A lot of newer OP Chains are considering fast bridges, so this new page (content below) will hopefully provide more guidance for this process while encouraging those teams to work with us.
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Evaluating 3rd party bridges for OP Chains
Bridge types overview
Fast bridges are typically implemented in 2 ways:
- Maintaining liquidity on both sides of the “fast bridge” (liquidity bridge) eg. Hop, etc
- Bypassing the security of the rollup completely (messaging bridge) eg. LayerZero, etc.
Liquidity bridges are typically used for vanilla ERC20s that are minted by the rollup standard bridge, hence require liquidity to bypass the 7-day withdrawal.
Messaging bridges, however, mint and burn with their own messaging mechanism. Today, that messaging mechanism is likely a validator set or multisig.
(Note: ETH on OP Stack rollups are currently only mintable by the standard bridge, which has a 7-day withdrawal and is secured by the fault proof. Fast ETH withdrawals are only achievable by liquidity bridges today.)
What to look out for
Bridge-security lock-in
Some messaging bridge token standards require a wrapping / full migration of these tokens to switch on and off of their security model. In Optimism’s roadmap, we plan to decentralize through ZK-proofs, which will one day provide fast(er) bridging secured by proofs instead of multisigs. We would advise to be cautious with bridge token standards that require full migrations to switch mint and burn models.
Added trust assumptions
Expanding from above, apart from bridge neutrality, adding a fast bridge today, also means adding its trust assumptions. For most messaging bridges, this means a validator set that signs off on cross-chain messages. Evaluate the security of each bridge by what parties can “sign off” on a message passed on the bridge, and how those parties are secured.
Cost
Though we don’t yet have a comprehensive view of how much each bridge costs, the cost of liquidity bridges is usually higher than messaging bridges, as maintaining liquidity isn’t free.
What is cooking at Optimism
Multi-proof architecture
We’re committed to building multiple proofs for the Superchain (1 fault proof and [2 ZK validity proof RFPs in progress](https://github.com/ethereum-optimism/ecosystem-contributions/issues/61)). This means that OP Chains will be secured by diverse proofs, and are less likely to fail because of implementation bugs. With the ZK validity proofs and ZK proving optimizations, one day OP Chain bridging can be fast and trustless.
Superchain messaging interface
In the meantime, we understand developers want fast bridging sooner. We’re working with the [Grants Council to kickstart cross chain research](https://twitter.com/OptimismGrants/status/1699504801644699772) today, aiming to start working with existing bridges toward a unifying interface, allowing an upgrade to our multi-proof architecture later, while unlocking fast bridging now.