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reCAPTCHA
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The reCAPTCHA logo
The reCAPTCHA service is a user-dialogue system originally developed by Luis von Ahn, Ben Maurer, Colin McMillen, David Abraham and Manuel Blum at Carnegie Mellon University's main Pittsburgh campus. It uses the CAPTCHA interface, of asking users to enter words seen in distorted text images onscreen, to help digitize the text of books, while protecting websites from bots attempting to access restricted areas.[1] On September 16, 2009, Google acquired reCAPTCHA.[2] reCAPTCHA is currently digitizing the archives of The New York Times and books from Google Books.[3] As of 2009, twenty years of The New York Times had been digitized and the project planned to have completed the remaining years by the end of 2010.[4]
The reCAPTCHA service 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, which sends the results to the digitization projects.
The system has been reported as displaying over 100 million CAPTCHAs every day,[3] and among its subscribers are such popular sites as Facebook, TicketMaster, Twitter, 4chan, CNN.com, and StumbleUpon[5] while Craigslist began using reCAPTCHA in June 2008.[6] The U.S. National Telecommunications and Information Administration also used reCAPTCHA for its digital TV converter box coupon program website as part of the US DTV transition.[7]
Origin
Distributed Proofreaders was the first project to volunteer its time to decipher scanned text that could not be read by OCR. It worked with Project Gutenberg to digitize the public domain works.
The reCAPTCHA program originated with Guatemalan computer scientist Luis von Ahn,[8] and was aided by a MacArthur Fellowship. An early CAPTCHA developer, he realized "he had unwittingly created a system that was frittering away, in ten-second increments, millions of hours of a most precious resource: human brain cycles".[9]
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Operation
An example of a reCAPTCHA challenge from 2007, containing the words "following finding". The waviness and horizontal stroke were added to increase the difficulty of breaking the CAPTCHA with a computer program.
Scanned text is subjected to analysis by two different optical character recognition programs. Their respective outputs are then aligned with each other by standard string-matching algorithms and compared both to each other and to an English dictionary. Any word that is deciphered differently by both OCR programs or that is not in the English dictionary is marked as "suspicious" and converted into a CAPTCHA. The suspicious word is displayed, out of context, along with a control word already known. The system assumes that if the human types the control word correctly, then the response to the questionable word is accepted as probably valid. If enough users were to correctly type the control word, but incorrectly type the 2nd word which OCR had failed to recognize, then the digital version of documents could end up containing the incorrect word. Thus, due to human error in distinguishing between the word "Internet" and the French name "Infernet", references to Captain Infernet have occasionally become Captain Internet.[10] The identification performed by each OCR program is given a value of 0.5 points, and each interpretation by a human is given a full point. Once a given identification hits 2.5 points, the word is considered valid. Those words that are consistently given a single identity by human judges are later recycled as control words.[11]
The original reCAPTCHA method was designed to show the questionable words separately,