Trustless Bridge Technical Research
This post aims to conduct research on the development of a trustless bridge within the Polkadot ecosystem.
1. What is a trustless bridge?
A trustless bridge is a cross-chain bridge that uses cryptographic primitives—for example, zero-knowledge proofs—to validate information passed between blockchains instead of relying on economic incentives or honesty assumptions for security.
2. The architecture of trustless bridges
2.1 Light Clients
A light client synchronizing block headers of the source chain is implemented as a smart contract on the target chain. This ensures the target chain can verify information about the state of the source chain without relying on an external party.
Moreover, it allows anyone to prove the existence of certain transactions on the source chain in the context of the target chain (using Merkle proofs).
2.2 Relayers
The task of relaying block headers from the source chain to the target chain may be handled by a single operator, a federation, or a set of incentivized parties.
While the exact nature of a trustless bridge (permissioned or permissionless) doesn't affect its soundness, it can affect things like liveness and censorship resistance.
2.3 Validity proofs
Parties tasked with relaying the latest block headers are required to generate a zero-knowledge proof confirming the validity of relayed block headers to the light client contract. Note that this proof attests to the validity of the blockchain's state (represented in the block headers) according to the consensus protocol.
The validity proof generated for a new block header verifies that:
- The latest finalized block was signed by the majority of the validator set.
- The signatures on the latest block are valid and correspond to a publicly available set of public keys associated with the current validator set.
2.4 Bridge contracts
Trustless bridges will have contrarcts deployed on bridged blockchains to process deposits and withdrawals like traditional cross-chain bridges.
Trustless bridges only acknowledge withdrawals or deposits after receiving a header from the light client contract proving the existence of a deposit or withdraw transaction.
3 Trustless bridges in production
- Telepathy on Succinct Labs
- ZK-Bridge with the Research and Development Institute at the University of Berkeley
- ZK-IBC on Electron Labs
4. Trustless Bridges on Polkadot
4.1 A vital milestone: BEEFY
BEEFY stands for Bridge Efficiency Enabling Finality Yielder.
This protocol supports efficient bridging between Polkadot’s central Relay Chain and segregated “foreign” blockchains, such as Ethereum, which were not built with the Polkadot interchain operability in mind.
4.2 Hyperbridge
Hyperbridge (short for hyper-scalable bridge) is a technological advancement developed by Polytope Labs. Rather than being specifically built as a Polkadot-Ethereum bridge, it is crafted to enable secure and state-proof-based interoperability across all blockchains and all consensus protocols.
Conclusion
Blockchain Bridges are essential lifelines in the complex web of interconnected blockchain networks. Polkadot is committed to exploring these in a way that allows bridging to be implemented in a trustless and decentralized manner, while retaining Relay Chain-level security and scalability.