Ethereum has been used as a platform for a variety of applications of financial interest. Several of these have a need for randomness — e.g., to implement a lottery, a competitive game, or crypto-collectibles. Unfortunately, writing a random number generator on a public blockchain is hard: computation needs to be deterministic, so that it can be replayed in a decentralized way, and all data that can serve as sources of randomness are also available to an attacker. Several exploits of bad randomness have been discussed exhaustively in the past. Next, we discuss near-trivial exploits of bad randomness, as well as ways to obtain true randomness in Ethereum.
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