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How robots and spiders are causing issues, how to stop them. We can also talk about Completely Automated Public Turing Test To Tell Computers And Humans Apart - their use, their compliance issues, porn proxies, PWNtcha and other ways to defeat them. 
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Posted by: royfdvorak23
Date: September 21, 2008 11:27PM

I am a captcha newbie so I do not how sophisticated my idea for a captcha would be.

Using PHP:
Define a seed string of upper and lower case letters, plus numbers.
Generate a random string of 512 or 1024 or whatever length long.
Divide the length of the generated string by the number of characters in the captcha string - call this number 'Nindex'
Generate a random number between 0 and Nindex.
Use this new random number as an index into the random string.
From that index, pick out the number of characters required for the catcha string.
Now display these characters vertically for the form user to re-enter.

How easily could you experts break this captcha ?


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Re: captcha
Posted by: Matt Presson
Date: September 22, 2008 08:12AM

In breaking a CAPTCHA, the algorithm used to generate the CAPTCHA string is not what is generally attacked. Instead, a large quantity of images are downloaded from your site, say and then these images are run through OCR software. The OCR (optical character recognition) software runs various algorithms to determine if it can distinguish wha characters are present in the image. If a significant percentage of the characters are recognizable, then the CAPTCHA is considered broken.

In short, it is not necessarily the algorithm you use to generate the string, but how you display the characters to the user that can ultimately determine the strength of the CAPTCHA.


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Re: captcha
Posted by: winwalk
Date: September 24, 2013 04:03AM

CAPTCHA is considered broken.

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