Building zkBridge: How B² Network Uses BitVM Challenge Mechanism to Implement a Two-way Pipeline?
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Building zkBridge: How B² Network Uses BitVM Challenge Mechanism to Implement a Two-way Pipeline?
Modular design plus the unique BitVM challenger mechanism gives B numerous possibilities for practical technical implementation.
Navigating Web3 tides with focused insightsAuthor: Haotian
Unlike some BTC cross-chain solutions backed by the credit of centralized custodial institutions, BSquare—built around core components such as the modular B² Hub and BitVM challenger model—appears to favor a more technical approach. B² Buzz has already accumulated over $600 million in BTC assets. So how does @BSquaredNetwork leverage zkBridge to achieve secure two-way asset bridging from BTC to Layer2? Let’s break it down technically:
Typically, the simplest way to securely transfer assets between two chains is through a centralized custodian that handles custody and accounting. However, if pursuing a purely technical solution, special designs must be made based on each chain's native characteristics:
Homogeneous Chain Bridges
"Homogeneous chains" refer to both sides supporting smart contracts—for example, cross-chain interactions between B² and other EVM-compatible chains. Both chains can use unified interoperability standards, with smart contracts governing bidirectional fund flows. There are two primary liquidity management models:
1) Smart contract-managed liquidity pools: Open and transparent liquidity pools can be established on both Chain A and Chain B, operating in mirrored mode. When a user deposits assets on Chain A, those assets are locked into the liquidity pool on Chain A, while the corresponding mirror assets are immediately released to the user from the pool on Chain B. Both pools maintain identical total balances, with a centralized server monitoring inflows and outflows across both chains and managing real-time rebalancing.
This approach requires liquidity pools on both chains for normal operation. If Chain A lacks sufficient liquidity, users on that chain will be unable to bridge. A key drawback is the need for robust mechanisms to prevent double-spending and malicious attacks.
For instance, if a user holds assets on both Chain A and Chain B and simultaneously initiates transactions on both chains, a failure in monitoring or state synchronization between the pools could allow both transactions to go through, effectively letting the user spend non-owned assets from the pool.
2) ZK-based smart contract state management: Deploy smart contracts on both Chain A and Chain B, and build a zkBridge to manage cross-chain communication. When a user sends an asset from Chain A to Chain B, the Chain A contract burns the asset and generates a ZK Proof, which is sent to the Chain B contract. After verifying the proof, the Chain B contract mints an equivalent asset on its side. This leverages ZK technology’s trust mechanism to enable secure cross-chain transfers.
@PolyhedraZK, which recently raised funds at a $1 billion valuation, uses this approach—relying on smart contracts and ZK tech to deliver a decentralized multi-zkBridge solution. However, this method only works between homogeneous EVM chains and cannot be directly applied to BTC, which lacks smart contract support.
Heterogeneous Bridges (BTC and EVM-Compatible Chains)
Heterogeneous chains typically include BTC paired with EVM-compatible chains, or EVM versus non-EVM chains (like SUI or Aptos—chains that support smart contracts but differ in standards and data structures). We won’t delve into the latter here. The main challenge lies in applying zkBridge to bridges connecting with the Bitcoin mainnet. B² faces exactly this technical hurdle when handling two-way asset bridging channels between Bitcoin and its network.
1) For asset inflow from Bitcoin to B², B² adopts the zkBridge solution provided by Polyhedra. Specifically, B² publishes a trusted multi-signature address secured via MPC+TSS to receive BTC deposits. Once zkOracle nodes detect and record the incoming transaction, they generate a ZK Proof containing details like amount and recipient address. This proof is relayed to an AA smart contract deployed on B². Upon successful verification, the contract mints the corresponding asset to the user’s B² address.
2) For asset withdrawal from B² back to Bitcoin, B² collaborates with Polyhedra to customize a specialized bridge. When a user initiates a withdrawal request on B², the transaction is first submitted as a proposal within the B² Hub, where other Validators vote to approve or reject it.
As I previously analyzed, Validators in the B² Hub serve as consensus managers for the Layer2 network. If most validators vote "Approve," the withdrawal is deemed valid. The voting result is then converted into a ZK Proof and sent to the MPC+TSS multisig address, which then releases the BTC directly to the user.
What if a user's withdrawal request is wrongfully denied by validators? Since B² Hub nodes are bound by the BitVM challenge mechanism, the user can initiate a challenge within the B² Hub to prove the legitimacy of their withdrawal. In this case, the validator responsible for the incorrect decision would be penalized. The “customization” essentially involves integrating B²’s unique BitVM challenger mechanism on top of zkBridge, ensuring secure state handling when transferring assets to non-smart-contract platforms like Bitcoin.
In summary,
It’s clear that modular design combined with B²’s unique BitVM challenger mechanism opens up numerous possibilities for technical implementation, offering valuable reference points for future BTC Layer2 developers tackling similar cross-chain asset challenges.
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Haotian | CryptoInsight
