BFT

BFT (Byzantine Fault Tolerance) is the property of a distributed system to keep operating correctly even when some of its nodes fail or behave maliciously. The term traces back to the classical “Byzantine generals” problem: a group of participants, some of whom may be traitors, must agree on a common decision, and the remaining honest participants must end up with the same answer.

Why blockchains need it

Any public network must assume that some validators or miners will be dishonest. A consensus protocol must guarantee that:

• All honest nodes eventually agree on the same chain of blocks.
• An attacker cannot roll back a confirmed transaction without controlling a very large share of the network.
• The system keeps making progress even when part of the validator set is offline or actively misbehaving.

The classical BFT result is that safety holds as long as malicious nodes are less than one-third of the total.

BFT in TON: Catchain

TON’s validator-side consensus is Catchain, a protocol from the pBFT (Practical Byzantine Fault Tolerance) family adapted for high throughput and a sharded architecture. In each round:

• Validators are split into groups by shard.
• Inside each group a leader is selected and proposes a block.
• The other validators vote on the proposal; once a qualified majority of more than two-thirds signs off, the block becomes final.
• The masterchain aggregates finality proofs from all shards in parallel.

Safety holds as long as fewer than one-third of validators are compromised. The stake that validators lock up to participate makes attacks expensive: a double-signing attempt is punished by slashing the offender’s deposit.

BFT versus Nakamoto

Bitcoin and early Ethereum use what is called Nakamoto consensus — probabilistic: a block becomes “reliable enough” after several confirmations, but there is no strict moment of “it is definitely final now”. BFT systems are a different class: a block is either signed by a qualified majority or it is not, and rolling back a signed block requires a public protocol violation and the loss of stake. TON, Cosmos / Tendermint and Polkadot all belong to the BFT camp.

Practical consequences

• Finality. In BFT chains, confirmation arrives in a single block — no need to wait for “6 confirmations”.
• Fault tolerance. The network keeps producing blocks as long as fewer than one-third of validators are compromised or offline.
• Incentives. Honest behaviour is rewarded with block rewards, while misbehaviour is penalised by slashing the validator’s stake.

BFT is not a marketing label but a mathematically formalised boundary on what fraction of attackers a network can absorb without catastrophe.