Byzantine Fault Tolerance

Byzantine Fault Tolerance (BFT) is a system's ability to keep reaching correct agreement even when some participants fail arbitrarily — including lying, sending conflicting messages, or actively colluding to disrupt the network. The name comes from the "Byzantine Generals Problem," a 1982 thought experiment about army commanders who must coordinate an attack when some generals may be traitors sending contradictory orders.

This is the core problem every blockchain solves: thousands of strangers must agree on one transaction history while assuming some of them are malicious. Classical BFT protocols, descended from PBFT (Practical Byzantine Fault Tolerance), work by having a known set of validators exchange rounds of votes; they can tolerate up to one-third of validators being faulty. For example, Tendermint — the consensus engine behind Cosmos chains — finalizes a block once more than two-thirds of validator voting power signs it, so a block is irreversible the moment it is committed. Ethereum's Casper FFG finality gadget applies the same two-thirds principle.

A common misconception is that Bitcoin uses a BFT voting protocol; Bitcoin instead achieves Byzantine fault tolerance probabilistically through proof of work and the longest-chain rule, without validators voting at all — a distinct approach often called Nakamoto consensus. The practical difference shows up in finality: BFT-style chains offer instant, absolute finality after one block, while Nakamoto-style chains offer confidence that grows with each confirmation but never reaches mathematical certainty.