Ouroboros Chronos: Permissionless Clock Synchronization via Proof-of-Stake

Clock synchronization allows parties to establish a common notion of global time by leveraging a weaker synchrony assumption, i.e., local clocks with approximately the same speed. The problem has long been a prominent goal for fault-tolerant distributed computing with a number of ingenious solutions in various settings. However, despite intensive investigation, the existing solutions do not apply to common blockchain protocols, which are designed to tolerate variable—and potentially adversarial—participation patterns, e.g., sleepiness and dynamic availability. Furthermore, because such blockchain protocols rely on freshly joining (or re-joining) parties to have a common notion of time, e.g., a global clock which allows knowledge of the current protocol round, it is not clear if or how they can operate without such a strong synchrony assumption. In this work, we show how to solve the global synchronization problem by leveraging proof of stake (PoS). Concretely, we design and analyze a PoS blockchain protocol in the above dynamic-participation setting, that does not require a global clock but merely assumes that parties have local clocks advancing at approximately the same speed. Central to our construction is a novel synchronization mechanism that can be thought as the blockchain-era analogue of classical synchronizers: It enables joining parties— even if upon joining their local time is off by an arbitrary amount—to quickly calibrate their local clocks so that they all show approximately the same time. As a direct implication of our blockchain construction—since the blockchain can be joined and observed by any interested party—we obtain a permissionless PoS implementation of a global clock that may be used by higher level protocols that need access to global time.