In 2000, began to protect its signup page with a CAPTCHA and prepared to file a patent. One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. HELLO could become |-|3|_|_() or )-(3££0, and others, such that a filter could not detect all of them. To circumvent such filters, they replaced a word with look-alike characters. The first such people were hackers, posting about sensitive topics to Internet forums they thought were being automatically monitored on keywords. Since the 1980s-1990s, users have wanted to make text illegible to computers.

It takes the average person approximately 10 seconds to solve a typical CAPTCHA. Many websites use it to prevent bot spamming and raiding, it works well, and it is widely used. 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. This form of CAPTCHA requires entering a sequence of letters or numbers in a distorted image.

The most common type of CAPTCHA (displayed as Version 1.0) was first invented in 1997 by two groups working in parallel. The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. tʃ ə/ KAP-chə, a contrived 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 the user is human.

This CAPTCHA (Version 1) of 'smwm' obscures its message from computer interpretation by twisting the letters and adding a slight background color gradient.Ī CAPTCHA ( / ˈ k æ p.

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