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Cloak: Transitioning States on Legacy Blockchains Using Secure and Publicly Verifiable Off-Chain Multi-Party Computation
range of industries, e.g., supply chain and digital assets, the confidentiality of smart contracts has become a fundamental requirement for practical applications. While many efforts have been made to develop architectural capabilities for enforcing confidential smart contracts, some promising works have arisen to extend confidential smart contracts to support Multi-Party Program (MPP). However, existing solutions lack public verifiability and require O(n) transactions to secure multi-party negotiation or resist Byzantine adversaries, thus suffering from inefficiency and compromised security. In this paper, we propose Cloak, a framework for enabling publicly verifiable off-chain MPP on existing blockchains. We identify the pitfalls of securing off-chain MPP evaluation by harmonizing TEE and blockchain, then propose corresponding solutions in Cloak. Cloak secures the off-chain nondeterministic negotiation process (a party joins an MPP without knowing identities or the total number of parties until the MPP proposal is settled), achieves public verifiability (to publicly verifiably evaluate an off-chain MPP that interacts with multiple parties off-chain and reads/writes states on-chain), and resists Byzantine adversaries. According to our proof, Cloak achieves better security with only 2 transactions, which is superior to previous works that achieve compromised security at the significant cost of O(n) transactions. By evaluating examples and real-world MPP contracts, the gas cost of Cloak reduces by 32.4% on average. To the best of our knowledge, Cloak enables the most secure arbitrary off-chain MPP evaluation on existing blockchains with the lowest cost. Therefore, we believe that Cloak paves the way for publicly verifiable and reusable off-chain MPP.