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CAPTCHA (Completely Automated Public Turing test to tell Computers and Humans Apart) is a popular mechanism used in Web sites to ensure humans only are interacting with the security functions of the Web site. CAPTCHA does this by producing images that include embedded numbers and letters that are not easily interpreted by automation tools.
The term CAPTCHA was coined in 2000 by Luis von Ahn, Manuel Blum, Nicholas J. Hopper (all of Carnegie Mellon University), and John Langford (then of IBM).

A common type of CAPTCHA requires that the user type the letters or numbers of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. This technology can reduce the risk of your authentication scheme or customer registration system from being subjected to an automated denial of service operation

By far the most common type of captcha involves the use of letters that are arranged randomly and are distorted in some way with various background colours. These are the ones that you will most likely have seen when signing up for a web mail account.
An example of the use of these can be seen when you are signing up for a Hotmail...
An example of a CAPTCHA used in the Yahoo Mail signup
or Yahoo! web mail account.

Another example is the contact form on our website http://www.interspire.com/contact/.

Other alternatives do exist. Audio CAPTCHA's for the visually impaired are probably the second most common type of CAPTCHA. There are also other CAPTCHA's that require you to solve a problem that should be easy for a person but very hard for a computer to solve such as choosing which item in a list is not a bird for example but the problem with this is that you need to have a large number of questions before it really becomes effective.

The most common requirement of a generating a visual CAPTCHA is GD, which is a graphics library that PHP uses for generating images. To check if GD is installed on your PHP web server, create a phpinfo page by creating a new file on your server with the following PHP code:
<?php phpinfo(); ?>
Next, browse to this PHP file in your web browser. There should be a section called GD. If there is no GD section then GD is not enabled on your server and you won't be able to use most CAPTCHA systems.

Example GD section from a phpinfo page
Also in this section you can see if FreeType support is enabled. If FreeType is enabled then you will be able to use TrueType fonts in your CAPTCHA's which will give you a much bigger range of fonts you can use to generate your CAPTCHA, thus reducing the effectiveness of scripts that attempt to automatically recognize letter and number shapes.

If you're targetting your site towards people with disablities, specifically visually impared users, then using a visual CAPTCHA is a bad idea since they will obviously have trouble entering the characters in the image.
Also, there are various ways to defeat CAPTCHA's, so they should only be considered one line in your defence against spam. Other tecniques can include blocking web site visiors by I.P. address or by the content in a form submission.


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