On Fri, Feb 28, 2014 at 12:07 PM, Mansi Gokhale <gokhalemans...@gmail.com>wrote:
> The image idea where users are asked to spot the odd one out like > demonstrated or find all the similar images like mentioned in > here<https://www.mediawiki.org/wiki/CAPTCHA> > If you display 8 images and the user has to pick one, then even by random guessing the attacker has a 12.5% chance of passing the captcha. That's not good at all. Finding "all matching" is slightly better since it reduces the guessability (1/256 for 8 images), but still not very good. A traditional captcha using only A-Z is 1/308915776. To do as well with image picking, you'd need to ask the user to choose the matches from a set of about 28. Adding in numbers 2-9 is 1/1544804416, needing a set of about 31 images. The set of possible images also needs to be very large and the categorization private. https://www.mediawiki.org/wiki/Talk:Requests_for_comment/CAPTCHA#Issue:_image_classification_CAPTCHAs_need_a_secret_corpusgoes into much more detail on this issue. Then there's the issue of different interpretation. Take for example https://www.mediawiki.org/wiki/File:Find-all-captcha-idea.png. Is the second image wearing glasses? Or is that a lorgnette or something like opera glasses, both of which are held in front of the eyes rather than worn? https://www.mediawiki.org/wiki/File:Find-the-different-captcha-idea.png has a similar problem. The first image is the only one with a cigarette, and the only one with non-realistic coloring. The second is the only bald one, and the only one with something resembling a lorgnette, and the only one not looking in the general direction of the camera, and the only one with a book. The fourth is the only child. The sixth is the only obvious female (I'm not sure about the cat). The eighth is the only one smiling, and the only one with visible teeth. Also a picture with a part chipped in could be shown and chipped pictures > could be given as options like find the missing part from a jigsaw puzzle. > > The image which would be shown is http://imgur.com/uefeb08 > > http://imgur.com/KEJqCg3 is the picture which would be the correct option. > > The other options could be rotated versions of this , which would not be > so easy for the bot to match. (unless it somehow worked some digital > processing algorithm and matched the color gradients or something like > that). > That seems very simple for a computer to solve. Just find the option with minimal difference along the join edges, which is probably easier than what they already do for OCRing text captchas. As far as captchas, I still think https://xkcd.com/810/ is the way to go. -- Brad Jorsch (Anomie) Software Engineer Wikimedia Foundation _______________________________________________ Wikitech-l mailing list Wikitech-l@lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l
