LibraryConsensus2017Design paperCorpus record
Casper the Friendly Finality Gadget
Casper FFG. Vitalik Buterin and Virgil Griffith.
A finality overlay. Validators cast two rounds of votes so that a chain of checkpoints becomes justified and then final. Equivocation can be slashed. The gadget does not, by itself, propose blocks.
Casper FFG finalises checkpoints on top of some other proposal rule. Validators make supermajority links. A checkpoint that is justified and then final can be reversed only by slashing a quorum that signed both sides.
- Evidence
- Primary paper
- Re-measured
- No
- Assumptions
- 4
- Records linked
- 7
Code
Source, repository, and what is actually hosted
- arXiv:1710.09437https://arxiv.org/abs/1710.09437
- The source URL is not a GitHub repository. No code remote is invented.
- Full text is not mirrored. Public availability is not a licence to copy the file onto this site.
- Add to a reading list
01 Claim ledger
What the paper is allowed to say
Each row is a sentence already in the study. The status is the same on every row: a model claim, not a live measurement.
Claim 01 · Paper model
It is a gadget
The paper does not propose blocks. Something else builds a chain. Casper looks at checkpoints on that chain and decides which of them an accountable set has committed to.
Claim 02 · Paper model
Justify, then finalise
A supermajority link from an earlier justified checkpoint to a later one justifies the target. A further link can finalise the earlier checkpoint. The two-step is what creates a slashable conflict.
Claim 03 · Paper model
Accountable safety
If two conflicting checkpoints both finalise, at least one third of the stake signed illegally. You can show the signatures. You cannot un-break the applications that already acted.
Claim 04 · Paper model
Gasper is not this PDF
Ethereum's post-merge finality uses a Casper-style gadget beside LMD-GHOST. Client rules, inactivity leaks, and the validator set are later specifications.
02 Three cuts
Observation, model, falsifier
A desk does not stop at the summary. Each claim is cut three ways, using only this study's own assumptions and checks. Nothing here is a new figure.
01 It is a gadget
Observation
What the study says
The paper does not propose blocks. Something else builds a chain. Casper looks at checkpoints on that chain and decides which of them an accountable set has committed to.
Model
What has to hold
The proposal mechanism keeps producing checkpoints. The gadget does not conjure them.
Falsifier
What would retire it
What proposes blocks, and what only finalises them?
02 Justify, then finalise
Observation
What the study says
A supermajority link from an earlier justified checkpoint to a later one justifies the target. A further link can finalise the earlier checkpoint. The two-step is what creates a slashable conflict.
Model
What has to hold
Fewer than one third of the stake signs conflicting votes. A larger coalition can finalise two histories.
Falsifier
What would retire it
How many rounds of votes sit between the head and a final checkpoint?
03 Accountable safety
Observation
What the study says
If two conflicting checkpoints both finalise, at least one third of the stake signed illegally. You can show the signatures. You cannot un-break the applications that already acted.
Model
What has to hold
Fewer than one third of the stake signs conflicting votes. A larger coalition can finalise two histories.
Falsifier
What would retire it
Which votes are slashable, and is the stake actually locked?
04 Gasper is not this PDF
Observation
What the study says
Ethereum's post-merge finality uses a Casper-style gadget beside LMD-GHOST. Client rules, inactivity leaks, and the validator set are later specifications.
Model
What has to hold
Clients wait for finality when they need it. The protocol does not force every light client to do so.
Falsifier
What would retire it
Is the live system Casper FFG or a later combination?
03 Sequence
One action, as an operating tape
- 01A proposal mechanism produces blocks. Epoch or checkpoint boundaries are defined on that chain.
- 02Validators vote for a link between a source checkpoint and a target checkpoint.
- 03If a supermajority votes for the link, the target is justified.
- 04If a justified checkpoint is itself the source of a further supermajority link, it can become final.
- 05A validator that votes for two conflicting links, or that surrounds one vote with another, is slashed.
04 Load-bearing
The argument, and where a pitch drops it
Finality is slower than the head on purpose
The cut
The head of the proposal chain can move every slot. The final checkpoint waits for two rounds of votes. A product that calls the head final has left the paper.
Why it carries weight
If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.
Where it is dropped
Finality lags the head of the chain on purpose. The head can still reorg before the checkpoint.
Slashing identifies, it does not repair
The cut
Accountable safety gives you someone to punish after a catastrophe. It is not a refund for the transactions that sat on the losing checkpoint.
Why it carries weight
If this cut is skipped, the paper's name is being used without the mechanism that makes the name mean anything.
Where it is dropped
Finality lags the head of the chain on purpose. The head can still reorg before the checkpoint.
05 Register
What has to be true
Stake and threshold · Not re-measured
Fewer than one third of the stake signs conflicting votes. A larger coalition can finalise two histories.
Stake and threshold · Not re-measured
Votes are attributable to stake that can actually be destroyed. A vote from an empty identity is not a slash.
Model · Not re-measured
The proposal mechanism keeps producing checkpoints. The gadget does not conjure them.
Model · Not re-measured
Clients wait for finality when they need it. The protocol does not force every light client to do so.
06 Divergence
What happened after the paper
Ethereum's research used this gadget and then specified Gasper, which pairs it with a different fork choice and with inactivity penalties. Other chains borrowed the words friendly finality without the vote rules. The 2017 paper is the definition of the two-phase link, not a map of a live validator set.
A later client, parameter set, or reward formula is a different object. Cite this paper for the mechanism. Cite a primary release for the network. This desk has not re-run the proof.
07 Pre-mortem
What to check before you use the idea
- 0 of 4 marked on this browser. A mark is a reading note, not a pass, a rating, or a recommendation.
08 Anatomy
The paper, in the order a builder needs
The problem it names
A longest-chain rule gives probabilistic finality. Casper FFG asks for a checkpoint that an accountable set has signed, so reversing it requires showing that a quorum broke the rules.
What the design proposes
- A proposal mechanism underneath. The gadget only finalises checkpoints of that proposal.
- A supermajority link from one checkpoint to a later one justifies the target.
- A justified checkpoint with a further link can be finalised.
How the mechanism is specified
- Safety is the usual quorum intersection, plus the slashing condition on two votes that violate the fork rule.
- Accountable safety means you can point at the keys that signed both sides. It does not mean the reversed blocks become harmless.
- Ethereum's later Gasper combines a fork choice with this style of finality. Gasper is not this paper.
What this page does not treat as proven
Why the desk still reads it
Ask whether the team has a proposal rule and a finality rule, and whether they have confused the two. Then ask which keys would be slashed if both sides finalised.
09 Lexicon
Terms, opened into the record
- Supermajority link
- A vote by at least two thirds of the stake from one checkpoint to a later one.
- Accountable safety
- Conflicting final checkpoints imply identifiable stake that broke the voting rules.
10 Repository
Every linked record on this page
Underlined words open a page that already exists: a concept, a protocol profile, a failure record, or another paper. If a word is not underlined, this desk does not have a record for it.
This is Blockchain Lab's reading of a public design paper. It is not the paper, not a copy of it, and not an offer of tokens, equity, custody or a partnership. Later network behaviour can diverge from the text. Nothing here is investment, legal or technical advice.
Research status: Design paper. Last reviewed: 1 October 2026. This is a reading of a public paper, not investment, legal or security advice.