Part II · 3 — Finality and attacks
Consensus only matters for the guarantee it delivers: when a transaction becomes irreversible. And every guarantee has an attack price. This section closes out consensus with what actually matters to the user — and to the adversary.
3.1 The two types of finality
Finality is the guarantee that a transaction will not be reverted. There are two regimes, inherited from the mechanism:
- Probabilistic (PoW, and Nakamoto-style PoS) — never 100%, but grows with
every block. In Bitcoin, 6 confirmations (1h) are treated as final; reverting would require overtaking the accumulated work.
- Deterministic (BFT — Tendermint, etc.) — a block finalized by ⅔ of the
validators is irreversible by protocol. Reverting would require ≥⅓ to cheat and be slashed.
The choice has a direct practical effect: an exchange accepts a deposit after N confirmations on Bitcoin, but instantly on a BFT chain.
3.2 Forks and the chain-choice rule
When there are competing branches, the fork-choice (chain-choice rule) decides the canonical chain:
- PoW — heaviest chain (most work).
- Ethereum PoS — LMD-GHOST (the subtree with the most attestation
weight), combined with the Casper FFG finality gadget.
Forks are also intentional: a soft fork tightens the rules in a backward-compatible way; a hard fork changes the rules incompatibly, creating a new chain (that's how BCH was born from BTC, and today's Ethereum from the fork of The DAO).
3.3 How consensus is attacked
| Attack | Applies to | Mechanics | Defense |
|---|---|---|---|
| 51% | PoW | a hashrate majority rewrites recent blocks (double-spend) | hardware/energy cost; deep reorgs are expensive |
Nothing-at-stake |
naive PoS | a validator signs every fork (signing is free) | slashing punishes double-signing |
| Long-range | PoS | rewrite from an old point with leaked keys | weak subjectivity (trusted recent checkpoints) |
| Grinding | sortition | trying to bias proposer selection | sortition with verifiable randomness (VRF) |
Notice how each PoS defense relies on economic punishment (slashing) or unpredictable randomness (CSPRNG/VRF) — the cryptography from earlier parts underpinning the game theory.
3.4 MEV: ordering is power too
A risk cutting across all mechanisms: MEV (Maximal Extractable Value). Whoever orders a block's transactions can insert, reorder, or censor transactions to profit — for example, "sandwiching" a user's buy order (front-run + back-run). It is an invisible tax on users and a centralizing force (whoever extracts MEV gains an advantage). Mitigations (proposer-builder separation, private mempools) are an area of active research.
Dense reference:
fork-choice(LMD-GHOST/Casper), attack taxonomy, and MEV in03-consensusand14-incidents. End of Part II. Part III — L1 and L2 (under construction) applies all of this to real chains: Bitcoin, Ethereum, and the rollups that scale them.