SmartFly: Fork-Free Super-Light Ethereum Classic Clients for the Internet of Things

The use of blockchains in the Internet of Things (IoT) is extremely promising, as it gives connected things the possibility to send and receive payments or tamper-proof data. In the last years, FlyClient has emerged in the literature as a technique for allowing resource-constrained devices to verify blockchain transactions. FlyClient is based on Merkle mountain ranges (MMRs) and probabilistic sampling, and it allows us to develop blockchain clients whose resource consumption is sublinear with the length of the chain. However, this comes at the cost of a change in the blockchain format, which leads to forks that are politically expensive, because they require 51% consensus. In this article, we explore the possibility of forkfree FlyClient verification methods that leverage smart contract programming. Smart contracts are able to add functionalities to a blockchain without needing forks. This raises several and novel technical issues that we address in the article. We show that fork-free sublinear clients are feasible without trusting the nodes that invoke the smart contract methods, as long as the smart contract language provides a means to access the most recent block or its hash. As a proof of concept we propose SmartFly, a fork-free FlyClient verification system for the ethereum classic (ETC) blockchain. We measure several performance metrics of SmartFly, proving that it is succinct in storage and bandwidth consumption and economically bearable (about 38 euros per day to maintain the whole system).