How non-custodial swaps move assets between chains

You hold bitcoin. You want ether on another chain. You do not want to hand your coins to a company, create an account, or trust a bridge contract that might be drained next month.

A non-custodial swap lets you exchange one asset for another across chains without any intermediary holding your funds at the same time. The mechanism is older than most bridge protocols and relies on cryptographic contracts rather than custodial promises.

What actually happens

The process has three phases: match, lock, and claim. None of them require you to deposit into a wallet controlled by the service.

Match. You tell the swap service what you have and what you want. The service finds a counterparty who wants the opposite trade - someone holding ether who wants your bitcoin. How does a swap service match you with someone moving the opposite direction? Usually by maintaining an order book or a liquidity pool of standing offers. The service does not take the other side of the trade itself; it connects two independent parties.

Lock. Both parties deposit their assets into separate hash time-locked contracts on their respective chains. A hash time-locked contract is a smart contract that holds coins until either a secret is revealed or a deadline passes. The same cryptographic hash is embedded in both contracts. Your bitcoin goes into a contract on Bitcoin’s chain. The counterparty’s ether goes into a contract on Ethereum’s chain. Neither contract can be spent by the swap service. No bridge contract holds your coins in a single pool. What does a swap look like when no bridge contract holds your coins? It looks like two independent escrows that share one secret.

Claim. The counterparty reveals the secret to claim your bitcoin. That secret is the preimage of the hash in both contracts. Once the counterparty publishes it on Bitcoin’s chain to take your coins, the secret becomes visible. You can now use that same secret to claim the ether from the contract on Ethereum’s chain. The atomic property - either both claims happen or neither does - is enforced by the timelocks.

What can go wrong

The swap timer is the critical variable. Each contract has a time window. Your contract on Bitcoin might give the counterparty two hours to claim. The counterparty’s contract on Ethereum might give you four hours. What happens if the swap timer expires before both sides confirm? The answer depends on who let it expire. If the counterparty fails to claim your bitcoin within their window, the bitcoin returns to your wallet automatically after the timelock expires. If you fail to claim the ether within your window, the ether returns to the counterparty. The service cannot recover either side’s funds; the contracts enforce the refunds programmatically.

But there is a catch. If the counterparty claims your bitcoin near the end of their window, then waits instead of revealing the secret promptly, your window to claim the ether shrinks. You might run out of time. This is called a griefing attack. It does not steal your funds - your bitcoin returns to you - but it wastes your time and leaves you without the ether you wanted. Reputable swap services set generous windows on both sides and monitor the chain for secret revelations, but you should check the contract parameters yourself before sending.

What the service needs from you

The form asks for your addresses, not your identity. What information does a swap site need from you to route between two chains? Your source address where the funds currently sit, your destination address where you want the new asset to arrive, and often a refund address on the destination chain.

Why do some non-custodial swaps ask for a refund address on the destination chain? Consider a swap where you send bitcoin and expect ether. If the counterparty fails to lock their ether, the swap cannot proceed. The service needs somewhere to return your bitcoin. But bitcoin refunds are straightforward - the contract returns to your sending address. The destination chain refund address matters when the counterparty locks their asset but you cannot claim it because of a network issue or a mistake in your destination address. The service cannot reverse a transaction on the destination chain. They need a fallback address on that chain to forward the locked funds to if the swap fails after the counterparty has committed.

How to tell who is really non-custodial

Not every service that claims to be non-custodial actually is. How can you tell if a swap provider is really non-custodial or just saying so? Check two things.

First, verify that you never send funds to an address the service controls. In a genuine non-custodial swap, your coins go directly into a contract whose code you can read on the block explorer. If the service asks you to deposit into a wallet they manage, even temporarily, they hold custody.

Second, check whether the service can abort the swap unilaterally. A real non-custodial swap gives the service no power to release funds early or cancel the trade. If the service can interrupt the process, they control the funds. The hash time-locked contract is the guarantee: neither party can move the coins except by revealing the secret or waiting for the timelock.

The difference from peer-to-peer order books

Some non-custodial swaps use a different model. How do peer-to-peer order books settle cross-chain trades? Instead of matching you instantly with a specific counterparty through the service, you post an offer to an open order book. Another user accepts it. The two of you then execute the hash time-locked contract directly, without the service holding any keys. The order book is just a bulletin board. The settlement is still atomic, but the matchmaking is decentralized.

The service described on this page uses a different approach. It matches you proactively and facilitates the contract creation, but never takes custody. You still control your funds until the moment you reveal the secret or the timelock expires.

The important distinction is that no matter which model is used, the core protection is the same: cryptographic contracts replace trust in a third party. You verify the hash. You check the timelock. You confirm the refund path. Then you send.