Introducing the Bitcoin Messaging Protocol with zkBridge

News

Polyhedra Network is excited to present the Bitcoin messaging protocol with zkBridge that aims to significantly improve the interoperability of Bitcoin. This initiative is designed to make the Bitcoin network interactable with other layer-1 and layer-2 networks.

Bitcoin as sender chain: using zkBridge to trustlessly access any data on Bitcoin

To facilitate interactions between Bitcoin and other networks, it’s essential to implement functionality in two key directions: Bitcoin as both a sender chain and a receiver chain. The first direction, Bitcoin as a sender chain, is fully compatible with our current zkBridge framework. zkBridge enables the updater contract (i.e., the light client contract) on the receiver chain to directly verify the consensus of Bitcoin, as well as each transaction on Bitcoin through verifying the Merkle proof. This compatibility ensures that both the consensus proof and the transaction Merkle proof on Bitcoin can be secured comprehensively by zkBridge.

Our Bitcoin zkBridge significantly enhances the interoperability of the Bitcoin ecosystem. Our system allows the transmission of all the current and historical data on Bitcoin to be accessible by any layer-1 and layer-2 networks (e.g., Ethereum, BNB Chain, and Arbitrum) and any applications. Our solution is further compatible with the latest innovations on Bitcoin, such as Ordinals, BRC-20, and Atomicals. This enables smart contracts on other blockchain networks to trustlessly access Bitcoin data and protocols, and make all kinds of computing logics, and the security is guaranteed by zero-knowledge proofs.

Bitcoin as receiver chain: Write remote chain data to Bitcoin with decentralized and economic trust

This innovative approach is designed to optimize the interoperability for the Bitcoin network, thereby enhancing overall blockchain security. However, when we want to send messages and cross-chain transfer assets from another blockchain to Bitcoin, we encounter a significant challenge: Bitcoin lacks native smart contract functionality. Consequently, Bitcoin cannot serve as a standard zkBridge receiver chain. For the sake of simplicity in this blog post, we will use Ethereum as the representative sender chain with Bitcoin as the receiver chain.

To ensure the security of these cross-chain transactions, we have adopted a Proof of Stake (PoS)-style mechanism. This involves inviting validators to stake native Ethereum tokens. If the sender chain is BNB Chain, then the validators need to stake BNB token. These stake owners are then authorized to write data on the Bitcoin network. To reach a consensus on the correct message to be sent, these validators will employ a Multi-Party Computation (MPC) protocol. It’s important to note that the use of the MPC protocol aims to enhance the robustness and reliability of our protocol.

At the core of our proposal is a distinctive feature: the security of our protocol is not dependent on the accuracy of the MPC protocol. In scenarios where a malicious entity gains control over a majority of the MPC protocol members and tampered with the correct message, users have a safeguard. They can initiate a zkBridge request to send this malicious message from Bitcoin to Ethereum. Upon receiving the message, the slashing contract on Ethereum assesses its validity and, if necessary, executes a slash on the MPC committee. A portion of the slashed stakes is then awarded to the user as compensation for any loss. By introducing decentralized and economic trust through staking/restaking mechanisms, our solution ensures that user assets remain secure despite potential malfeasance within the MPC protocol.

Our detailed scheme unfolds as follows:

  1. Stakers initially deposit tokens on Ethereum as a form of collateral.
  2. In the event of malicious actions by these stakers, any honest staker or user can report the fraud action on Ethereum. Then we utilize the bridge to pass related transactions on Bitcoin to Ethereum. The offending information is instantaneously transmitted to Ethereum through the established relay network. In particular, the structure of a unidirectional bridge with Bitcoin as the source chain is straightforward. We have a decentralized blockheader relay network. Any node in the network can pass the current blockheader of the Bitcoin network to Ethereum with zero-knowledge proofs. Then Ethereum can validate any bridge transaction appearing on the Bitcoin network with these blockheaders.
  3. Concurrently, a dedicated smart contract on Ethereum stands ready to autonomously verify the validity of the messages transferred from the Bitcoin network. Upon confirmation, the contract executes the withdrawal of the dishonest staker’s deposit.
  4. This ensures that as long as at least one committee member or one user remains honest on the Bitcoin network, our staking mechanism maintains its integrity as secure, trustless, and decentralized.

In essence, the staking/restaking mechanism not only augments the utility of bitcoins but also reinforces the trust and security across PoS blockchains, marking a significant advancement in the realm of digital assets and decentralized finance.

Summary

In conclusion, Polyhedra Network’s introduction of the Bitcoin interoperability protocol marks a significant leap in blockchain technology. By enabling Bitcoin to function both as a sender and receiver with zkBridge, this initiative paves the way for unprecedented interaction between Bitcoin and various blockchain networks. For Bitcoin as the sender chain, zkBridge allows the transmission of all the current and historical data on Bitcoin to be accessible by any layer-1 and layer-2 networks. For Bitcoin as the receiver chain, our system not only ensures robust security through a staking and slashing mechanism but also enhances the utility and reach of Bitcoin. This innovation not only taps into the latent potential of idle bitcoins but also fortifies the trust and security across proof-of-stake blockchains, heralding a new era in digital assets and decentralized finance.