Jeremy Elson John R. Douceur Jon Howell Jared Saul Asirra: a CAPTCHA that exploits interest-aligned manual image categorization. 366-374 2007 ACM Conference on Computer and Communications Security http://doi.acm.org/10.1145/1315245.1315291 conf/ccs/2007 db/conf/ccs/ccs2007.html#ElsonDHS07