The intents bridge: How Across uses optimistic verification to lower costs
Tldr; Cross-chain bridges need a way to verify transfers, and they can do so optimistically or through onchain messages. As Across is built using an intents-based architecture, it uses optimistic verification, which results in fewer onchain actions and lower gas costs.
Key takeaways:
- Cross-chain bridges can verify transfers optimistically or through onchain messages.
- As onchain verification involves sending and validating many messages, it consumes a lot of gas.
- Across only uses onchain actions when relayer repayments are disputed, improving the system’s gas efficiency.
The cross-chain ecosystem is home to many cross-chain bridge solutions. Bridges vary in scope, but at their core, they all serve a similar purpose: helping users move their assets from A to B. Regardless of the bridge a user opts for, they have to make a deposit at the origin chain, something has to happen in the middle, and then they can receive their funds at the destination chain.
The key differentiator between bridges lies in the step between the user making a deposit and collecting their funds. In our recent features on Across’ architecture, we detailed its intents-based approach, which uses offchain liquidity and repayment bundling for fast, cost-effective transfers between the origin and destination.
In most bridges, transfers must be verified before the funds can be collected at the destination. Bridges have different ways of verifying transfers. Many of them use gas-intensive onchain verification. But in Across’ system, relayers front orders and get repaid in bundles. Their repayments are verified optimistically, which saves on costs and improves efficiency. This approach speaks to the strength of optimistic systems and intents-based bridging. We go deep on the topic below.
What is optimistic verification?
Optimistic systems “optimistically” process blockchain data. They require only one honest actor to flag invalid data within a challenge window, using token-based incentives to settle rare dispute cases. They have proven effective: the Layer 2 rollups Arbitrum and Optimism use optimistic designs to scale Ethereum, and UMA’s optimistic oracle has gained adoption as the go-to solution for trustlessly recording any verifiable truth onchain.
Cross-chain bridges can optimistically verify transactions. Optimistic verification assumes transactions are valid until someone raises a dispute, and dispute cases are rare because users are incentivized to act honestly. Across adopts this model to verify relayer repayments, using UMA to settle any disputed transactions.
Across’ optimistic approach differs from other bridge designs. In many cases, bridges use gas-intensive onchain messages to verify deposits. As dispute cases are rare, Across’ optimistic verification mechanism only requires sending messages in outlier cases.
As dispute cases are rare, optimistic verification drastically reduces the number of onchain actions required to confirm transfers.
How relayers carry risk on behalf of users
Importantly, optimistic verification acts as a system’s security mechanism. In turn, this impacts the user experience. As optimistic designs have a challenge window for disputes, they add time-latency. So while optimistic bridging requires less cross-chain messaging, someone is burdened with time-latency until the transfer is settled.
But in Across’ case, relayers front their assets on behalf of users. This means users do not suffer from any latency. When a user wants to move their assets, they make a deposit and signal their intent, i.e. where they want to collect their assets. Relayers compete to fulfill users requests. The relayer chooses to lend out their assets at the destination and then waits to be repaid once UMA’s 120-minute challenge window passes. They take on this risk because they trust UMA’s system — and they can earn an interest rate for making a short-term loan.
With Across’ optimistic intents-based system, relayers take on risk when they lend out their assets. Users do not suffer from any latency as relayers front orders and rely on UMA’s system to ensure they’ll be repaid.
Minimizing gas expenditure
Across makes gas savings throughout its system. The contracts are simple, so they’re extremely gas efficient. When someone makes a deposit, the contracts record the action, and then the protocol matches the deposits to relayer fills. Repayments are then proposed and optimistically verified to relayers that made valid fills. As this process happens offchain, it doesn’t incur gas costs.
As optimistic verification only requires onchain transactions when repayments get disputed, it’s much more gas-efficient than other bridging methods. Rather than using onchain computation to verify every individual transaction, optimistic verification minimizes onchain actions to exceptional cases. Across bridging data shows that disputes are rare.
As we discussed in our previous feature in this series, Across makes gas savings on deposits, fills, and relayer repayments. One of the key features of its gas optimization is bundling. UMA verifies that the relayer filled orders, and then the relayer gets repaid for multiple fills in one transaction. The gas cost for repaying relayers is amortized across many transactions, so the actual cost for the user covering the destination gas fee is very low.
As Across uses optimistic verification and disputes are rare, it saves on gas costs. Bundling repayments to relayers further improves the system’s gas efficiency.
The drawbacks of onchain verification
The alternative to Across’ optimistic verification is to use onchain verification. With this approach, onchain messages verify individual bridge transactions in place of a dispute process.
Many of today’s most widely used bridges use onchain verification. Each of these designs differs in the way it handles the “something in the middle” part of the bridging process. But they all require individual messages to be verified and recorded onchain. This process can be very gas-intensive.
In short, as onchain verification requires individual messaging for transfers, it has much higher gas consumption than optimistic verification.
Intents and the optimistic future
While bridges take different approaches to serve users, they are all similar in that their main job is to help the user get their assets from A to B. In most cases, the bridge must verify the transaction after the user deposits at A and before they arrive at B.
Across takes a different approach. It stands out with its optimistic system, which reduces gas costs and enables relayers to take on risk for users. The key to this approach is Across’ intents-based architecture, where users simply express their intent and third parties front orders on their behalf. While the system uses onchain actions for verification, it only sends them when disputes occur.
Optimistic verification and intents present clear advantages over alternative bridging methods. Across is already proving that by offering the fastest, cheapest, and most gas-efficient way to move through the cross-chain ecosystem.
Across has steadily gained adoption throughout 2023 as its fundamentals have shone, so it’s not hard to imagine a future where intents systems continue to grow. In cross-chain bridging, intents-based systems abstract away complexity, minimize gas consumption, and lower costs, creating a better user experience. Across is leading the way in cross-chain interoperability because it embraces optimistic verification and intents. The multi-chain future is optimistic, and Across is set to shape it for a long time yet.
This piece was compiled with contributions and editing from Ryan Carman, Across Product Lead.
