The usability of CAPTCHAs on smartphones

Completely Automated Public Turing tests to tell Computers and Humans Apart (CAPTCHA) are challenge-response tests used on the web to distinguish human users from automated bots (von Ahn et al., 2004). In this paper, we present an exploratory analysis of the results obtained from a user study and a heuristic evaluation of captchas on smartphones; we aimed to identify opportunities and guide improvements for captchas on smartphones. Results showed that existing captcha schemes face effectiveness and user satisfaction problems. Among the more severe problems found were the need to often zoom and pan, and too small control buttons. Based on our results, we present deployment and design guidelines for captchas on smartphones.