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[[File:captcha.jpg|upright=1.35|thumb|This CAPTCHA ([[reCAPTCHA v1]]) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.]]
A '''CAPTCHA''' ({{IPAc-en|ˈ|k|æ|p|.|tʃ|ə}} {{respell|KAP|chə}}) is a type of [[challenge–response authentication|challenge–response]] test used in [[computing]] to determine whether the user is human in order to deter bot attacks and spam.<ref>{{Cite web|title=The reCAPTCHA Project – Carnegie Mellon University CyLab|url=https://www.cylab.cmu.edu/partners/success-stories/recaptcha.html|url-status=dead|archive-url=https://web.archive.org/web/20171027203659/https://www.cylab.cmu.edu/partners/success-stories/recaptcha.html|archive-date=2017-10-27|access-date=2017-01-13|website=www.cylab.cmu.edu}}</ref>
The term was coined in 2003 by [[Luis von Ahn]], [[Manuel Blum]], Nicholas J. Hopper, and [[John Langford (computer scientist)|John Langford]].<ref name="abhl" /> It is a [[contrived acronym]] for "Completely Automated Public [[Turing test]] to tell Computers and Humans Apart."<ref>{{Cite web |title=What is CAPTCHA? |url=https://support.google.com/a/answer/1217728 |url-status=live |archive-url=https://web.archive.org/web/20200806173938/https://support.google.com/a/answer/1217728 |archive-date=6 August 2020 |access-date=2022-09-09 |website=Google Support |publisher=Google Inc. |quote=CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a [...]}}</ref> A historically common type of CAPTCHA (displayed as [[reCAPTCHA v1]]) was first invented in 1997 by two groups working in parallel. This form of CAPTCHA requires entering a sequence of letters or numbers in a distorted image. Because the test is administered by a computer, in contrast to the standard Turing test that is administered by a human, CAPTCHAs are sometimes described as [[reverse Turing test|reverse Turing tests]].<ref>{{Cite journal |last=Mayumi Takaya |last2=Yusuke Tsuruta |last3=Akihiro Yamamura |date=2013-09-30 |title=Reverse Turing Test using Touchscreens and CAPTCHA |url=http://isyou.info/jowua/papers/jowua-v4n3-3.pdf |journal=Journal of Wireless Mobile Networks, Ubiquitous Computing, and Dependable Applications |volume=4 |issue=3 |pages=41–57 |doi=10.22667/JOWUA.2013.09.31.041|archive-date=22 August 2017|archive-url=https://web.archive.org/web/20170822001858/http://isyou.info/jowua/papers/jowua-v4n3-3.pdf|url-status=live}}</ref>
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