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Eighth International Network Conference (INC 2010)

Eighth International Network Conference (INC 2010)
Heidelberg, Germany, July 6-8, 2010
ISBN: 978-1-84102-259-8

Title: Shortcomings in CAPTCHA Design and Implementation: Captcha2, a Commercial Proposal
Author(s): Carlos Javier Hernandez-Castro, Jonathan D Stainton-Ellis, Arturo Ribagorda, Julio Cesar Hernandez-Castro
Reference: pp159-168
Keywords: CAPTCHA, HIP, Captcha2, shortcomings, flaws
Abstract: Many CAPTCHA proposals have shortcomings in their design or implementation
that make them much weaker than intended. In this paper we study Captcha2,
a commercial algorithm, as a means of showing typical flaws that make many CAPTCHAs
prone to successful low-cost attacks. The attack we present makes no use of any
AI techniques, not affecting the resilience of the original AI problem this CAPTCHA
is (supposedly) based upon. That’s why it can be considered a pure side-channel attack.
We conclude with some tips for improving this CAPTCHA, which can be also
used as general guidelines for avoiding a certain family of very common flaws.
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