An On-Chain Smart Contract Protocol for Tangle

Chain-structured blockchains (e.g., Bitcoin and Ethereum) are often criticized for resource waste, low scalability, and high transaction fees. Tangle has been proposed to overcome these drawbacks by adopting a directed acyclic graph structure, new consensus mechanisms, etc. Particularly, Tangle defines a transaction-processing rule, which requires that new incoming transactions should approve several existing transactions before being attached to Tangle, to exclude miners and transaction fees. However, this rule makes it difficult to support smart contract (SC) in Tangle, an essential component of numerous decentralized applications, because the execution and verification of SC usually require transaction fees as incentive awards. In this work, we propose an 'on-Tangle' SC protocol called equivalent-exchange-based smart contract (EESC), which runs on the Tangle core, to address this challenge. EESC extends the transaction-processing rule to SC and hence maintains Tangle's advantages of no fees and no mining. In EESC, a user should verify other users' SCs before submitting its SC. The workload of verifying these existing SCs is greater than that of verifying the newly submitted one. Extensive simulations verify that EESC is fast and efficient and can well achieve our goal.