Statechain

Statechains transfer coins the way you hand over a sealed envelope: the coin never moves on chain, the right to spend it does. Ruben Somsen described the design in 2018, CommerceBlock shipped a working version in 2021, and a blinded rewrite called Mercury Layer followed in 2024. You get instant transfers of whole outputs, plus one new party you have to trust to delete something.

How it works

A statechain coin sits in an output that takes two signatures, yours and a server's, combined into what looks on chain like an ordinary single-key spend.

Selling the coin does not spend that output. You send your key share to the buyer, and the server rotates its own share so the combined key stays exactly the same. The address never changes, the blockchain sees nothing, and the buyer now holds half of a two-party key over a coin that has been sitting untouched since it was funded. Transfers cost no mining fee and settle as fast as two messages.

The safety net is a chain of pre-signed backup transactions. Each owner receives one that can claim the coin on chain if the server disappears, and every transfer shortens its timelock, so the newest owner can always exit before any previous owner. That ordering is what stops a former owner from racing you to the coin. It also means the receiving wallet must verify the whole history itself, checking that the sequence of locktimes really does step down, because nobody else will do it for you.

Then there is the assumption nobody can remove. If the server keeps a copy of an old key share and colludes with the person who used to own the coin, the two of them can spend it. There is no cryptographic proof of deletion, anywhere, ever. Mercury Layer narrows the damage by blinding the server so it never learns which on-chain coin it is signing for, but blinding hides the target, it does not force the forgetting.

One structural limit shapes everything else: a statechain moves whole outputs. You cannot send part of one, so wallets issue coins in a fixed ladder of sizes, which also makes them look alike and improves the anonymity of a transfer.

Where you see it

Statechains are a niche tool, and the honest advice is that most buyers will never touch one.

Where they earn their place is moving a fixed-size coin instantly with no fee and no on-chain footprint, and as a way to hand over things that are awkward to hand over: the design also allows transferring the funding output of a Lightning channel, so a channel itself can change owners. Mercury Layer wallets are the working implementation.

If you are buying bitcoin to hold, this is not the tool. Coins in a statechain depend on a server behaving well after it has stopped being useful to you, which is a strange thing to rely on for savings. Learn the term for a different reason: off chain is not one thing, and the shape of the trust varies wildly between a Lightning channel, a statechain and a custodial app.

Statechain vs payment channel

A payment channel splits one balance between two parties who can rewrite the split thousands of times; a statechain never splits anything and simply reassigns one whole coin to a new owner. Their safety rests on different foundations. A channel is protected by a revocation you can enforce on chain against your counterparty, which is a mechanism. A statechain is protected by a server's claim to have erased data, which is a promise. Channels let you pay any amount you like; a statechain lets you pay exactly one amount, the size of the coin.