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\begin{document}

\title{Conservation of CSP Noninterference Security\\under Sequential Composition}
\author{Pasquale Noce\\Security Certification Specialist at Arjo Systems, Italy\\pasquale dot noce dot lavoro at gmail dot com\\pasquale dot noce at arjosystems dot com}
\maketitle

\begin{abstract}
In his outstanding work on Communicating Sequential Processes, Hoare has defined
two fundamental binary operations allowing to compose the input processes into
another, typically more complex, process: sequential composition and concurrent
composition. Particularly, the output of the former operation is a process that
initially behaves like the first operand, and then like the second operand once
the execution of the first one has terminated successfully, as long as it does.

This paper formalizes Hoare's definition of sequential composition and proves,
in the general case of a possibly intransitive policy, that CSP noninterference
security is conserved under this operation, provided that successful termination
cannot be affected by confidential events and cannot occur as an alternative to
other events in the traces of the first operand. Both of these assumptions are
shown, by means of counterexamples, to be necessary for the theorem to hold.
\end{abstract}

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