reCAPTCHA is a free CAPTCHA service provided by Google that helps to digitize books, newspapers and old time radio shows.
To archive human knowledge and to make information more accessible to the world, multiple projects are currently digitizing physical books that were written before the computer age. The book pages are being photographically scanned, and then transformed into text using “Optical Character Recognition” (OCR). One problem is that OCR is not perfect.
reCAPTCHA improves the process of digitizing books by sending words that cannot be read by computers to the Web in the form of CAPTCHAs for humans to decipher.
Each new word that cannot be read correctly by OCR is given to a user in conjunction with another word for which the answer is already known. The user is then asked to read both words. If they solve the one for which the answer is known, the system assumes their answer is correct for the new one. The system then gives the new image to a number of other people to determine, with higher confidence, whether the original answer was correct.
The technical backround of this technique has been published in a paper in the Science Magazine, Vol 321, 12 September 2008.
CAPTCHA is standing for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. Because other computers are supposedly unable to solve the CAPTCHA, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human.
A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.
The term “CAPTCHA” was launched in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University).
A very interesting contribution about CAPTCHA (in french) has been published by Nicolas Kerschenbaum, a security consultant of the french company Xmco Partners.