Finality¶

This chapter closes the Ethereum section by returning to a concept introduced generally in Finality: Ethereum's specific finality mechanism, Casper FFG (Friendly Finality Gadget), and exactly how it differs from Bitcoin's purely probabilistic model.

Checkpoints, justification, and finalization¶

Casper FFG operates on checkpoints, specifically, the first block of each epoch (see Proof of Stake). Validators' attestations don't just vote on the current chain head; they also vote on which checkpoint they consider valid, building toward two successive thresholds:

Justified: a checkpoint becomes justified once attestations representing at least two-thirds of total staked ETH vote for it (technically, for the specific (source, target) checkpoint pair required by the protocol's exact voting rules).
Finalized: a checkpoint becomes finalized once the next checkpoint after it also becomes justified, two consecutive justified checkpoints finalize the earlier one.

Epoch N checkpoint    Epoch N+1 checkpoint
   justified?    ──►      justified?
       │                      │
       └──── both justified ──┘
                  │
                  ▼
      Epoch N checkpoint is now FINALIZED

What finalization actually guarantees¶

Once a checkpoint is finalized, reverting it would require an attacker to have controlled at least one-third of total staked ETH and be willing to have that entire stake slashed (see Slashing), since reverting a finalized checkpoint necessarily requires validators to violate the same double-voting rules slashing is specifically designed to catch and punish. This gives Ethereum's finality a meaningfully different character than Bitcoin's: it's not "increasingly improbable with more confirmations" (Bitcoin's model, covered with its actual formula in Probabilistic Finality) but "would require deliberately, provably destroying a specific, enormous, quantifiable amount of real capital", the economic finality concept introduced generally in Finality.

Deterministic versus probabilistic guarantees, compared¶

	Bitcoin	Ethereum (post-Merge)
Finality type	Probabilistic	Justified/finalized checkpoints (economic finality)
Typical time to strong confidence	Multiple confirmations (often ~1 hour for high value)	Two epochs (~12.8 minutes) for finalization
What reversal requires	More cumulative hash power than the honest chain, sustained	Roughly 1/3+ of total stake, willing to be slashed
Guarantee style	Probability approaches (never reaches) zero	Reversal is possible but economically self-destructive past finalization

Why this two-tier design (justified, then finalized), not a single step¶

Requiring two consecutive justified checkpoints, rather than finalizing immediately upon a single checkpoint reaching the two-thirds threshold, is a deliberate safety margin: it protects against a scenario where a checkpoint reaches the threshold due to a temporary, unusual pattern of attestations (perhaps during a network partition or unusual validator behavior) that doesn't reflect a durable, sustained consensus, requiring the next checkpoint to also independently reach the same threshold provides confirmation that the justified state reflects a stable, ongoing agreement, not a one-off statistical or adversarial anomaly.

Common misconceptions¶

Ethereum's finalized blocks are not mathematically, absolutely irreversible in the same sense a formal proof establishes something as logically impossible. Reverting them is possible in principle, but only at a specific, enormous, and precisely quantifiable economic cost (the slashing of roughly a third of all staked ETH), which is a different, though in practice extremely strong, kind of guarantee than mathematical impossibility.

"Confirmed" (a block simply being added to the chain) and "finalized" (surviving the full justification-then-finalization process) are not the same thing on Ethereum. An unfinalized, recently added block can still, in principle, be reorganized away under some circumstances, similar in spirit (though different in exact mechanism) to Bitcoin's shallow reorgs covered in Chain Reorganizations; finalization is the stronger, later guarantee.