Crypto glossary
Byzantine fault tolerance (BFT)
Byzantine fault tolerance is the ability of a distributed network to keep agreeing on one state even when some participants fail or deliberately lie.
The idea comes from the Byzantine generals problem: several commanders must agree on a plan by messenger, knowing some of them may be traitors. A system is Byzantine fault tolerant if honest participants still reach the same decision despite faulty or malicious members. Classic BFT protocols work as long as fewer than one third of the participants misbehave.
Blockchains are BFT systems by design, since anyone may run a node. Many proof-of-stake networks use BFT-style voting in which validators exchange signed messages and a block is final once two thirds of the stake approves it. The Cosmos ecosystem builds on Tendermint, now CometBFT, and Ethereum's finality gadget follows the same two-thirds rule.
BFT voting gives fast, explicit finality, but message traffic grows with the number of validators, which pushes designs toward smaller or committee-based validator sets. If more than one third goes offline, the chain can halt rather than finalize blocks.
Example
In a network of 100 equally weighted validators, a block becomes final once 67 sign it, and the chain stays safe as long as no more than 33 act maliciously.
