For those who don't know what a CAPTCHA is, it's a Completely Automated Public Turing test to tell Computers and Humans Apart.  It's those images that web sites sometimes display that contain distored characters or words that you are required to enter as text in order to verify that you are, in fact, human and not a computer program.

I've always liked the idea behind CAPTCHA, being very anti-SPAM myself.  But what constantly bugs me are the dozens of different kinds of CAPTCHAs I encounter that seem so easily defeatable.  For one thing, they make the text a totally different color from the background so that it could easily be isolated from it.  Second, they usually barely distort the text.  You know, I haven't the slightest clue how OCR (optical character recognition) works, but I bet I could write from scratch a program that could defeat 90% of the CAPTCHAs I've seen on the web.  But instead of egotistically trying to prove my programming prowess, I decided to design my idea of what an undefeatable CAPTCHA would be.  So I just quickly wrote up a rough draft.

Basically, I would think that a program to defeat CAPTCHAs would try to find patterns in the image that resemble characters, right?  My idea is that a CAPTCHA should not have any patterns or follow any rules that could be programmed against.  It should be completely random.  But how could even a human recognize the image if it were completely random?  Probability. 

The idea is that every pixel in the image is a random color; however, the random number generator is designed such that there is a slightly higher probability of the random color being darker where the text is.  So any color can appear anywhere in the image, but where the text is, it's a little more likely to be a dark color, even though it could even still be white there.  Humans can easily pick up on this, because our advanced brains are infinitely more intelligent than any software ever could be.  In fact, the entire makeup of our brain is about making connections even between seemingly unrelated things, whereas computers have to deal with descrete values.

Ok, so I guess this algorithm technically creates a pattern.  But it's the most random, least pattern-like pattern I could come up with.  Anyways, I bet you're just dying to see an image of what my algorithm generated...  Here it is:

It can easily be fine-tuned.  Plus this is a draft.  I would definitely add some distortion of the text before applying the color distortion.  In any case, it may seem easy to read or even defeatable, but look at it extremely close.  No two pixels next to each other are the same color, there are equally dark pixels in the background as the foreground and equally light pixels in the foreground as the background.

Please, I encourage you to open this image in your favorite graphics program and try to isolate the text from the static.  Try using software to defeat this CAPTCHA.  I'm curious to see how easily this can be defeated.

Comments?  Criticism?