  Quantum Key Distribution allows two parties to establish a secret key that is
secure against computationally unbounded adversaries. To extend the distance
between parties, quantum networks, and in particular repeater chains, are
vital. Typically, security in such scenarios assumes the absolute worst case:
namely, an adversary has complete control over all repeaters and fiber links in
a network and is able to replace them with perfect devices, thus allowing her
to hide her attack within the expected natural noise. In a large-scale network,
however, such a powerful attack may be infeasible. In this paper, we analyze
the case where the adversary can only corrupt a contiguous subset of a repeater
chain connecting Alice and Bob, while some portion of the network near Alice
and Bob may be considered safe from attack (though still noisy). We derive a
rigorous finite key proof of security assuming this attack model and show that
improved performance and noise tolerances are possible.
