Author(s): Gerardo Reynaga, Sonia Chiasson, Paul C. van Oorschot

Download: Paper (PDF)

Date: 7 Feb 2015

Document Type: Briefing Papers

Additional Documents: Slides

Associated Event: NDSS Symposium 2015


Completely Automated Public Turing tests to tell Computers and Humans Apart (captchas) are challenge-response tests used as a security mechanism on the web to distinguish human users from automated programs. While captchas are often necessary to stop abuse of resources, most existing schemes are intended for traditional desktop computing environments rather than for mobile device usage. In this paper we present a comparative user study of nine captcha schemes on smartphones to assess whether alternative input mechanisms help improve the usability of captchas in smartphones, and to evaluate the usability of modified schemes intended to be more suitable for smartphones. The results show that although participants find virtual keyboards on smartphones prone to errors they prefer them as input mechanism over other alternatives. We also found that the content of the challenge is highly relevant in users’ perceptions when it comes to captchas on smartphones. Based on our experiences, we offer a set of ten specific recommendations for the implementation of captchas on smartphones.