  In this paper we explore how recent technologies can improve the security of
optical networks. In particular, we study how to use quantum key distribution
(QKD) in common optical network infrastructures and propose a method to
overcome its distance limitations. QKD is the first technology offering
information theoretic secret-key distribution that relies only on the
fundamental principles of quantum physics. Point-to-point QKD devices have
reached a mature industrial state; however, these devices are severely limited
in distance, since signals at the quantum level (e.g. single photons) are
highly affected by the losses in the communication channel and intermediate
devices. To overcome this limitation, intermediate nodes (i.e. repeaters) are
used. Both, quantum-regime and trusted, classical, repeaters have been proposed
in the QKD literature, but only the latter can be implemented in practice. As a
novelty, we propose here a new QKD network model based on the use of not fully
trusted intermediate nodes, referred as weakly trusted repeaters. This approach
forces the attacker to simultaneously break several paths to get access to the
exchanged key, thus improving significantly the security of the network. We
formalize the model using network codes and provide real scenarios that allow
users to exchange secure keys over metropolitan optical networks using only
passive components. Moreover, the theoretical framework allows to extend these
scenarios not only to accommodate more complex trust constraints, but also to
consider robustness and resiliency constraints on the network.
