Casper is the name of a family of proof-of-stake research protocols developed for Ethereum in the mid-2010s. The two main branches were Casper FFG (Friendly Finality Gadget), led by Vitalik Buterin with co-author Virgil Griffith, and Casper CBC (Correct-by-Construction), led by researcher Vlad Zamfir. Casper FFG's finality rules became part of Gasper, the consensus protocol that has run on Ethereum's Beacon Chain since December 2020 and has secured the whole network since the Merge in September 2022.
The name should not be confused with Casper Network, a separate proof-of-stake blockchain with its own token (CSPR), which launched in 2021 and drew on ideas from the CBC research line. On Ethereum, "Casper" today usually refers to the finality mechanism that lets validators mark blocks as irreversible, the job described in more detail under consensus layer.
Casper in plain terms
Think of a long document being written by a committee, one page at a time. Anyone can suggest the next page, and for a while there may be two competing versions. Every so often, the committee holds a formal vote on a page. If two-thirds of the members approve it in two rounds, that page and everything before it are locked and can never be rewritten.
Casper is the set of voting rules for those formal checkpoints. Members put down a large deposit to join, and if any of them is caught voting for two contradictory versions, part of their deposit is confiscated. That penalty makes cheating very expensive, which is how the system replaces the electricity spent by miners with money that validators put at risk.
History
Ethereum's founders intended from early on to move away from proof of work. Zamfir began Casper research in 2014, and in 2017 Buterin and Griffith published the Casper FFG paper, which described a finality layer that could sit on top of any block-producing chain. A 2018 plan to add FFG as an overlay to Ethereum's proof-of-work chain, a form of hybrid PoW/PoS consensus, was shelved in favour of building a separate proof-of-stake chain with sharding in mind. That chain became the Beacon Chain, which combined Casper FFG with a fork-choice rule called LMD GHOST. The combination is known as Gasper.
How Casper FFG works
Casper FFG treats the first block of each epoch, a period of 32 slots, as a checkpoint. Validators vote on links between checkpoints, and the protocol applies a few rules:
- Justification. A checkpoint becomes justified when validators holding at least two-thirds of the total stake vote for a link to it from an earlier justified checkpoint.
- Finalization. A justified checkpoint becomes finalized when the next checkpoint is also justified on top of it, after which the blocks before it are treated as permanent.
- Slashing conditions. A validator is penalised if it casts two different votes for the same height or a vote that "surrounds" another of its own votes, the two ways it could help finalize conflicting histories.
Under these rules, finalizing two conflicting checkpoints would require at least one-third of all stake to break a slashing condition, so an attacker would lose a very large sum. This property is called accountable safety. If more than one-third of validators go offline, finality stalls, and an "inactivity leak" gradually reduces their stake until the online majority can finalize again.
Casper CBC
Casper CBC took a different approach, deriving a protocol from a formal specification of the safety it should guarantee. It produced research on estimators and on finality that does not depend on fixed rounds, which influenced later designs, including the Highway protocol used by Casper Network, but it was not adopted for Ethereum's main chain.
Significance and criticism
Casper FFG helped show that proof of stake could offer explicit, economic finality, a stronger guarantee than the probabilistic finality of proof-of-work chains such as Bitcoin. Critics argue that staking concentrates influence among large holders and that finality taking about two epochs, around 13 minutes, is slow for some uses. Ethereum researchers are working on single-slot finality and other changes to address these points. The wider network is described under Ethereum Blockchain, and its native coin under Ethereum (ETH). Related ideas appear under Byzantine fault tolerance and staking.