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A captcha (an acronym for "completely automated public Turing test to tell computers and humans apart") is a type of challenge-response test used in computing to determine whether or not the user is human. The term was coined in 2000 by Luis von Ahn, Manuel Blum, and Nicholas J. Hopper of Carnegie Mellon University, and John Langford of IBM. A common type of captcha requires that the user type the letters of a distorted and/or obscured word that appears on the screen.

Example captcha image

They are used to prevent bots from using various types of computing services. Applications include preventing bots from taking part in online polls, registering for free email accounts (which may then be used to send spam), and, more recently, preventing bot-generated spam by requiring that the (unrecognized) sender successfully pass a captcha test before the email message is delivered.

By definition, captchas have the following characteristics:

There has been some criticism that the use of visually-oriented captchas prevents access by visually-impaired users. However, as pointed out by the inventors of the concept, captchas do not necessarily have to be visual. Any hard artificial intelligence problem, including an aural one, can be used as the basis of a captcha. Some implementations of captchas have already begun permitting users to opt for an aural captcha if they are visually impaired. The development of aural captchas appears to have lagged behind that of visual captchas, however, and presently may not be as effective.

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