  In this paper we present an information flow security property for
stochastic, cooperating, processes expressed as terms of the Performance
Evaluation Process Algebra (PEPA). We introduce the notion of Persistent
Stochastic Non-Interference (PSNI) based on the idea that every state reachable
by a process satisfies a basic Stochastic Non-Interference (SNI) property. The
structural operational semantics of PEPA allows us to give two
characterizations of PSNI: the first involves a single bisimulation-like
equivalence check, while the second is formulated in terms of unwinding
conditions. The observation equivalence at the base of our definition relies on
the notion of lumpability and ensures that, for a secure process P, the steady
state probability of observing the system being in a specific state P' is
independent from its possible high level interactions.
