This extension generates an image which contains an obfuscated text string that a user has to repeat in a form field in order to gain access to a website service. The technique assumes that humans can read an repeat the string while a spam-bot cannot, thus preventing guestbooks, tip-a-friend forms etc from being spammed by non-human clients.

The techniques are discussed here: http://en.wikipedia.org/wiki/Captcha

This captcha is meant as a resource other TYPO3 plugins can use. In doing so the overall site security level is improved when this captcha is improved in security. People interested in captcha security is more than welcome to continue development on this initial implementation!

This is how the captcha is used in “tipafriend” extension:

The user submitting the form must repeat the letters in the image in the field below it. If the string is right the form is submitted. If the string is wrong the form is shown again with an error and the user will have another chance to enter the string.