Blockchain, what time is it? Trustless Datetime Synchronization for IoT

Time synchronization among IoT devices is a fundamental requirement for efficient and reliable communication on a global scale. Common synchronization schemes such as NTP operate on a trust-based client-server model, which does not scale well in a decentralized network because single server failures can lead to a severe downtime before re-establishing synchronization. Public blockchains such as Ethereum provide a trustless network and tamper-proof time-stamped data that is freely available. In this paper, we leverage the availability of time information in the block headers, which are very small (several hundreds of bytes) compared to the full blocks and can be validated without participation in the mining process. Our approach uses two estimators that are fed with the timestamps from block headers as well as the elapsed time between consecutive block receptions to estimate the true time to an accuracy of one second. We evaluate our approach by extensive validation on blockchain data from different geographical locations across the globe and show that global synchronization can be established despite the non-deterministic behavior of blockchains such as mining difficulty, network latencies and forks.