Tuesday, March 22, 2011

CAPTCHA: Telling Humans and Computers Apart Automatically

A CAPTCHA is a program that protects websites against bots by generating and grading tests that humans can pass but current computer programs cannot. This has the benefit of distinguishing humans from computers. It also creates incentive to further develop artificial intelligence of computers.

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.

CAPTCHAs are used in attempts to prevent automated software from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail, Hotmail, and Yahoo Mail.
CAPTCHAs are also used to minimize automated posting to Blogs, forums and wikis. whether as a result of commercial promotion or harassment and vandalism. CAPTCHAs also serve an important function in rate limiting. Automated usage of a service might be desirable until such usage is done to excess and to the detriment of human users. In such cases, administrators can use CAPTCHA to enforce automated usage policies based on given thresholds.
As of 2010, most CAPTCHAs display distorted text that is difficult to read by character recognition software. The alternative implementations may include various tests, such as identifying an object that does not belong in a particular set of objects, locating the center of a distorted image, or identifying distorted shapes.

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 (including Wikipedia) offering only the visual CAPTCHA, with or without providing the option of generating a new image if one is too difficult to read.

Use reCAPTCHA on Your Site
reCAPTCHA helps prevent automated abuse of your site (such as comment spam or bogus registrations) by using a CAPTCHA  to ensure that only humans perform certain actions.
  • It's Free! Yep, reCAPTCHA is free.
  • It's Useful. Why waste the effort of your users? reCAPTCHA helps to digitize books.
  • It's Accessible. reCAPTCHA has an audio test that allows blind people to freely navigate your site.
  • It's Secure. Most other CAPTCHA implementations can be easily broken.
  • It's Popular. Over 100,000 sites use reCAPTCHA, including household names like Facebook, Ticketmaster, and Craigslist.
  • It's Easy. reCAPTCHA is a Web service. As such, adopting it is as simple as adding a few lines of code on your site. For many applications and programming languages such as Wordpress and PHP we also have easy-to-install plugins available. We generate and check the distorted images, so you don't need to run costly image generation programs.

Captcha Fun: Guess Appropriate word from Pictures and write it in Answer Box.

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