Michael Backes Agostino Cortesi Riccardo Focardi Matteo Maffei A calculus of challenges and responses. 51-60 2007 FMSE http://doi.acm.org/10.1145/1314436.1314444 conf/ccs/2007fmse db/conf/ccs/ccs2007fmse.html#BackesCFM07