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A CAPTCHA (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 was first invented in 1997 by Mark D. Lillibridge, Martin Abadi, Krishna Bharat, and Andrei Z. Broder. 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 term is ambiguous because it could also mean a Turing test in which the participants are both attempting to prove they are the computer. This user identification procedure has received many criticisms, especially from disabled people, but also from other people who feel that their everyday work is slowed down by distorted words that are illegible even for users with no disabilities at all.〔(Disabled Australian starts petition to kill CAPTCHA ), an article by Tim Schiesser (August 5, 2013, 9:30 AM)〕 == Origin and inventorship == Since the early days of the Internet, users have wanted to make text illegible to computers. The first such people could be hackers, posting about sensitive topics to online forums they thought were being automatically monitored for keywords. To circumvent such filters, they would replace a word with look-alike characters. ''HELLO'' could become or , as well as numerous other variants, such that a filter could not possibly detect ''all'' of them. This later became known as leetspeak. Subsequent to that work, two teams of people have claimed to be the first to invent the CAPTCHAs used throughout the Web today. The first team consists of Mark D. Lillibridge, Martín Abadi, Krishna Bharat, and Andrei Broder, who used CAPTCHAs in 1997 at AltaVista to prevent bots from adding URLs to their search engine. Looking for a way to make their images resistant to 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 inventorship of CAPTCHAs consists of Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford, who 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, including many different examples of CAPTCHAs. The controversy of inventorship has been settled by the existence of a 1998 patent by Lillibridge, Abadi, Bharat, and Broder,〔U.S. Patent 6,195,698. Method for selectively restricting access to computer systems. Filed on Apr 13, 1998 and granted on Feb 27, 2001. Available at http://www.google.com/patents/US6195698〕 which predates other publications by several years. Though the patent does not use the term CAPTCHA, it describes the ideas in detail and precisely depicts the graphical CAPTCHAs used in the Web today. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「captcha」の詳細全文を読む スポンサード リンク
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