Blockchain Interoperability

Blockchain interoperability is the ability of separate blockchains to communicate: to pass messages, prove what happened on another chain and move value between them. Each blockchain keeps its own ledger and rules and cannot see the others. Interoperability tools connect them, so that a token issued on one network can be used on another, or an action on one chain can trigger an action on a second.

The topic is the crypto counterpart of interoperability in payments, where banks and payment systems connect through shared standards and messaging networks such as SWIFT. It covers the tools described in cross-chain protocols, crypto bridges and cross-chain compatibility, and it is one of the most attacked areas of the crypto industry.

Think of different countries with different rail gauges. A train from one cannot run on the other's tracks, so goods are unloaded at the border and reloaded, or a special adapter is used. Blockchains are similar: an Ethereum token cannot travel to Solana on its own.

Interoperability tools are the border stations. Some lock the goods on one side and issue a receipt on the other. Some have officials on both sides who confirm the shipment. Some let traders swap directly at the border. Each method is only as trustworthy as whoever checks the paperwork, which is why these border crossings are where thieves most often strike.

Designs are usually compared on who has to be trusted to confirm that something really happened on the other chain: the chains' own validators, an outside group of signers, the token issuer, or nobody at all.

• Lock-and-mint bridges. Assets are locked in a contract on the source chain and wrapped tokens are issued on the destination chain. The wrapped token is only as good as the locked collateral.
• Burn-and-mint by the issuer. The issuer destroys tokens on one chain and creates them on another, so no wrapped copy is needed. Circle's Cross-Chain Transfer Protocol (CCTP), launched in 2023 and extended in CCTP V2, works this way for USDC.
• Light-client messaging. Each chain verifies the other's block headers directly. The Inter-Blockchain Communication protocol (IBC), which went live on the Cosmos (ATOM) network in 2021, is the main example.
• Shared security. Chains that share one validator set can message each other natively, as with parachains on Polkadot (DOT) or rollups in Optimism's Superchain.
• External verifier networks. A separate group of nodes, similar to a blockchain oracle, attests to events on one chain for another. Chainlink's Cross-Chain Interoperability Protocol (CCIP), from Chainlink (LINK), entered mainnet early access in July 2023.
• Atomic swaps. Two users exchange assets on two chains using hashed timelocks, so both legs complete or neither does, with no bridge custody. See atomic swaps.

Bridges hold large pools of locked assets, measured as bridged TVL, and rely on a small set of signers or complex code. That makes them prime targets for hacking. Three incidents in 2022 show the main failure types:

• Ronin Bridge (March 2022). Attackers gained control of enough validator keys to withdraw about $625 million. The FBI attributed the theft to North Korea's Lazarus Group.
• Wormhole (February 2022). A flaw in signature verification let an attacker mint about 120,000 wrapped ether on Solana without depositing collateral, worth roughly $320 million at the time.
• Nomad (August 2022). A faulty contract upgrade let anyone copy a valid withdrawal and replace the address with their own. About $190 million was drained by hundreds of addresses.

The lesson the industry drew is that the trust model matters more than speed: who verifies the message, how many keys are needed, and what happens if they are compromised.

Banks and market infrastructures face the same problem as tokenised assets and tokenised deposits spread across different ledgers. In 2023 SWIFT published results of experiments with Chainlink and several banks on sending instructions to multiple public and private chains through existing SWIFT connections. Older work on connecting ledgers includes the Interledger Protocol (ILP). Policy papers on interoperable money argue that a fragmented set of incompatible ledgers would undo the benefits of tokenisation.

• Different finality rules. A chain with probabilistic finality can reorganise after a bridge has already released funds on the other side.
• Fragmented liquidity. The same asset can exist as several wrapped versions that are not interchangeable.
• Lost composability. Cross-chain calls are asynchronous, so contracts cannot combine actions across chains as freely as within one chain. See composability.
• Different virtual machines. Connecting EVM and non-EVM chains requires translating formats, not just passing messages.

• Inter-Blockchain Communication Protocol (IBC)
• ethereum.org: Bridges