combination of threshold cryptography with BFT consensus gives the protocol both efficiency and strong cryptographic guarantees.

Threshold BFT has been running on the Internet Computer mainnet since its launch in May 2021. The Internet Computer is a blockchain platform developed by the DFINITY Foundation, founded by Dominic Williams, with the goal of serving as a decentralized cloud computing platform. The network is organized into subnets, each running its own instance of Threshold BFT, with a Network Nervous System (NNS) DAO governing the network's configuration. The Internet Computer hosts web-serving smart contracts (called canisters) that can serve HTTP requests directly, and the fast consensus finality is essential for providing responsive user experiences. DFINITY invested years of research into threshold cryptography to enable this consensus design.