Ethereumlottery.io is an Ethereum smart contract that uses Bitcoin blocks to decide the winner of a lottery.
The deterministic nature of blockchains makes it difficult to provide a secure source of randomness. Oracles can be one solution to this, but depending on the exact setup usually require trust in the integrity of the oracle. Other, more secure approaches tend to be so complex as to make it hard to transparently demonstrate how a specific result was reached. This lottery instead uses Bitcoin blocks as a source of randomness. While they are not beyond the reach of tampering, Bitcoin's longer block time and higher exchange rate ensure more safety than a single Ethereum block would. Furthermore, anyone can easily double-check the result using nothing but a block explorer and some simple math. The smart contract accesses this data via BTCRelay and uses it to decide on the outcome of the lottery.
