Elizabeth Stobert Alain Forget Sonia Chiasson Paul C. van Oorschot Robert Biddle Exploring usability effects of increasing security in click-based graphical passwords. 79-88 2010 ACSAC http://doi.acm.org/10.1145/1920261.1920273 conf/acsac/2010 db/conf/acsac/acsac2010.html#StobertFCOB10