A CAPTCHA (/ˈkæp.ə/ KAP-chə, a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart"[1]) is a type of challenge–response test used in computing to determine whether the user is human.[2]

This CAPTCHA (Version 1) of "smwm" obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.

The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford.[3] The most common type of CAPTCHA (displayed as Version 1.0) 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, a CAPTCHA is sometimes described as a reverse Turing test.[4]This test has received many criticisms, from people with disabilities, but also many websites use it to prevent bot spamming and raiding, and it works effectively, and its usage is widespread. Most websites use hCaptcha or reCAPTCHA.[5][6] It takes the average person approximately 10 seconds to solve a typical CAPTCHA.[7]

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