Accurate and decentralized timestamping using smart contracts on the Ethereum blockchain

Timestamps allow us to identify a date and time when a piece of data existed or an event took place. For example, we use timestamps to establish the date when we grant a patent. Services that offer trusted timestamps on the blockchain exist, where one creates a timestamp on a value by sending the blockchain a transaction containing the value, which is eventually confirmed in a block a miner creates and timestamps. Our work focuses on the accuracy of such timestamps created on the Ethereum blockchain. We estimate that their mean accuracy is half of the average time between consecutive blocks. However, Ethereum clients geth and parity allow accuracy to vary in up to a few tens of seconds. Surprisingly, we found out empirically that miners can backdate data using the timestamp on a block, which is a serious flaw in timestamping services. To address this problem, we propose a new decentralized timestamping service that combines smart contracts and distinct time providers. The service offers timestamps with higher accuracy of milliseconds. We conduct a cost analysis of our service, which shows that Ethereum transaction fees drive our timestamps costs when the service scales up. Finally, we discuss alternatives to cut costs. •In theory, the mean accuracy of Ethereum timestamps can vary in up to tens of seconds.•In practice, Ethereum block timestamps can allow to backdate transactions.•A smart contract and distinct witnesses together can issue decentralized timestamps.•Decentralized timestamps can provide proof of existence with accuracy of milliseconds.•Decentralized timestamps can be more reliable than RFC3161 timestamps.