{{Cleanup rewrite|date=November 2022|it feels like an essay criticising CAPTCHA|section=no}}
[[File:captcha.jpg|upright=1.35|thumb|This CAPTCHA (Version 1{{clarifyme|date=June 2023}}) 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}}, 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 |website=Google Support |publisher=Google Inc. |access-date=2022-09-09 |quote=CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a [...] |archive-date=6 August 2020 |archive-url=https://web.archive.org/web/20200806173938/https://support.google.com/a/answer/1217728 |url-status=live }}</ref>) 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" /> TheIt mostis 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 Version 1.0{{clarifyme|date=June 2023}}) 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, aCAPTCHAs CAPTCHA isare sometimes described as a [[reverse Turing test|reverse Turing tests]].<ref>{{cite web|url=http://isyou.info/jowua/papers/jowua-v4n3-3.pdf|title=Reverse Turing Test using Touchscreens and CAPTCHA∗|author1=Mayumi Takaya|author2=Yusuke Tsuruta2|author3=Akihiro Yamamura1|publisher=Akita University|access-date=10 January 2019|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>
Two Manywidely websitesused useCAPTCHA itservices to prevent bot spamming and raiding. Most websites useare hCaptcha or reCAPTCHA.,<ref>{{Cite web |title=Websites using hCaptcha |url=https://trends.builtwith.com/websitelist/hCaptcha |access-date=2022-11-10 |website=trends.builtwith.com |archive-date=10 November 2022 |archive-url=https://web.archive.org/web/20221110020408/https://trends.builtwith.com/websitelist/hCaptcha |url-status=live }}</ref> an independent company,<ref>{{Cite web |title=hCaptcha - About Us |url=https://www.hcaptcha.com/about |access-date=2023-07-20 |website=www.hcaptcha.com |language=en}}</ref> and reCAPTCHA, offered by Google.<ref>{{Cite web |title=What is reCAPTCHA? - reCAPTCHA Help |url=https://support.google.com/recaptcha/answer/6080904?hl=en |access-date=2023-07-20 |website=support.google.com}}</ref><ref>{{Cite web |last=Sulgrove |first=Jonathan |date=2022-07-07 |title=reCAPTCHA: What It Is and Why You Should Use It on Your Website - TSTS |url=https://www.tsts.com/blog/recaptcha-what-it-is-and-why-you-should-use-it-on-your-website/ |access-date=2022-11-10 |website=Twin State Technical Services |language=en-US |archive-date=10 November 2022 |archive-url=https://web.archive.org/web/20221110020410/https://www.tsts.com/blog/recaptcha-what-it-is-and-why-you-should-use-it-on-your-website/ |url-status=live }}</ref> It takes the average person approximately 10 seconds to solve a typical CAPTCHA.<ref>{{cite book|last1=Bursztein|first1=Elie|last2=Bethard|first2=Steven|last3=Fabry|first3=Celine|last4=Mitchell|first4=John C.|last5=Jurafsky|first5=Dan|title=2010 IEEE Symposium on Security and Privacy |chapter=How Good Are Humans at Solving CAPTCHAs? A Large Scale Evaluation |access-date=March 30, 2018|chapter-url=https://web.stanford.edu/~jurafsky/burszstein_2010_captcha.pdf|year=2010|journal=Proceedings of the 2010 IEEE Symposium on Security and Privacy|pages=399–413|doi=10.1109/SP.2010.31|citeseerx=10.1.1.164.7848|isbn=978-1-4244-6894-2|s2cid=14204454|archive-date=8 August 2018|archive-url=https://web.archive.org/web/20180808033552/https://web.stanford.edu/~jurafsky/burszstein_2010_captcha.pdf|url-status=live}}</ref>
