🔒 Ethereum Casper Casper-FFG
Casper-FFG, the 'Friendly Finality Gadget,' is the part of Ethereum's proof-of-stake system that finalizes blocks: it permanently locks in the chain's history so old transactions can no longer be reversed.
🔒 The simple version — a notary stamp for history
Picture a class keeping one shared record book. Two-thirds of the class raise their hands to approve a page, and once the next page is also approved on top of it, the earlier page gets stamped, sealed, and can never be torn out. Casper is that notary stamp for Ethereum. It does not pick which transactions happen; it decides which blocks are permanent. Once a block is finalized, exchanges, wallets, and bridges can safely treat the transactions inside it as done.
🧩 Justified, then finalized — two steps
Finality works on special blocks called checkpoints (the first block of each time period, called an epoch). The process has two stages:
| Step | What has to happen |
|---|---|
| ✋ Justified | More than two-thirds of all staked ETH votes in favor of a checkpoint |
| 🔒 Finalized | A second checkpoint is justified directly on top of a justified one, sealing the earlier block forever |
This two-step pattern is borrowed from a classic computer-science design called BFT (specifically PBFT), where agreement also happens in a prepare step and then a commit step. Casper's justify-then-finalize is the same idea in new clothes.
⚙️ Casper plus LMD-GHOST = Gasper
Ethereum's live consensus is named Gasper, and it has two jobs split between two parts. Casper-FFG is the finality gadget that locks blocks in. LMD-GHOST is the fork-choice rule that picks the current chain head when there is more than one candidate. So while you grow the chain, LMD-GHOST keeps choosing which block is the tip, and Casper keeps stamping older blocks as permanent behind it.
🪙 Who does the voting? Validators with skin in the game
The votes come from validators, and they only get a vote by putting money on the line. To propose and attest to blocks, a validator stakes a minimum of 32 ETH. Vote honestly and you keep earning; cheat or try to rewrite finalized history and you can be slashed — part of your stake is destroyed and you are ejected from the network. That penalty is what makes the two-thirds vote trustworthy.
📅 Live since The Merge
Casper has secured Ethereum since The Merge on September 15, 2022. Before then, Ethereum ran on proof of work, where miners burned electricity to add blocks. The Merge swapped that for proof of stake, cutting Ethereum's energy use sharply and putting Casper-FFG in charge of finality. The canonical place you meet it is the Ethereum network itself: when a transaction's status reaches 'finalized,' that is Casper at work.
❓ FAQ
- Is Casper the same thing as Ethereum's proof of stake?
- Not quite. Casper-FFG is only the part that finalizes blocks. Ethereum's full consensus is called Gasper, which pairs Casper-FFG with a fork-choice rule called LMD-GHOST. Casper decides what becomes permanent; LMD-GHOST decides which block is the current chain head.
- What does it mean for a block to be 'finalized'?
- A finalized block is locked into the chain's history for good. First a checkpoint block becomes 'justified' when more than two-thirds of all staked ETH vote for it. When a second checkpoint is justified directly on top of it, the earlier one is finalized and can no longer be reversed.
- When did Casper go live on Ethereum?
- Casper has secured Ethereum since The Merge on September 15, 2022. That upgrade replaced energy-hungry proof-of-work mining with proof of stake, where validators put up coins instead of burning electricity.