Implicit CAPTCHAs

ABSTRACT We propose a design methodology for \implicit" CAPTCHAs to relieve drawbacks of present technology. CAPTCHAs are tests administered automatically over networks that can distinguish between people and machines and thus protect web services from abuse by programs masquerading as human users. All existing CAPTCHAs’ challenges require a signican t conscious eort by the person answering them | e.g. reading and typing a nonsense word | whereas implicit CAPTCHAs may require as little as a single click. Many CAPTCHAs distract and interrupt users, since the challenge is perceived as an irrelevant intrusion; implicit CAPTCHAs can be woven into the expected sequence of browsing using cues tailored to the site. Most existing CAPTCHAs are vulnerable to \farming-out" attacks in which challenges are passed to a networked community of human readers; by contrast, implicit CAPTCHAs are not \fungible" (in the sense of easily answerable in isolation) since they are meaningful only in the specic context of the website that is protected. Many existing CAPTCHAs irritate or threaten users since they are obviously tests of skill: implicit CAPTCHAs appear to be elementary and inevitable acts of browsing. It can often be dicult,to detect when CAPTCHAs are under attack: implicit CAPTCHAs can be designed so that certain failure modes are correlated with failed bot attacks. We illustrate these design principles with examples. Keywords: human interactive proofs, CAPTCHAs, abuse of web sites and services, implicit CAPTCHAs, usability, legibility