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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.<\/jats:p>","DOI":"10.1145\/3397312","type":"journal-article","created":{"date-parts":[[2020,6,15]],"date-time":"2020-06-15T22:30:37Z","timestamp":1592260237000},"page":"1-26","update-policy":"https:\/\/doi.org\/10.1145\/crossmark-policy","source":"Crossref","is-referenced-by-count":14,"title":["SenCAPTCHA"],"prefix":"10.1145","volume":"4","author":[{"given":"Yunhe","family":"Feng","sequence":"first","affiliation":[{"name":"Department of Electrical Engineering &amp; Computer Science, University of Tennessee, Knoxville, TN, USA"}]},{"given":"Qing","family":"Cao","sequence":"additional","affiliation":[{"name":"Department of Electrical Engineering &amp; Computer Science, University of Tennessee, Knoxville, TN, USA"}]},{"given":"Hairong","family":"Qi","sequence":"additional","affiliation":[{"name":"Department of Electrical Engineering &amp; Computer Science, University of Tennessee, Knoxville, TN, USA"}]},{"given":"Scott","family":"Ruoti","sequence":"additional","affiliation":[{"name":"Department of Electrical Engineering &amp; Computer Science, University of Tennessee, Knoxville, TN, USA"}]}],"member":"320","published-online":{"date-parts":[[2020,6,15]]},"reference":[{"key":"e_1_2_2_1_1","doi-asserted-by":"publisher","DOI":"10.1109\/ICDAR.2005.205"},{"key":"e_1_2_2_2_1","volume-title":"Pattern Recognition","author":"Bongard M.M."},{"key":"e_1_2_2_3_1","doi-asserted-by":"publisher","DOI":"10.1007\/s11042-017-4883-7"},{"key":"e_1_2_2_4_1","doi-asserted-by":"publisher","DOI":"10.5555\/2817912.2817913"},{"key":"e_1_2_2_5_1","unstructured":"John Brooke et al. 1996. 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