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A type of test used to determine whether a requestto a web site comes from a human or a computer program,typically by asking the user to perform some kind of imagerecognition task such as reading distorted text. The termwas coined in 2000 by Luis von Ahn, Manuel Blum, NicholasJ. Hopper (all of Carnegie Mellon University) and JohnLangford (of IBM) as a contrived acronym for "CompletelyAutomated Public Turing test to tell Computers and HumansApart". CAPTCHA aims to prevent software tools fromperforming actions which might degrade the service, such asregistering user accounts or automating the playing of a game.