A CAPTCHA is a variation of the Turing test, in which a challenge is used to distinguish humans from computers (`bots') on the internet. They are commonly used to prevent the abuse of online services. CAPTCHAs discriminate using hard articial intelligence problems: the most common type requires a user to transcribe distorted characters displayed within a noisy image. Unfortunately, many users and them frustrating and break rates as high as 60% have been reported (for Microsoft's Hotmail). We present a new CAPTCHA in which users provide three words (`tags') that describe a video. A challenge is passed if a user's tag belongs to a set of automatically generated ground-truth tags. In an experiment, we were able to increase human pass rates for our video CAPTCHAs from 69.7% to 90.2% (184 participants over 20 videos). Under the same conditions, the pass rate for an attack submitting the three most frequent tags (estimated over 86,368 videos) remained nearly constant (5% over the 20 videos, roughly 12.9% over a separate sample of 5146 videos). Challenge videos were taken from YouTube.com. For each video, 90 tags were added from related videos to the ground-truth set; security was maintained by pruning all tags with a frequency 0.6%. Tag stemming and approximate matching were also used to increase human pass rates. Only 20.1% of participants preferred text-based CAPTCHAs, while 58.2% preferred our video-based alternative. Finally, we demonstrate how our technique for extending the ground truth tags allows for different usability/security trade-offs, and discuss how it can be applied to other types of CAPTCHAs.

Library of Congress Subject Headings

CAPTCHA (Challenge-response test)--Evaluation; Metadata--Evaluation; User interfaces (Computer systems)--Evaluation; Computer security--Research; Pattern recognition systems--Evaluation

Publication Date


Document Type


Department, Program, or Center

Computer Science (GCCIS)


Canosa, Roxanne

Advisor/Committee Member

Butler, Zack


Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works. Physical copy available through RIT's The Wallace Library at: Q341 .K58 2008


RIT – Main Campus