[isabelle] 2 new AFP entries

Transitive closure according to Roy-Floyd-Warshall
by Makarius Wenzel

This formulation of the Roy-Floyd-Warshall algorithm for the
transitive closure bypasses matrices and arrays, but uses a more direct
mathematical model with adjacency functions for immediate predecessors and
successors. This can be implemented efficiently in functional programming
languages and is particularly adequate for sparse relations.


Noninterference Security in Communicating Sequential Processes
by Pasquale Noce

An extension of classical noninterference security for deterministic
state machines, as introduced by Goguen and Meseguer and elegantly
formalized by Rushby, to nondeterministic systems should satisfy two
fundamental requirements: it should be based on a mathematically precise
theory of nondeterminism, and should be equivalent to (or at least not
weaker than) the classical notion in the degenerate deterministic case.

This paper proposes a definition of noninterference security applying
to Hoare's Communicating Sequential Processes (CSP) in the general case of
a possibly intransitive noninterference policy, and proves the
equivalence of this security property to classical noninterference
security for processes representing deterministic state machines.

Furthermore, McCullough's generalized noninterference security is shown
to be weaker than both the proposed notion of CSP noninterference security
for a generic process, and classical noninterference security for processes
representing deterministic state machines. This renders CSP noninterference
security preferable as an extension of classical noninterference security
to nondeterministic systems.




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