Birthday paradox
In probability theory, the birthday paradox states that given a group of 23 (or more) randomly chosen people, the probability is more than 50% that some pair of them will have the same birthday. For 57 or more people, the probability is greater than 99%, although it cannot be exactly 100% unless there are at least 367 people. Calculating this probability (and related ones) is the birthday problem. The mathematics behind it has been used to devise a well-known cryptographic attack named the birthday attack.
