Friday, July 17, 2009

Probability Theory, the Birthday Paradox, Web Security and Free Beer

On the off-chance that the above doesn't make it crystal clear for you, what I am trying to say is 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 both have been born on the same day. For 57 or more people, the probability is more than 99%, and it reaches 100% when the number of people reaches 366 (by the pigeon hole principle, ignoring leap years). The mathematics behind this problem leads to a well-known cryptographic attack called the birthday attack, often discussed as a potential weakness of the internet's domain-name service system and digital signatures.

Actually, I don't really care about all of that, but was trying to figure the odds of winning a bar bet and a free beer by exploiting the stats.

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