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μcaptcha: Human Interaction Proofs Tailored to Touch-Capable Devices via Math Handwriting. International Journal of Human-Computer Interaction, 2015. https://dx.doi.org/10.1080/10447318.2015.1038124Online services are often protected with captchas that typically must be solved by typing on a keyboard. Now that smartphones and tablets are increasingly being used to browse the web, new captchas best suited to touch-capable devices should be devised, since entering text on soft keyboards is usually uncomfortable an error-prone. We contribute to solving this issue with μcaptcha, a novel captcha scheme to tell humans and computers apart by means of math handwriting input. Instead of entering text with a keyboard, the user retypes a mathematical expression on a touchscreen using e.g. the finger, a stylus, or an e-pen. Further, as a byproduct of solving μcaptcha challenges, a valuable labeled dataset of online handwritten math expressions is collected. Our studies reveal that μcaptcha is accurate, fast, and easy to perform, and that users find it to be both useful and enjoyable. Ultimately, this work informs our understanding of designing better web security measures.