Bridge
Cross-chain Bridge
Protocol enabling asset transfers between different blockchains
Definition
A bridge moves value between chains that cannot talk to each other natively. Three designs dominate: lock-and-mint, where the asset is locked on the source chain and a representation is minted on the destination; burn-and-mint, where a canonical issuer such as Circle destroys supply on one side and issues it on the other; and liquidity networks such as Across or Hop, where a relayer fronts you funds on the destination chain and is reimbursed later. Security varies enormously between them. A rollup's canonical bridge inherits Ethereum's security, at the cost of a seven-day challenge window on optimistic rollups; a third-party bridge is only as safe as its own validator set or multisig, which is why bridges have been the single largest source of losses in crypto.
A bridge lets an asset on one chain be used on another, usually by locking the original and issuing a claim on the other side. The claim is only as good as whoever holds the lock.
Example
Bridging 10 ETH from Ethereum to Arbitrum through the canonical bridge takes a few minutes inbound and roughly seven days outbound, because the withdrawal must clear the fraud-proof window. Through a liquidity network such as Across, the same withdrawal settles in minutes for a fee around 0.05%, because a relayer takes on the delay for you. The cost of that convenience is visible in the record: Ronin lost $624M in 2022 when five of nine validator keys were compromised, Wormhole $326M through a signature verification flaw, and Nomad $190M when a bad initialisation let anyone replay a valid proof.
How it works
Lock-and-mint locks on the source and mints a representation on the destination. Burn-and-mint has the issuer destroy and reissue. Liquidity networks have a relayer front you funds on arrival and settle later.
Why it matters
Bridges concentrate more value behind fewer keys than almost anything else in DeFi, and the historical loss record reflects it. Which bridge you use is a security decision, not a UX preference.
What to check
Identify the trust model: canonical rollup bridge, external validator set, or liquidity network. Check the multisig threshold and who holds the keys, the withdrawal delay, and whether the wrapped asset you receive is canonical or a bridge IOU.
Risks to Consider
- Validator-set or multisig compromise is the dominant failure mode, and it drains the entire locked balance at once, not a position
- A wrapped asset is only worth the bridge behind it; if the bridge is drained, the representation on the destination chain goes to zero while the original stays locked
- Liquidity-network bridges carry relayer and rebalancing risk, and can quote badly or stall when flows are one-directional
- Optimistic rollup withdrawals take about seven days through the canonical path, which is a real constraint during volatile markets
Common Questions
Canonical bridge or third-party bridge?
Canonical when you are not in a hurry and the amount matters: it has no extra trust assumption beyond the rollup itself. Third-party when you need speed, in which case you are paying a fee to take on someone else's security model for the duration. A reasonable rule is to size the amount you bridge through a third party to what you would be willing to lose to that bridge's multisig.
Why do bridges get hacked so often?
Because they hold a single pooled balance backing every wrapped unit on the other side, and because their security usually reduces to a small set of keys or a custom verification routine rather than the underlying chain's consensus. That makes them the highest value-per-line-of-code target in the industry. Incentives compound the problem: bridges compete on speed and chain coverage, which pushes toward simpler and weaker trust models.
Do I even need to bridge?
Often not. Withdrawing directly from a centralised exchange to the destination chain is frequently cheaper, faster and avoids the bridge risk entirely, since the exchange is already carrying that exposure. Bridging is worth it when you hold an asset on-chain already, when the exchange does not support the target network, or when you need an asset that only exists on one side.