Harder, Better, Faster, Stronger... Techniques for an image-based CAPTCHA?

Answer 1

Make each letter/number out of a pattern, I.E. unconnected dots. Meaning the computer has no way of knowing that a dot is part of a letter other than pattern recognition (which they don't have yet.) Then the usual distortions and random lines.

How you do this is the challenge.

EDIT: Also, bonus points for patterns of different shapes, and try alpha transparency on the characters (on the edges or the whole character), so they merge with the background.

Answer 2

I really hate CAPTCHA on sites, they just annoy me, but if you want to try and make a robust one try the following:

• Ability to get a new image without submitting
• Spoken version for the visually impaired
• Non-uniform characters

I've used Recaptcha on a few sites, it's a nice and robust solution.

Or if you want to be really funky about it check out this: http://research.microsoft.com/asirra/

Answer 3

Algorithms that try to break captcha are pattern matchers that work by a few different ways: scaling and skewing the symbols that they already know about, finding and tracing edges, and counting interior holes to help. If you can break the letter up into pieces, vary the letter quality, or add strong lines or “scratches” along the letters these techniques will help. However all of this is fairly moot considering we have recaptcha for this purpose and it’s a wonderful third party app for this. Additionally captcha will help the security of your site, but will not stop those who are truly enticed.

Answer 4

What if the color of the letters faded into other colors... for instance the 5 can start off as yellow on top and fade into blue or something. The colors chosen should be random.

With the multicolored background it might make it hard for the computer to pickup where the background ends and the character begins.. and hopefully it would not be too difficult for the human to actually pick up the pattern.

Answer 5

I like the idea of KittenAuth and Microsoft's Asirra project. The idea is that, while OCR will eventually evolve to break your traditional captcha, the ability to distinguish a kitten from a dog is many orders of magnitude more complex a problem, while absolutely trivial for humans.

This solution, while probably the sexiest captcha idea ever, has the limitation of not being easily portable to hearing-impaired methods.

Answer 6

Make letters difficult to separate. Use handwriting-like font or add lines that join letters. Decrease and randomize spacing between letters.

Add wave distortion in other axis too. Distortion in one axis only can be relatively easily analyzed and reversed.

Don't bother with color background at all. It's super-easy to automatically filter black from other colors. Your background hinders only humans.

Don't add scratches or other noise unless it has the same thickness as letters. Noise-removal algorithms can easily remove things that are thinner than letters.

Answer 7

What about shearing and shuffling bands to mangle display and mouse-only input?

Start by taking your sine-wave morphed text, divide into horizontal bands or maybe even a grid.

That makes optical recognition harder and might allow you to avoid the kind of nasty background games that make some captchas hard for humans.

For a site where you can rely on local drag in the browser, instead of typing in an entry use shuffling requiring the user to re-order pieces (just in sloppy order, not like one of those puzzles). Or, if you wanted to use clicks alone, the classic sliding tile puzzle.

Note, I've run into a captcha where you had to identify which of N cartoons had an animal in them which succeeded in blocking me!

Wellington Grey sums up the AI CAPTCHA race nicely.

Answer 8

You could add a random array of fonts so that GD renders each character using a different one.