Proposal for "Text_CAPTCHA"

» Metadata » Status
» Description
Implementation of CAPTCHA (completely automated public Turing test to tell computers and humans apart) images, a concept conceived by Luis von Ahn, Manuel Blum, Nicholas J. Hopper and John Langford.

This technique is used on many websites (including eBay, Yahoo! and others) in order to prevent automated form submissions.

You probably have seen this effect before: before submitting a form, you are presented an image with some characters on it and you have to enter these characters in a textbox before the form is submitted. The "Image" driver of Text_CAPTCHA creates these images.

The text on the CAPTCHA is created using Text_Password. Currently, the obfuscation algorithm in order the make the life of OCR algorithms harder is rather simple. I evaluated Image_Text in order to add the text on the image but will have to wait until the package supports rotation in order to be able to use it. I have also experimented with other effects like color gradients. The good thing is that the algorithm can change without breaking BC. :-)

(please note that currently .phps files are not syntax-highlighted on my server; I am working on that)
» Dependencies » Links
  • Text_Password
  • Image_Text
  • gd extension w/ ttf support
» Timeline » Changelog
  • First Draft: 2004-05-27
  • Proposal: 2004-05-28
  • Call for Votes: 2004-07-23
  • Christian Wenz
    [2004-06-11 08:59 UTC]

    * now uses Image_Text
    * new constructor includes
    - option to set the CAPTCHA phrase (as suggested by Philippe)
    - font options for Image_Text
    * some minor fixes
  • Christian Wenz
    [2004-06-11 14:52 UTC]

    some CS tweaks (thanks, Martin!)
  • Christian Wenz
    [2004-07-21 19:38 UTC]

    # changed to driver model
    # changed name to Text_CAPTCHA
    # when more drivers are added (we are working on that), the gd extension and Image_Text will be optional; till then, they are mandatory

    Since the structure has changed, please do comment if I did it right!
  • Christian Wenz
    [2004-07-22 06:20 UTC]

    how embarrassing ... now also the driver's source code is visible :-)