Casper FFG (Gasper)

Casper the Friendly Finality Gadget (FFG) works on checkpoints, the first block of each epoch. Validators vote for a link between a source checkpoint and a target checkpoint. When validators holding two-thirds of the staked ETH support a link, the target becomes justified, and when a justified checkpoint is followed by a justified child, it becomes finalised. Under normal conditions that takes about two epochs, roughly 13 minutes.

Ethereum pairs FFG with LMD-GHOST, a fork-choice rule that picks the head of the chain from the latest votes, and the combination is called Gasper. LMD-GHOST keeps blocks flowing every slot, while FFG adds the economic guarantee that reverting a finalised block would require at least one-third of all stake to be slashed. Exchanges and bridges often wait for finality before crediting large deposits.

Finality is not instant, and when more than a third of validators go offline the chain keeps producing blocks but stops finalising until the inactivity leak restores a two-thirds majority. Research on single-slot finality aims to shrink the wait, but it is a roadmap item, not a live feature.