SenCAPTCHA: A Mobile-First CAPTCHA Using Orientation Sensors

CAPTCHAs are used to distinguish between human- and computer-generated (i.e., bot) online traffic. As there is an ever-increasing amount of online traffic from mobile devices, it is necessary to design CAPTCHAs that work well on mobile devices. In this paper, we present SenCAPTCHA, a mobile-first CAPTCHA that leverages the device's orientation sensors. SenCAPTCHA works by showing users an image of an animal and asking them to tilt their device to guide a red ball into the center of that animal's eye. SenCAPTCHA is especially useful for devices with small screen sizes (e.g., smartphones, smartwatches). In this paper, we describe the design of SenCAPTCHA and demonstrate that it is resilient to various machine learning based attacks. We also report on two usability studies of SenCAPTCHA involving a total of 472 participants; our results show that SenCAPTCHA is viewed as an "enjoyable" CAPTCHA and that it is preferred by over half of the participants to other existing CAPTCHA systems.