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A Captcha - a completely automatic public Turing test to tell computers and humans apart - is a test that humans can pass but computer programs cannot; such tests are becoming key to defending e-commerce systems. By using a Captcha, for example, IT systems can permit only real people-rather than a spammer's script-to create a free e-mail account. This article explains the various types of Captchas and discusses their strengths and weaknesses as a security measure. It also lists sources for more information on the formal research into Captchas.