reCAPTCHA is a system developed at Carnegie Mellon University which utilizes CAPTCHA to assist in the process of digitizing the text of books, while protecting websites from bots attempting to access restricted areas.

reCAPTCHA supplies subscribing websites with images of words that optical character recognition (OCR) software has been unable to read. The subscribing websites (whose purposes are generally unrelated to the book digitization project) present these images for humans to decipher as CAPTCHA words, as part of their normal validation procedures. They then return the results to the reCAPTCHA service, thereby contributing to the digitization project. The result is that the university receives approximately 3,000 man hours per day of free labor to help in the preservation of books.

reCAPTCHA has the same goal as Distributed Proofreaders, although DP uses conventional proofreaders. The system is reported to deliver 30 million images every day (as of December 2007), [ [ Facebook takes the Captcha rap] , "The Register", 13th December 2007] and counts such popular sites as Facebook, TicketMaster, Twitter and StumbleUpon amongst subscribers. [ Spam weapon helps preserve books] — BBC news report by Paul Rubens, 2007-10-02.] Craigslist began using reCAPTCHA in June 2008. [ "Fight Spam, Digitize Books"] — Craigslist Blog, 2008-06.] The U.S. National Telecommunications and Information Administration also uses reCAPTCHA for its digital TV converter box coupon program website as part of the US DTV transition. [ [ TV Converter Box Program] ]


Most implementations use reCAPTCHA in order to validate a website registration. For example, before allowing a visitor to post on a website forum, the website can require the visitor to complete a registration. Usually this registration will require the visitor to have a valid email and, in the case of reCAPTCHA, solve the CAPTCHA image.

In order to verify that humans can decipher these previously unrecognisable words correctly, two words are displayed; one is a word which OCR software has been unable to read, and the other is a word which several other human users have already been able to identify. If the user recognises the identified word, this gives confidence that they were also correct about the new word. The same unknown words are sent to two different people; if they agree then the decipherment is assumed correct, otherwise it is sent to more people until agreement is reached. [cite web |url= |title=May 24: Carnegie Mellon Project Boosts Book Digitization Efforts - Carnegie Mellon University |accessdate=2007-06-23 |format= |work=]


reCAPTCHA tests are taken from the central site of the reCAPTCHA project [ [ The reCAPTCHA project] - part of the Carnegie Mellon School of Computer Science at Carnegie Mellon University.] as they are supplying the undecipherable words. This is done through a Javascript API with the server making a callback to reCAPTCHA after the request has been submitted. The reCAPTCHA project provides libraries for various programming languages and applications to make this process easier. reCAPTCHA is a free service (that is, the CAPTCHA images are provided to websites free of charge, in return for assistance with the decipherment). [ [ Recaptcha Faq ] ]


reCAPTCHA has also created project Mailhide [ [ reCAPTCHA Mailhide: Free Spam Protection ] ] which protects email addresses from being harvested by spambot. The email address is converted into a format that does not allow a crawler to see the full email address. For example, the email "" would be converted to "" The visitor would then click on the "..." and solve the CAPTCHA in order to obtain the full email address.


External links

* [ The reCAPTCHA project]

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