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We introduce a new kind of abstract machine based on the chemical metaphor used in the l? language of Ban%tre & al. States of a machine are chemical solutions where floating molecules can interact according to reaction rules. Solutions can be stratified by encapsulating subsolutions within membranes that force reactions to occur locally. We illustrate the use of this model by describing the operational semantics of the TCCS and CCS process calculi. We also show how to extract a higher-order concurrent &calculus out of the basic concepts of the chemical abstract machine.The situation is much less clear in the field of concurrent programming.Models such as Petri Nets, CommunicatingAutomata, or Data Flow Networks can be considered as abstract machines, but certainly they lack expressive power. More expressive models such as Algebraic Process Calculi (18,6] are intended to be specification formalisms for distributed systems rather than abstract machines. Implementation models of Concurrent ProgrammingLanguages such as CSP [IS] are conceptually based on standard sequential machine models augmented with scheduling facilities, not on specific abstract machines. @ 1990 ACM 089791-3434/90/0001/0081 $1.50 81

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.

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