MODELLING DYNAMIC OPACITY USING PETRI NETS WITH SILENT ACTIONS

摘要

In a previous work, we presented a Petri Net based framework in which various confidentiality properties may be expressed in terms of predicates over system state and abstraction mappings from the reachable states and transitions of the underlying Petri Net. Here we extend that work by generalising these mappings by allowing them to be state dependent. This provides a natural framework in which to model various situations of importance in security, for example key compromise and refresh, downgrading of secrecy labels and conditional anonymity. We also show how global changes in the abstraction mappings can be used to model how some secrecy requirements depend on the status of the observer. We illustrate this by modelling the various flavours of anonymity that arise in the dining cryptographers example. A further development on the earlier work is to provide a more complete treatment of silent actions. We also discuss the expressiveness of the resulting framework and the decidability of the associated verification problems.