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Yet another captcha implementation in PHP. This one is written with the current state of captcha-defeating research (as of 2007) in mind. Apart from a mathematical letter distortion algorithm that is more than just rotating the letters (just like on the big websites such as Google and Yahoo), the clutter is designed to make segmentation of the image into separate letter glyphs hard to do automatically. Finally, the images contain the name of the originating website to discourage re-using them on porn sites.

If you think this is paranoid, then check out the following references:

• Microsoft research papers by Kumar Chellapilla: Computer programs are better than humans at identifying extremely distorted letters.
• OCR Research team's gallery of weak captchas
• aiCaptcha, a step-by-step description of how to defeat a distorted-letter captcha.
• Breaking a visual CAPTCHA: The old captcha at Yahoo defeated.