CAPTCHA

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CAPTCHA is an acronym for "completely automated public Turing test to tell computers and humans apart". Typically a graphic is generated that many humans can read but computers cannot. In web design, it is used to prevent bots from accessing website functionality such as creating an account or checking a web-based whois service.

There are several open source libraries available for web development which are primarily PHP. Yahoo.com and the CMU Captcha Project (http://www.captcha.net/) jointly developed the version used on Yahoo.com.

Software Development Canada (SDC) has a comparison of open source CAPTCHAs (http://www.softwaredevelopment.ca/captchas.shtml) on their site.

Several CATPCHAs have already been defeated. This is discussed here:

  • PWNtcha (http://sam.zoy.org/pwntcha/)
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Perl CAPTCHA

  • Authen::Captcha (http://search.cpan.org/~unrtst/Authen-Captcha-1.023/Captcha.pm)

According to the SDC comparison, the PHP CAPTCHAs use TrueType fonts and are more flexible than Authen::Captcha. A useful project may porting a PHP CAPTCHA to Perl.

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PHP CAPTCHAs

  • Captcha (http://hn273.users.phpclasses.org/browse/package/1163.html)
  • Captcha-TT (http://www.luluware.com/index.php?option=com_docman&task=view_category&Itemid=36&subcat=4&catid=5&limitstart=0&limit=5): one of the more comprehensive libraries.
  • Forms Validator (http://hn273.users.phpclasses.org/browse/package/1769.html)
  • HN Captcha (http://hn273.users.phpclasses.org/browse/package/1569.html)
  • OCR Captcha (http://hn273.users.phpclasses.org/browse/package/1538.html)
  • VeriWord (http://hn273.users.phpclasses.org/browse/package/1768.html): used by Wikipedia, WordPress, and Drupal
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