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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. By the pigeonhole principle, the probability reaches 100% when the number of people reaches 367 (including February 29 births). But perhaps counterintuitively, 99% probability is reached with just 57 people, and 50% probability with 23 people. These conclusions are based on the assumption that each day of the year (except February 29) is equally probable for a birthday.
The mathematics behind this problem led to a wellknown cryptographic attack called the birthday attack.
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