Randomness beacons are services that periodically emit a random number, allowing users to agree on the same random outcome without trusting anyone: ideally, the randomness beacon is secure (cannot be influenced) and transparent (can be monitored by users). Hence, such randomness beacons can serve as an important primitive for smart contracts in a variety of contexts. In this paper we aim to bridge the gap between theory and practice of public beacon design inspired by the unicorn protocol of Lenstra and Wesolowski using verifiable delay functions. We first present a structured security analysis, based on which we design, implement, and evaluate a trustworthy and efficient randomness beacon allowing users to join at any time. We then compare different implementation and deployment options on distributed ledgers, and report on a Ethereum smart contract-based lottery using our beacon.

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