Introducing zkBridge Fast Finality for opBNB

opBNB is a high-performance layer-2 solution on BNB Chain, built using the OP Stack. Leveraging its block size of 100M, opBNB’s gas fees remain stable and cost-effective (can be as low as 0.005 USD per transaction), with only a 1-second layer-2 block time. Therefore, opBNB provides a great solution for widespread adoption across multiple digital environments such as games, social networks, the metaverse, and high-frequency trading.

While achieving high-performance throughput in opBNB, we should also consider enhancing its user experience in cross-chain and cross-rollup interoperability. However, due to the limitations of the current optimistic rollup technology, cross-chain transactions from an optimistic rollup are slow because they require waiting for a challenge period. This period can extend to as long as 7 days for layer-2 blockchains like opBNB, OP mainnet, and Arbitrum One.

While optimistic rollups such as opBNB provide a high transaction throughput, they represent a formidable challenge to zkBridge.

There are proposals to reduce cross-chain latency without compromising security. Some suggestions include converting fraud proof into ZK-based validity proof or amalgamating multiple proofs. However, these require substantial modifications to the underlying OP stack. In this blog, we introduce how we enable high-security, low-latency cross-chain user experience powered by zkBridge on opBNB.

zkBridge for opBNB: Classic design

zkBridge provides the most secure design for general message passing and asset bridging between various L1 and L2 networks. With zkBridge, any state transition that happens on a source chain is trustless verified on a destination chain without reliance on an intermediary protocol’s security. With zero-knowledge proofs, zkBridge offers strong security without relying on external assumptions and a significant reduction in on-chain verification costs.

If we hope zkBridge to maintain the same security assumptions as opBNB, then zkBridge must track finalized states from L2-to-L1 withdrawal transactions and L2-to-L1 messages in order to generate the state proofs for L2, and also verify the consensus for BNB Chain for the settlement chain (i.e., BNB Chain) for opBNB.

The above classic approach for building zkBridge from opBNB first triggers an L2-to-L1 withdrawal transaction or a L2-to-L1 messaging, and then waits for the fraud proof to finalize (could be as long as 7 days for opBNB). zkBridge needs to prove both the consensus of the BNB Chain and the finalized state for the L2-to-L1 transaction, so that the rollup block written on the settlement chain can be used. While this is secure enough, it creates a high latency for cross-chain applications.

This latency presents a significant challenge when aiming to bridge these L2 transactions to other blockchains. The reason is that the bridge transaction is obligated to wait for the L2 transactions’ finalization on L1.

zkBridge fast finality for opBNB with decentralized trust

The above classic design is secure for the cross-chain transactions from opBNB to other networks. However, the user experience of the applications has been significantly affected by the long challenge period.

To further improve the user experience for cross-chain interoperability on opBNB to achieve both fast finality and maximum security, we introduce zkBridge fast finality design for opBNB with decentralized trust. We have implemented the end-to-end system for zkBridge fast finality on opBNB (https://zkbridge.com/opbnb), offering a smooth cross-chain user experience with the high-security and low-latency zkBridge on opBNB.

To implement the zkBridge fast finality on opBNB, we introduce an auxiliary validation network with decentralized trust. This network incorporates hundreds of validators that attest to opBNB’s state, matching the security levels of opBNB’s fraud proof challenges. With decentralized trust, the validators rapidly simulate the fraud-proof process by replaying the transactions of the opBNB sequencer, achieving security nearly equivalent to the 7-day challenge period.

Like our original design of zkBridge, zkBridge is dedicated to validating the signatures efficiently from the validation network with hundreds of nodes, and then presenting zero-knowledge proofs to the receiver chain. Then on the receiver chain side, the zkBridge updater contract will verify the zero-knowledge proofs from the provers. There are two key components of building zkBridge fast finality for opBNB:

1. zkBridge verifies the signatures from the validator networks after an opBNB layer-2 block reaches the safe state.
2. zkBridge verifies the BNB chain consensus for the settlement layer-1 of opBNB.

To further reduce the proof verification cost, we can support the batch proof for multiple batches in opBNB, which helps save the gas cost on the receiver chain for zkBridge proof verification.

Through integrating the state proofs from the validation network with zkBridge, we can achieve fast finality for opBNB with decentralized trust. The efficiency is guaranteed by our advanced proof system, deVirgo. With deVirgo, zkBridge can generate the zero-knowledge proof for hundreds of signatures from the validation network within one second for each opBNB block, which is fast enough to catch up with the block time for opBNB. Furthermore, we can incorporate the crypto-economic security from staking and restaking mechanisms to achieve stronger security guarantees with both decentralized and economic trust.

User experience for zkBridge fast finality

zkBridge now offers seamless and production-ready interoperability between opBNB mainnet and BNB Chain mainnet at https://zkbridge.com/opbnb. Users can effortlessly transfer both NFTs and tokens between these two blockchain networks.

For transfers from BNB Chain to opBNB, we have implemented the zkBridge consensus proof design, ensuring secure and efficient transactions. Conversely, when moving from opBNB to BNB, users have the option to select between the Classic Mode and the Lightning Mode, depending on their preferences and requirements.

Classic Mode:

The classic mode follows the classic design, maintaining exactly the same level of security assumption of opBNB, where users need to wait for the 7-day challenge period for opBNB to conclude before receiving the assets.

Lightning Mode:

For users seeking a more streamlined experience and cost-effective transfers, zkBridge offers the Lightning Mode. Powered by zkBridge fast finality solution, this mode significantly reduces the bridging finality period, allowing users to complete asset transfers from opBNB to BNB in as little as 3 minutes with just one contract interaction.

Conclusion

zkBridge is a pivotal innovation in addressing the challenges of cross-chain interoperability, particularly within the context of opBNB. Through its meticulously designed architectures — both classic and the groundbreaking fast finality design — zkBridge offers unparalleled security and high efficiency. The synergy between opBNB and zkBridge not only enhances user experiences in cross-chain interactions but also sets the stage for a more integrated and secure digital blockchain future.