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A captcha (Completely Automatic 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 is an acronym for "completely automated public Turing test to tell computers and humans apart" and 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:

  • They are completely automated. This avoids the necessity for human maintenance or intervention in the test, with obvious benefits in cost and reliability.
  • The algorithm used is made public. This is stipulated so as to require that breaking a captcha requires the solution of a hard problem in the field of artificial intelligence (AI) rather than just the discovery of the (secret) algorithm, which could be obtained through reverse engineering or other means.

Captchas are actually reverse Turing tests, since the goal is to cause the bot to fail the test, not pass it, as is the case with standard Turing tests.

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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