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Concept
CAPTCHA
Summary
A CAPTCHA (ˈkæp.tʃə ) is a type of challenge–response test used in computing to determine whether the user is human in order to deter bot attacks and spam.
The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. It is a contrived acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart." A historically 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, CAPTCHAs are sometimes described as reverse Turing tests.
Two widely used CAPTCHA services are hCaptcha, an independent company, and reCAPTCHA, offered by Google. It takes the average person approximately 10 seconds to solve a typical CAPTCHA.
CAPTCHAs' purpose is to prevent spam on websites, such as promotion spam, registration spam, and data scraping, and bots are less likely to abuse websites with spamming if those websites use CAPTCHA. Many websites use CAPTCHA effectively to prevent bot raiding. CAPTCHAs are designed so that humans can complete them, while most robots cannot. Newer CAPTCHAs look at the user's behaviour on the internet, to prove that they are a human. A normal CAPTCHA test only appears if the user acts like a bot, such as when they request webpages, or click links too fast.
Since the 1980s–1990s, users have wanted to make text illegible to computers. The first such people were hackers, posting about sensitive topics to Internet forums they thought were being automatically monitored on keywords. To circumvent such filters, they replaced a word with look-alike characters. HELLO could become -3__() or )-(3££0, and others, such that a filter could not detect all of them. This later became known as leetspeak.
One of the earliest commercial uses of CAPTCHAs was in the Gausebeck–Levchin test. In 2000, idrive.
Official source
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