captcha

Completely Automated Public Turing test to tell Computers and Humans Apart". – captcha

A 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 a computer asking a user to complete a simple test which the computer is able to grade. These tests are designed to be easy for a computer to generate, but difficult for a computer to solve, so that if a correct solution is received, it can be presumed to have been entered by a human. A common type of CAPTCHA requires the user to type letters or digits from a distorted image that appears on the screen, and such tests are commonly used to prevent unwanted internet bots from accessing websites.

The term "CAPTCHA" was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper, and John Langford (all of Carnegie). It is an 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.

A CAPTCHA is sometimes described as a reverse Turing test, because it is administered by a machine and targeted at a human, in contrast to the standard Turing test that is typically administered by a human and targeted at a machine.

Computer character recognition

A number of research projects have attempted (often successfully) to beat visual

CAPTCHAs by creating programs that contain the following functionality:

1.     Pre-processing: Removal of background clutter and noise.

2.     Segmentation: Splitting the image into regions which each contain a single character.

3.     Classification: Identifying the character in each region.

Steps 1 and 3 are easy tasks for computers.  The only step where humans still outperform computers is segmentation. If the background clutter consists of shapes similar to letter shapes, and the letters are connected by this clutter, the segmentation becomes nearly impossible with current software. Hence, an effective CAPTCHA should focus on the segmentation.

Several research projects have broken real world CAPTCHAs, including one of Yahoo's early CAPTCHAs called "EZ-Gimpy”, the CAPTCHAs used by popular sites such as PayPal, iveJournal, phpBB, the e-banking CAPTCHAs used by a lot of financial institutions, and CAPTCHAs used by other services. In January 2008 Network Security Research released their program for automated Yahoo! CAPTCHA recognition. Windows Live Hotmail and Gmail, the other two major free email providers, were cracked shortly after.

In February 2008 it was reported that spammers had achieved a success rate of 30% to 35%, using a bot, in responding to CAPTCHAs for Microsoft's Live Mail service and a success rate of 20% against Google's Gmail CAPTCHA. A Newcastle University research team has defeated the segmentation part of Microsoft's CAPTCHA with a 90% success rate, and reported that this could lead to a complete crack with a greater than 60% rate.

Applications

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.

Accessibility

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 still not usable for deaf blind people or for users of some text-based web browsers.

Due to the sound distortion present in audio CAPTCHAs and visual distortion present in visual CAPTCHAs, offering one as an alternative to the other does not help people with impairments in both areas. While deaf blind is a small group, having some degree of impairment in both areas is actually common, and very common amongst older people.

ReCAPTCHA :

The reCAPTCHA service is a user-dialogue system originally developed 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.

Implementation

The reCAPTCHA tests are displayed from the central site of the reCAPTCHA project, which supplies the words to be deciphered. 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), but the reCAPTCHA software itself is not open source.