On declassification and the non-disclosure policy

  • Authors:

  • Ana Almeida Matos;Gérard Boudol

  • Affiliations:

  • SQIG - Instituto de Telecomunicações, and DEI - IST TU Lisbon, Lisbon, Portugal;INRIA Sophia Antipolis, France

  • Venue:
  • Journal of Computer Security - 18th IEEE Computer Security Foundations Symposium (CSF 18)
  • Year:
  • 2009

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Abstract

We address the issue of declassification in a language-based security approach. We introduce, in a Core ML-like language with concurrent threads, a declassification mechanism that takes the form of a local flow policy declaration. The computation in the scope of such a declaration is allowed to implement information flow according to the local policy. To take into account declassification, and more generally dynamic flow policies, we introduce a generalization of non-interference, that we call the non-disclosure policy, and we design a type and effect system for our language that enforces this policy. Besides dealing with declassification, our type system improves over previous systems for checking information flow in two directions: first, we show that the typing of terminations leaks can be largely improved, by particularizing the case where the alternatives in a conditional branching both terminate. Moreover, we also provide a quite precise way of approximating the confidentiality level of an expression, by ignoring the level of values that are only used for side-effects.