CAPTCHA

A CAPTCHA (/kæp.tʃə/, an 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 or not the user is human. The term was coined in 2003 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford. 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 that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. 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 user identification procedure has received many criticisms, especially from people with disabilities, but also from other people who feel that their everyday work is slowed down by distorted words that are difficult to read. It takes the average person approximately 10 seconds to solve a typical CAPTCHA. Since the early days of the Internet, 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 get pass filters, they replaced a word with look-alike characters. HELLO could become |-|3|_|_() or )-(3££0, as well as numerous other variants, such that a filter could not possibly 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.com began to protect its signup page with a CAPTCHA and prepared to file a patent on this seemingly novel technique. In 2001, PayPal used such tests as part of a fraud prevention strategy in which they asked humans to 'retype distorted text that programs have difficulty recognizing.' PayPal cofounder and CTO Max Levchin helped commercialize this early use. A popular deployment of CAPTCHA technology, reCAPTCHA, was acquired by Google in 2009. In addition to preventing bot fraud for its users, Google used reCAPTCHA and CAPTCHA technology to digitize the archives of The New York Times and books from Google Books in 2011. Two teams have claimed to be the first to invent the CAPTCHAs used widely on the web today. The first team with Mark D. Lillibridge, Martín Abadi, Krishna Bharat, and Andrei Broder, used CAPTCHAs in 1997 at AltaVista to prevent bots from adding Uniform Resource Locator (URLs) to their web search engine. Looking for a way to make their images resistant to optical character recognition (OCR) attack, the team looked at the manual of their Brother scanner, which had recommendations for improving OCR's results (similar typefaces, plain backgrounds, etc.). The team created puzzles by attempting to simulate what the manual claimed would cause bad OCR. The second team to claim to be the first to invent CAPTCHAs with Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford, first described CAPTCHAs in a 2003 publication and subsequently received much coverage in the popular press. Their notion of CAPTCHA covers any program that can distinguish humans from computers.