CAPTCHA (standing for Completely Automated Public Turing test to tell Computers and Humans Apart) must have seemed like a good idea when it was first invented in 2000. Spam was beginning to become a major problem on the web and a method was needed to fight back. CAPTCHA at first glance seems ideal: a distorted image that would be instantly recognisable by humans yet incomprehensible to machines. Place some letters in the distorted image and get the user to type them back and bingo: you’ve stopped your spam problem.

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