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Saturday, August 15, 2009

Math captcha dont work... why textual captcha are FAIL

there are tons of captcha solvers and sites that break images like here but i have been seeing a rise in math captchas so i wanted to real quick discuss something that i thought was kinda funny. its the idea that simple math problems are difficult for bots to solve. i found math captcha or a text based captcha that i thought would be really easy to solve so i decided i would break the captcha real quick.

basically an image captcha had a "textual riddle" version of the code in its alt tag.

one of the most difficult lines it returned was
(((((??? - 1) - 7) - 8) * 8) - 4) = -76
but i knew it had to be a number from 0 to 9 so i wrote a function to spit this out

computers are really good at math this takes no time to create and execute this simple vb
code:

so now this captcha whose images are actually kind of hard to segment and classify is broken because of 4 lines of vb code

im working on cracking my second image captcha this time the letters aren't fixed with and have a rotation. i plan to use a feed forward back propagation learning neural net so ill let you know how that goes and hopefully get to post again about another captcha ive cracked

you may also find my breaking of image captcha article interesting too...

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Math captcha dont work... why textual captcha are FAIL