Difference between revisions of "CAPTCHA"

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CAPTCHA is most often used for registration of new users on web sites. A CAPTCHA is a type of challenge-response test used in computing to determine whether the user is human. "CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart", trademarked by Carnegie Mellon University. A CAPTCHA involves one computer (a server) which asks a user to complete a test. While the computer is able to generate and grade the test, it is not able to solve the test on its own. Because computers are unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. The term CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University), and John Langford (of IBM). A common type 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.

Currently, reCAPTCHA is recommended by the CAPTCHA creators as an official CAPTCHA implementation.

|thumb|290px|A modern CAPTCHA. Rather than attempting to create a distorted background and high levels of warping on the text, this CAPTCHA focuses on making segmentation difficult by adding an angled line

Origins

The first discussion of automated tests which distinguish humans from computers for the purpose of controlling access to web services appears in a 1996 manuscript of Moni Naor from the Weizmann Institute of Science, entitled "Verification of a human in the loop, or Identification via the Turing Test". Primitive CAPTCHAs seem to have been later developed in 1997 at AltaVista by Andrei Broder and his colleagues to prevent bots from adding URLs to their search engine. Looking for a way to make their images resistant to OCR (Optical Character Recognition) attack, the team looked at the manual to their scanner, which had recommendations for improving OCR results (similar typefaces, plain backgrounds, etc.). The team created puzzles by attempting to simulate what the manual claimed would cause bad OCR. In 2000, von Ahn and Blum developed and publicized the notion of a CAPTCHA, which included any program that can distinguish humans from computers. They invented multiple examples of CAPTCHAs, including the first CAPTCHAs to be widely used (at Yahoo!).

Applications

CAPTCHAs are used to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. Although CAPTCHAs are most often deployed as a response to encroachment by commercial interests, the notion that they exist to stop only spammers is mistaken. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail, Hotmail, and Yahoo. CAPTCHAs have also found active use in stopping automated posting to blogs or forums, whether as a result of commercial promotion, or harassment and vandalism. CAPTCHAs also serve an important function in rate limiting, as automated usage of a service might be desirable until such usage is done in excess, and to the detriment of human users. In such a case, a CAPTCHA can enforce automated usage policies as set by the administrator when certain usage metrics exceed a given threshold. http://recaptcha.net/

What is reCAPTCHA?

Some of the original inventors of the CAPTCHA system have implemented a means by which some of the effort and time spent by people who are responding to CAPTCHA challenges can be harnessed as a distributed work system. This works by including "solved" and "unrecognized" elements (images which were not successfully recognized via OCR) in each challenge. The respondent thus answers both elements and roughly half of his or her effort validates the challenge while the other half is captured as work.

This reCAPTCHA system is being used to aid in the conversion of printed works (scanned images) into digital text. The approach is similar to one of the techniques by which CAPTCHA systems can be circumvented (in that the respondents are performing human intelligence to accomplish small amounts of work in a highly distributed way).

The reCAPTCHA maintainers estimate that existing CAPTCHA systems represent approximately 150,000 hours of labor per day that could be transparently tapped into via their revised system. That's approximately 75 years of normal, full-time work accomplished every day.

Source: http://en.wikipedia.org/wiki/CAPTCHA