MAP the Blockchain World: A Trustless and Scalable Blockchain Interoperability Protocol for Cross-chain Applications

TL;DR

This paper proposes MAP, a trustless blockchain interoperability protocol that relays cross-chain transactions across heterogeneous chains with high scalability, and develops a novel cross-chain relay architecture that integrates a unified relay chain and on-chain light clients of source chains, allowing the trustworthy retrieval and verification of heterogeneous cross-chain transactions.

Abstract

Blockchain interoperability protocols enable cross-chain asset transfers or data retrievals between isolated chains, which are considered as the core infrastructure for Web 3.0 applications such as decentralized finance protocols. However, existing protocols either face severe scalability issues due to high on-chain and off-chain costs, or suffer from trust concerns because of centralized designs. In this paper, we propose \texttt{MAP}, a trustless blockchain interoperability protocol that relays cross-chain transactions across heterogeneous chains with high scalability. First, within \texttt{MAP}, we develop a novel \textit{cross-chain relay} technique, which integrates a unified relay chain architecture and on-chain light clients of different source chains, allowing the retrieval and verification of diverse cross-chain transactions. Furthermore, we reduce cross-chain verification costs by incorporating an optimized zk-based light client scheme that adaptively decouples signature verification overheads from inefficient smart contract execution and offloads them to off-chain provers. For experiments, we conducted the first large-scale evaluation on existing interoperability protocols. With \texttt{MAP}, the required number of on-chain light clients is reduced from $O(N^2)$ to $O(N)$, with around 35\% reduction in on-chain costs and 25\% reduction for off-chain costs when verifying cross-chain transactions. To demonstrate the effectiveness, we deployed \texttt{MAP} in the real world. By 2024, we have supported over six popular public chains, 50 cross-chain applications and relayed over 200K cross-chain transactions worth over 640 million USD. Based on rich practical experiences, we constructed the first real-world cross-chain dataset to further advance blockchain interoperability research.

Figures (5)

• Figure 1: To connect three chains A, B, and C, LC-based protocols must deploy the LCs of chains B and C on chain A to allow it to verify transactions from those chains (and same for chains B and C), resulting in total 3*2=6 LCs needed ($O(N^2)$). Besides, it also poses heavy on-chain or off-chain costs when verifying transactions.
• Figure 2: Overview of MAP: We introduce a unified relay chain as a framework to facilitate cross-chain communications, which continually retrieves and verifies cross-chain transactions from source blockchains, including their blocks, transactions, and related proofs. This procedure is executed by the normal on-chain light client and our hybrid light clients based on zk-SNARKs, which are implemented by smart contracts with off-chain provers.
• Figure 3: Our hybrid light client overperforms conventional light clients by adoptive offloading. We move the on-chain verification workloads to off-chain provers through zk-SNARKs. Meanwhile, we keep the hash operations on-chain to minimize the circuits size and proof generation time.
• Figure 4: Cross-chain latency under different size of validators
• Figure 5: Historical statistics of MAP: The maximum value of any single cross-chain transaction is significantly smaller than the security boundary of the relay chain

Theorems & Definitions (2)

• Theorem 1
• Theorem 2