justification and finalization

Justification and finalization are the two stages by which Casper FFG promotes a checkpoint from 'agreed upon' to 'irreversible.' Both operate on checkpoints — the first block of each epoch — and both depend on supermajority links, where validators controlling more than two-thirds of total stake cast attestations linking a source checkpoint to a target checkpoint. The two-step structure is what gives Ethereum its hard finality without ever requiring a single instantaneous global vote.

A checkpoint becomes justified the moment a supermajority link points to it as the target: more than two-thirds of stake has agreed it belongs on the canonical chain. Justification is strong but not yet permanent — it can, in rare adversarial conditions, be skipped over. Finalization is the second click of the ratchet: when a justified checkpoint is itself the source of a new supermajority link that justifies the very next checkpoint (its direct child), the earlier checkpoint is finalized. In other words, two consecutive justified checkpoints with a supermajority link between them finalize the first.

Finalization is the strongest guarantee Ethereum offers: undoing a finalized checkpoint would require an attacker to violate a slashing condition with at least one-third of all staked ETH, getting that stake provably burned — the accountable-safety property. Under normal conditions a block finalizes within about two epochs, roughly 12.8 minutes, which is why exchanges and bridges wait for finality rather than a fixed number of confirmations. If finalization stalls because too much stake is offline, the inactivity leak gradually restores the conditions for a supermajority link to form again.