In probability theory, the birthay problem, or birthday paradox, pertains to the probability that in a set of randomly chosen people some pair of them will have the same birthday. In a group of 23 (or more) randomly chosen people, there is more than 50% probability that some pair of them will have the same birthday. For 57 or more people, the probability is more than 99%, tending toward 100% as the pool of people grows. The mathematics behind this problem leads to a well-known cryptographic attack called the birthday attack.