William G. Morein Angelos Stavrou Debra L. Cook Angelos D. Keromytis Vishal Misra Dan Rubenstein Using graphic turing tests to counter automated DDoS attacks against web servers. 8-19 2003 conf/ccs/2003 ACM Conference on Computer and Communications Security http://doi.acm.org/10.1145/948109.948114 db/conf/ccs/ccs2003.html#MoreinSCKMR03