Block Finality

Found in Proof-of-Work (PoW) chains like Bitcoin, finality is not absolute but increases exponentially with each subsequent block. The probability of a reorganization becomes negligible after a sufficient number of confirmations (e.g., 6 blocks).

• Mechanism: Relies on the cumulative weight of the longest chain.
• Example: A transaction with 1 confirmation has a non-zero chance of being orphaned. After 6 confirmations, it is considered practically final.

Achieved by Proof-of-Stake (PoS) networks using BFT-style consensus, where a block is finalized as soon as a supermajority of validators votes for it. Once finalized, it is cryptographically guaranteed to be irreversible.

• Mechanism: Uses a two-phase voting process (e.g., pre-vote, pre-commit).
• Example: Networks like Ethereum (post-merge), BNB Smart Chain, and Cosmos provide absolute finality, typically within one block.

A stronger form of finality where reverting a block would be economically irrational, as it would require validators to destroy a significant amount of staked capital. This is a core security feature of modern Proof-of-Stake systems.

• Mechanism: Enforced via slashing penalties, where malicious validators lose their staked assets.
• Implication: An attack becomes prohibitively expensive, as the cost to revert a block far exceeds any potential gain.

Used in optimistic rollups, this model assumes transactions are valid by default (optimistically) but includes a challenge period (e.g., 7 days) during which they can be disputed. Finality is achieved only after this window passes without a successful fraud proof.

• Trade-off: Enables high throughput and low cost but introduces a delay to absolute finality for cross-chain withdrawals.
• Example: Arbitrum and Optimism use this model for their Layer 2 chains.

Auxiliary protocols that enhance a blockchain's native finality. They allow a chain with probabilistic finality to achieve stronger guarantees.

• Casper FFG: The finality gadget used by Ethereum, which operates alongside the LMD-GHOST fork choice rule. It periodically finalizes checkpoints (groups of blocks).
• GRANDPA: The finality gadget used by Polkadot, which provides unconditional finality for entire chains of blocks at once.

The finality time is the latency between a transaction's submission and its irreversible confirmation. It is a critical metric for user experience and directly impacts a blockchain's perceived throughput (TPS).

• Key Trade-off: There is often a tension between fast finality, high decentralization, and high throughput.
• Example: A chain with 12-second block times and instant finality (like some BFT chains) has a lower finality time than a chain with 10-second blocks and 6-confirmation rules.

In Proof-of-Work chains, finality is probabilistic, meaning the likelihood of a block being reverted decreases exponentially as more blocks are built on top of it. This is often measured in confirmation blocks.

• Example: A Bitcoin transaction with 6 confirmations is considered settled, as the probability of a reorganization that deep is astronomically low.
• Mechanism: Finality is achieved through the longest chain rule and the cumulative proof-of-work security assumption.

These blockchains use Byzantine Fault Tolerant (BFT) consensus to achieve instant finality upon block production. Once a block is added to the chain, it is immediately irreversible.

• Example: In Algorand, a block is finalized within a single round (typically ~4 seconds) via its Pure Proof-of-Stake and cryptographic sortition mechanism.
• Key Feature: Eliminates the need for confirmation waits, enabling real-time settlement for DeFi and payments.

In Layer 2 scaling solutions like Optimism and Arbitrum, finality has two layers. Transactions achieve fast, soft finality on the L2, but must undergo a challenge period (e.g., 7 days) on Ethereum L1 for hard finality.

• Mechanism: This fraud-proof window allows anyone to challenge invalid state transitions, after which the result is cemented on L1.
• Trade-off: Enables high throughput and low cost while inheriting L1 security, but introduces a withdrawal delay.

In permissioned or centralized systems, finality is absolute and deterministic, governed by a known set of authorities.

• Example: A Hyperledger Fabric channel with a solo ordering service achieves finality the instant its sole node creates a block.
• Characteristic: There is no probability or economic cost; finality is a function of administrative control and trust in the pre-selected validators.

Probabilistic finality, used by Nakamoto Consensus (e.g., Bitcoin), means the probability of a block being reverted decreases exponentially with each subsequent confirmation. Absolute finality, achieved by protocols like Tendermint or Ethereum's finality gadget, is a cryptographic guarantee that a block is permanently settled and cannot be reverted without slashing a large portion of the validator stake.

A reorganization occurs when a longer, competing chain overtakes the current canonical chain, causing previously confirmed blocks to be orphaned. This is a primary attack vector against probabilistic finality. The risk depends on the attacker's hashrate (PoW) or stake (PoS). A 51% attack is a severe reorg where an attacker with majority control can double-spend by secretly mining a longer chain.

To enhance security, some chains implement finality gadgets. Ethereum's Casper FFG (Friendly Finality Gadget) adds a finality layer on top of its LMD-GHOST fork choice rule. Validators periodically vote to finalize checkpoints (epoch boundaries). Once finalized, reversion requires at least one-third of the total staked ETH to be slashed, making it economically prohibitive.

A long-range attack targets Proof-of-Stake chains with weak subjectivity. An attacker acquires old validator keys (e.g., from a past epoch) to create an alternative history from a point far in the past. Defenses include weak subjectivity checkpoints (clients must sync from a recent trusted block) and slashing for equivocation, which makes creating a parallel chain from an old state detectable and punishable.

Time to Finality is the latency from transaction submission to irreversible inclusion. It's a key security-performance trade-off. A shorter TTF improves user experience but may compromise security if the finality mechanism is too fast. Liveness—the chain's ability to continue producing new blocks—can conflict with safety. Some BFT protocols may halt (no new blocks) if they cannot achieve finality to preserve safety.

Economic finality is the practical assurance that reverting a block is financially irrational. In PoS, this is enforced via slashing, where malicious validators lose their staked assets. The cost of attack must exceed the potential profit. For high-value settlements (e.g., cross-chain bridges), applications often wait for a higher number of confirmations or for cryptographic finality to be reached.

A finality model where the probability of a block being reverted decreases exponentially as more blocks are built on top of it. This is the standard model for Proof-of-Work (PoW) chains like Bitcoin.

• Mechanism: Finality is not absolute but is inferred from the depth of the chain. A transaction with 6+ confirmations is considered practically final.
• Trade-off: Provides robust security against moderate attacks but offers no absolute guarantee, enabling chain reorganizations.

A finality model where a block is irreversibly finalized the moment it is added to the canonical chain, with zero probability of reversion. This is achieved through explicit consensus steps.

• Mechanism: Used by Proof-of-Stake (PoS) chains with BFT-style consensus (e.g., Tendermint, Ethereum's Casper FFG). Validators vote in rounds to finalize blocks.
• Benefit: Provides strong, immediate security guarantees, crucial for high-value cross-chain bridges and settlements.

A concept where a block is considered final because the cost to revert it (e.g., via a 51% attack) would be economically prohibitive, exceeding any potential profit from the attack.

• Foundation: Central to both PoW and PoS security models. In PoS, it's enforced through slashing, where malicious validators lose their staked assets.
• Purpose: Creates a strong disincentive against attempting to reverse transactions, making the network secure under rational actor assumptions.

A property where a transaction is finalized within one consensus round, typically in a single block, with no waiting period for confirmations. Often synonymous with absolute finality in fast BFT systems.

• Examples: Chains like Solana (Tower BFT) and Avalanche (Snowman++) aim for sub-2-second finality.
• Contrast: Differs from probabilistic models (e.g., Bitcoin's 10-minute blocks) and even from Ethereum's ~12-minute finality under its hybrid model.

An event where a blockchain's canonical history changes, as nodes converge on a different, longer, or heavier chain. This directly challenges finality.

• Cause: In PoW, caused by natural propagation delay or a 51% attack. In PoS, can be caused by consensus failures or deliberate attacks.
• Impact: Transactions in orphaned blocks lose their finality. Deep reorgs can undermine trust and enable double-spending attacks.

A mechanism where a trusted source (e.g., a foundation or a super-majority of validators) periodically attests to a specific block hash, making all preceding blocks practically irreversible.

• Historical Use: Early PoW chains sometimes used hard-coded checkpoints in client software to prevent deep reorgs.
• Modern Use: In PoS Ethereum, finalized checkpoints are created every two epochs (~12 minutes) by the consensus layer, providing a rolling point of absolute finality.