Cryptology ePrint Archive: Report 2019/838

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

Christian Badertscher and Peter Gaži and Aggelos Kiayias and Alexander Russell and Vassilis Zikas

Abstract: 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.

Category / Keywords: cryptographic protocols / Blockchain, Consensus, Clock Synchronization

Date: received 18 Jul 2019, last revised 1 Jun 2020

Contact author: christian badertscher at iohk io, peter gazi at iohk io, akiayias at inf ed ac uk, acr at cse uconn edu, vassilis zikas at ed ac uk

Available format(s): PDF | BibTeX Citation

Note: Revised overall structure of the paper and improved presentation of the results.

Version: 20200601:112621 (All versions of this report)

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