CAPTCHA has been widely used for preventing malicious programs to access web resources automatically. In this paper, a new type CAPTCHA system will be proposed. The proposed scheme, named Click spell, combined the features of text-based and image-based CAPTCHAs. Click spell asks users to spell a randomly chosen word by clicking distorted letters for passing the test. Users can learn the definition(s) of the chosen word. In addition, Click spell can add an advertisement image optionally. Thanks to the advertisement image, Click spell improved the capability of resistance to the attack by malicious programs. Our preliminary test showed that Click spell is practical in the aspects of security and usability.
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