Dinan Gunawardena Jacob Scott Alf Zugenmaier Austin Donnelly Countering Automated Exploits with System Security CAPTCHAS. 162-169 2005 conf/spw/2005 Security Protocols Workshop http://dx.doi.org/10.1007/978-3-540-77156-2_19 db/conf/spw/spw2005.html#GunawardenaSZD05