@misc{backes07calculus-wits,
  author       = {Backes, Michael and Cortesi, Agostino and Focardi, Riccardo and Maffei, Matteo},
  title        = {A Calculus of Challenges and Reponses},
  howpublished = {7th International Workshop on Issues in the Theory of Security (WITS)},
  month        = {April},
  year         = {2007}
}