How Crypto Bridges Work

About how crypto bridges work

To bridge a token, you send it to a bridge contract on the source chain, and a matching wrapped token arrives at your address on the destination chain. Blockchains run in isolated environments with distinct rules, so they cannot talk directly to each other or pass tokens back and forth natively. You pay the source chain's network fee to send, plus any fee the bridge adds. Using the token after it arrives costs the destination chain's fee. The bridge route introduces distinct security risks because the wrapped token is only as good as the locked funds behind it.

How it actually works

A lock-and-mint bridge holds your native deposit in a smart contract on the source network. It verifies that deposit and mints an equivalent wrapped token on the destination network. To return, a burn-and-release bridge reverses that path. It burns your wrapped tokens on the destination network. The bridge then releases the original locked tokens back to your address on the source network. For an explanation of the security flaws that target this logic, read our guide on how cross-chain bridge exploits work.

Consider a worked transfer for illustration. You hold 1 token A on chain X and want to use it on chain Y. You deposit that 1 token A into the bridge contract on chain X, which locks the asset in place. The bridge mechanism verifies the event and mints 1 wrapped token A to your address on chain Y. In this flow, you pay chain X's network fee for the initial deposit and any fee the bridge adds. You later pay chain Y's network fee when you interact with applications on chain Y. The wrapped token you receive on chain Y is only ever as good as the locked reserve backing it inside the contract on chain X. You can inspect network costs ahead of time by exploring what is gas and gwei or checking live network rates with our crypto fee tools.

Bridges differ sharply in how they secure those locked funds and verify incoming transfers. Trusted bridges depend on a central entity or system for their operations. You rely on the operator's reputation. You also face censorship risk if operators block transfers, and custodial risk if operators collude to steal funds. Trustless bridges rely instead on smart contracts and algorithms, meaning your security matches the underlying blockchain and you keep control of your assets throughout the process. Many bridges fall somewhere between these two extremes. You can inspect market share, bridge architecture types, destination networks, and specific risk vectors before moving funds by reviewing L2BEAT's bridges summary.

ethereum.org reports that 120k wETH ($325 million USD) was stolen in a single bridge hack, and that many of the top blockchain hacks involved bridges. Bridges face smart contract risk from code bugs, alongside technology risk from software failures, spam, or malicious attacks. Funds are also at risk if you make an error or the underlying blockchain is hacked. Holding wrapped bitcoin on another network is not the same as holding native BTC directly. You can see the differences in our explainer on what is wrapped bitcoin. A centralized exchange can also convert assets between different networks, though doing so usually requires more intermediate trade steps than a direct bridge.

ethereum.org names four uses: lower transaction fees, apps on other chains, exploring another chain's ecosystem, and holding a native asset. For example, ethereum.org notes that moving ETH from Ethereum Mainnet to an Ethereum Layer 2 rollup can reduce transaction fees. It also gives the example of lending a stablecoin through an app on another chain, where the rate may be higher than on Mainnet. Before a move onto a rollup, read what is Ethereum layer 2 and DeFi risks and regulation. Some bridges add their own fees on top of standard network gas fees. Always confirm the quote from the bridge before you execute the transfer.

Under Internal Revenue Service (IRS) Notice 2014-21, virtual currency is property for US federal tax purposes, so a disposal can create a gain or loss. The IRS has not clearly ruled on wrapping or bridging, so positions vary and audit risk exists. Keep records of every transfer and ask a tax professional before bridging a large amount.

Based on ethereum.org's bridges guide and L2BEAT's bridge risk data, bridging lets you access other apps or reduce fees on rollups. Every bridge adds smart contract risk, and a trusted bridge adds operator risk on top of the two chains. Pick the bridge with the fewest trust assumptions you can check on L2BEAT, and bridge only the amount you need. Keep fee money on the destination network and start with a small test transfer. Record every transaction and consult a tax professional for large amounts because federal tax rules remain unsettled.

Start here

1. 1Look up the bridge on L2BEAT to review its architecture type, destination networks, and operational risks.
2. 2Check that your source and destination networks, token contract addresses, and recipient wallet addresses match your planned route.
3. 3Ensure your destination wallet already holds enough native network coins to cover gas fees for post-bridge transactions.
4. 4Submit a small test transaction through the bridge interface before moving the remainder of your balance.
5. 5Read the quote the bridge shows and check the network fee on each chain before you confirm. Save every transaction receipt for your tax records.

Common questions

Sources

• ↗ethereum.org Bridges Guide
• ↗L2BEAT Bridges Summary and Risk Analysis
• ↗IRS Notice 2014-21 Virtual Currency Guidance

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