Interweaving Unicode and Color to Enhance CAPTCHA Security

Many existing CAPTCHAs require users to identify characters in a static image and match them with their counterparts in another image. Requiring intelligent human interaction to submit matching task solutions poses a second challenge, which is straightforward for human users but difficult to emulate for Bots. In this paper, we develop several interactive matching tasks involving dynamic elements and demonstrate their impact on CAPTCHA security and usability in a series of tests and user studies. Our tests indicate that requiring intelligent human interaction can substantially decrease the likelihood of a CAPTCHA being broken in addition to diversifying its risk of being broken. Our results provide both a security and a usability benchmark for the development of interactive dual-challenge CAPTCHAs. Furthermore, we develop a profile of human users' mouse movement patterns that can be used as a third layer of security to distinguish human users from robots. The proposed interactions can be readily incorporated in several types of existing CAPTCHA to enhance their security.