Smart contracts are programs that permanently store and automatically execute on the blockchain system such as Ethereum. Due to the non-tamperable nature of the underlying blockchain, smart contracts are difficult to update once deployed, which requires redeploying the contracts and migrating the data. It means that the observation of smart contract evolution in the real world makes more sense. Hence, in this paper, we conducted the first large-scale empirical study to characterize the evolution of smart contracts in Ethereum. For evolution identification, we presented a contract similarity-based search algorithm, digEvolution, and evaluated its effectiveness with five different search strategies. Then we applied this algorithm to 80,152 on-chain contracts we collected from Ethereum, to dig out the evolution among these contracts. We then explored three research questions. We first studied whether the evolution of smart contracts is common (RQ1), then we studied how do the Gas consumption (RQ2) and the vulnerability (RQ3) of smart contracts vary during the evolution. Our research results show that the evolution of smart contracts is not very common. There are some contract components that have vulnerability but still be called by users. The Gas consumption of most smart contracts doesn’t vary during the evolution, contract is Gas-efficient before and after the evolution. The vulnerability of most smart contracts doesn’t vary during the evolution, both are secure before and after the evolution.

Smart contracts are programs that automatically execute on the blockchain system such as Ethereum. Everybody can write and deploy smart contracts on Ethereum, which causes a large collection of similar contracts via code reuse. In practice, code reuse in smart contract may amplify severe threats like security attacks, resource waste, etc. In this paper, we conduct an empirical study of code reuse in smart contracts for understanding the code reuse practice in the smart contract ecosystem. We first collect 146,452 open-source smart contract projects from Ethereum and then perform a detailed analysis. We first study how often the smart contract projects reuse and then we identify the top reused smart contracts and analyze how the developers revise smart contracts during reuse. Our research suggests that the code reuse in smart contract is quite frequent because about 26% contract code blocks are reused and the average time of reuse is 14.6. And the top reused contracts are almost all related to ERC20 token, which reveals that the current smart contract ecosystem is relatively homogenous. At last, we summarize 7 common types of code revision in smart contracts.