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SUMMARY:The robustness of CAPTCHAs - Jeff Yan\, Newcastle University
DTSTART:20081121T160000Z
DTEND:20081121T170000Z
UID:TALK15132@talks.cam.ac.uk
CONTACT:Andrew Lewis
DESCRIPTION:No matter whether you like or hate it\, CAPTCHA has found wide
 spread application on numerous commercial web sites - it is now almost a s
 tandard security mechanism for defending against undesirable or malicious 
 Internet bot programs. \n\nThis talk introduces our recent work on attacki
 ng numerous widely deployed CAPTCHAs. I will present new techniques of gen
 eral value to attack a number of text CAPTCHAs\, including the schemes des
 igned and deployed by Microsoft\, Yahoo and Google. In particular\, the Mi
 crosoft CAPTCHA has been deployed since 2002 at many of their online servi
 ces including Hotmail\, MSN and Windows Live. Designed to be segmentation-
 resistant\, this scheme has been studied and tuned by its designers over t
 he years. However\, our simple attack has achieved a segmentation success 
 rate of higher than 90% against this scheme. It took on average ~80 ms for
  the attack to completely segment a challenge on an ordinary desktop compu
 ter. As a result\, we estimate that this CAPTCHA could be instantly broken
  by a malicious bot with an overall (segmentation and then recognition) su
 ccess rate of more than 60%. On the contrary\, the design goal was that au
 tomated attacks should not achieve a success rate of\nhigher than 0.01%. F
 or the first time\, our work shows that CAPTCHAs that are carefully design
 ed to be segmentation-resistant are vulnerable to novel but simple attacks
 .\n\nOur experience suggests that CAPTCHA will go through the same process
  of evolutionary development as cryptography\, digital watermarking and th
 e like\, with an iterative process in which successful attacks lead to the
  development of more robust systems.        \n        \n
LOCATION:Computer Laboratory\, William Gates Building\, Room FW11
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