CAPTCHA  is a type of challenge-response test used in computing as an attempt to ensure that the response is generated by a person. The process usually involves one computer (a server) asking a user to complete a simple test which the computer is able to generate and grade. Because other computers are supposedly unable to solve the CAPTCHA, any user entering a correct solution is presumed to be human. Thus, it is sometimes described as a reverse Turing test, because it is administered by a machine and targeted to a human, in contrast to the standard Turing test that is typically administered by a human and targeted to a machine. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen.

 Early CAPTCHAs such as these, generated by the EZ-Gimpy program, were used on Yahoo!. However, technology was developed to read this type of CAPTCHA

The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie Mellon University). It is a contrived acronym based on the word "capture" and standing for "Completely Automated Public Turing test to tell Computers and Humans Apart". Carnegie Mellon University attempted to trademark the term, but the trademark application was abandoned on 21 April 2008.

Characteristics

A CAPTCHA is a means of automatically generating challenges which intends to:

• Provide a problem easy enough for all humans to solve.
• Prevent standard automated software from filling out a form, unless it is specially designed to circumvent specific CAPTCHA systems.

A check box in a form that reads "check this box please" is the simplest (and perhaps least effective) form of a CAPTCHA. CAPTCHAs do not have to rely on difficult problems in artificial intelligence, although they can.

This has the benefit of distinguishing humans from computers. It also creates incentive to further develop artificial intelligence of computers.

Applications

Because CAPTCHAs rely on visual perception, users unable to view a CAPTCHA due to a disability will be unable to perform the task protected by a CAPTCHA. Therefore, sites implementing CAPTCHAs may provide an audio version of the CAPTCHA in addition to the visual method. The official CAPTCHA site recommends providing an audio CAPTCHA for accessibility reasons, but it is not usable for deafblind people or for users of text web browsers. This combination is not universally adopted, with most websites offering only the visual CAPTCHA, with or without providing the option of generating a new image if one is too difficult to read.

Attempts at more accessible CAPTCHAs

Even audio and visual CAPTCHAs will require manual intervention for some users, such as those who have disabilities. There have been various attempts at creating more accessible CAPTCHAs, including the use of JavaScript, mathematical questions ("how much is 1+1") and common sense questions ("what colour is the sky on a clear day"). However, some types of CAPTCHAs do not meet the criteria for a successful CAPTCHA. They are not automatically generated and they do not present a new problem or test for each attack.

Also See :

reCAPTCHA

reCAPTCHA is a system originally developed at Carnegie Mellon University that uses CAPTCHA to help digitize the text of books while protecting websites from bots attempting to access restricted areas. On September 16, 2009, Google acquired reCAPTCHA. reCAPTCHA is currently digitizing the archives of The New York Times and books from Google Books. Twenty years of The New York Times have been digitized and the project planned to have completed the remaining years by the end of 2010.

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, which sends the results to the digitization projects.

The system is reported to display over 200 million CAPTCHAs every day, and among its subscribers are such popular sites as Facebook, TicketMaster,Twitter, 4chan, CNN.com, and StumbleUpon. Craigslist began using reCAPTCHA in June 2008. 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.