@inproceedings{backes07calculus,
  author       = {Backes, Michael and Cortesi, Agostino and Focardi, Riccardo and Maffei, Matteo},
  title        = {A Calculus of Challenges and Responses},
  booktitle    = {Proceedings of 5th ACM Workshop on Formal Methods in 
    Security Engineering (FMSE)},
  month        = {November},
  year         = {2007}
}