TrustVault : An Efficient Cross-Chain Protocol for Cryptocurrency-Backed Assets using Trusted Execution Environment

Abstract

Building a cross-chain asset exchange protocol is an important task to achieve interoperability between independent blockchains. Atomic cross-chain swap (ACCS), a notable technique for cross-chain asset exchange, has disadvantages of being slow, inefficient, and expensive because all participants must monitor all blockchains, and every cross-chain swap requires a long wait period and four on-chain transactions. Cryptocurrency-Backed Assets (CBAs) refers to assets issued in the same amount on one blockchain (an issuing blockchain) based on cryptocurrency locked on another blockchain (a backing blockchain). Swap through CBAs enables faster cross-chain asset exchange than ACCS. It can also be used for transaction offloading. However, since CBAs requires locking assets on a backing blockchain, CBAs should be working efficiently in blockchains that do not support smart contracts. A notable framework for CBAs, XCLAIM has the following limitations. First, Chain Relay Contract(i.e Blockchain Relays), which is used to verify the transaction event of the other blockchain, has to periodically submit the block header of the other blockchain, so the on-chain transaction cost is so high that it is not feasible to be used in the public blockchain. Second, in a blockchain that does not support smart contracts, the method of introducing collateral to enforce intermediary trust that can lock and refund cryptocurrencies instead has a problem that the amount of CBAs that can be issued is limited for safety. Also, there is a disadvantage that the number of on-chain transactions that occurred during the issue and redeem process of CBAs increased due to collateral. Finally, if the value of the locked cryptocurrency is higher than the collateral deposited by the intermediary, there is a risk that the intermediary may give up the collateral and take away the locked cryptocurrency. This thesis proposes TrustVault, an efficient cross-chain protocol for CBAs that introduces Trust Vault Server (TVS) using a trust execution environment (TEE). The protocol proposed has the following features. First, TVS safely per- forms the role of the Chain Relay Contract on the off-chain, thereby reducing the overall on-chain transaction cost of cross-chain protocol for CBAs. Second, it re- duces the transaction cost by reducing the size of the transaction inclusion proof. Third, it reduces the number of on-chain transactions of the protocol caused by collateral and removes the limit on the issuance amount of CBAs. Finally, it can also be applied to blockchains that do not support smart contracts that lock or redeem cryptocurrency according to the state of the issuing blockchain.