Zero-Collateral Lotteries in Bitcoin and Ethereum

We present cryptocurrency-based lottery protocols that do not require any collateral from the players. Previous protocols for this task required a security deposit that is $O(N^2)$ times larger than the bet amount, where $N$ is the number of players. Our protocols are based on a tournament bracket construction, and require only $O(\log N)$ rounds. Our lottery protocols thus represent a significant improvement, both because they allow players with little money to participate, and because of the time value of money. The Ethereum-based implementation of our lottery is highly efficient. The Bitcoin implementation requires an $O(2^N)$ off-chain setup phase, which demonstrates that the expressive power of the scripting language can have important implications. We also describe a minimal modification to the Bitcoin protocol that would eliminate the exponential blowup.