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Sketcha: A Captcha Based on Line Drawings of 3D Models

WWW 2010, April 2010

Steven A. Ross, J. Alex Halderman, Adam Finkelstein
Example captcha based on line drawings. The goal of the user is to rotate each image until it is upright, choosing among four orientations by clicking on the image. Each line drawing was automatically rendered from a 3D model using a randomized point of view, providing for many possible images from each model.

This paper introduces a captcha based on upright orientation of line drawings rendered from 3D models. The models are selected from a large database, and images are rendered from random viewpoints, affording many different drawings from a single 3D model. The captcha presents the user with a set of images, and the user must choose an upright orientation for each image. This task generally requires understanding of the semantic content of the image, which is believed to be difficult for automatic algorithms. We describe a process called covert filtering whereby the image database can be continually refreshed with drawings that are known to have a high success rate for humans, by inserting randomly into the captcha new images to be evaluated. Our analysis shows that covert filtering can ensure that captchas are likely to be solvable by humans while deterring attackers who wish to learn a portion of the database. We performed several user studies that evaluate how effectively people can solve the captcha. Comparing these results to an attack based on machine learning, we find that humans possess a substantial performance advantage over computers.

Steven A. Ross, J. Alex Halderman, and Adam Finkelstein.
"Sketcha: A Captcha Based on Line Drawings of 3D Models."
WWW 2010, April 2010.


   author = "Steven A. Ross and J. Alex Halderman and Adam Finkelstein",
   title = "Sketcha: A Captcha Based on Line Drawings of {3D} Models",
   booktitle = "WWW 2010",
   year = "2010",
   month = apr