Lessons Learned from Implementing a Privacy-Preserving Smart Contract in Ethereum

Real-world smart contracts which preserve the privacy of both, their users and their data, have barely been proposed theoretically, let alone been implemented practically. In this paper, we are the first to implement a privacy-preserving protocol from the energy domain as a smart contract in Ethereum. We elaborate on and present our implementation as well as our practical findings, including more or less subtle traps and pitfalls. Despite major optimizations to our implementation, we find that while it is currently possible, it is not feasible to implement a privacy-preserving protocol of modest complexity in the Ethereum blockchain due to the high cost of operation and the lack of privacy by design.