Jodie P. Boyer Ragib Hasan Lars E. Olson Nikita Borisov Carl A. Gunter David Raila Improving multi-tier security using redundant authentication. 54-62 2007 CSAW http://doi.acm.org/10.1145/1314466.1314475 conf/ccs/2007csaw db/conf/ccs/ccs2007csaw.html#BoyerHOBGR07