Birthday problem

In probability theory, the birthday problem, or birthday paradox^[1] 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 at least 23 randomly chosen people, there is more than 50% probability that some pair of them will have the same birthday. Such a result is counter-intuitive to many. For 57 or more people, the probability is more than 99%, and it reaches 100% when, ignoring leap-years, the number of people reaches 366 (by the pigeonhole principle). The mathematics behind this problem led to a well-known cryptographic attack called the birthday attack. http://en.wikipedia.org/wiki/Birthday_problem