Bugtraq mailing list archives
Re: Defeating CAPTCHAs via Averaging
From: Alexander Klimov <alserkli () inbox ru>
Date: Tue, 30 Jan 2007 17:12:04 +0200 (IST)
On Sat, 27 Jan 2007 noreply9871234 () ich-habe-fertig com wrote:
The detailed article (including sample images) is online here: http://www.cip.physik.uni-muenchen.de/~wwieser/misc/captcha/To extract a series of captchas with the same information (number) in them, it is sufficient to repeatedly call their captcha generator.
I am not sure I understand how you propose to build an automatic system to attack it: If you can tell that two images contain the same number then it is very likely that you can recognize the numbers themselves (there are only 10 different digits). OTOH, if you have a human in the loop, they can just use gimp to create the averaged figure images from a single image per figure, and then use these templates to calculate correlation in different places of a given challenge.
Do not produce images with noise-like distortions. For example, moving and rotaing individual letters by a large enough distance/angle will spoil averaging by reducing the contrast in averaged images.
If the number of distinct tuples (position, angle) is not a very large number, the correlation attack will work anyway (and I doubt that it is possible to make the number large, because the attacker can try to start with a scaled down image and templates). -- Regards, ASK
Current thread:
- Defeating CAPTCHAs via Averaging noreply9871234 (Jan 29)
- Re: Defeating CAPTCHAs via Averaging Alexander Klimov (Jan 30)
- Re: Defeating CAPTCHAs via Averaging Fred Leeflang (Jan 31)
- Re: Defeating CAPTCHAs via Averaging Lou Katz (Jan 31)
- Re: Defeating CAPTCHAs via Averaging Fred Leeflang (Jan 31)
- Re: Defeating CAPTCHAs via Averaging Alexander Klimov (Jan 30)