Definition
Byzantine Fault Tolerance (BFT) is the property that lets a distributed system reach correct agreement even when some of its participants behave arbitrarily, including crashing, going silent, or actively lying by sending different messages to different peers. It is the formal standard a consensus protocol must meet to be trustworthy in an adversarial environment.
The Byzantine Generals Problem
The name comes from a 1982 thought experiment by Lamport, Shostak, and Pease. Several army divisions, each led by a general, must jointly decide to attack or retreat, coordinating only through messengers. Some generals may be traitors who send conflicting orders. The challenge is for all loyal generals to settle on the same plan despite the traitors. This abstracts the core difficulty of agreement when participants cannot be assumed honest.
Limits and guarantees
A classic result is that deterministic BFT protocols need at least 3f+1 participants to tolerate f Byzantine faults, meaning a system can withstand up to roughly one-third malicious nodes. Protocols are judged on two properties: safety (honest nodes never disagree on a finalized result) and liveness (the system keeps making progress). Practical Byzantine Fault Tolerance (PBFT) made these guarantees efficient for small, known validator sets.
Relevance to Bitcoin
Bitcoin solves a harder, permissionless version of this problem: it achieves Byzantine tolerance among anonymous participants of unknown number by substituting proof-of-work for identity. See Nakamoto Consensus for how that substitution works.
In Simple Terms
Byzantine Fault Tolerance (BFT) is the property that lets a distributed system reach correct agreement even when some of its participants behave arbitrarily, including crashing,…