We investigate Quantum Key Distribution (QKD) relaying models. Firstly, we propose a novel quasi-trusted QKD relaying model. The quasi-trusted relays are defined as follows: (i) being honest enough to correctly follow a given multi-party finite-time communication protocol; (ii) however, being under the monitoring of eavesdroppers. We develop a simple 3-party quasi-trusted model, called Quantum Quasi-Trusted Bridge (QQTB) model, to show that we could securely extend up to two times the limited range of single-photon based QKD schemes. We also develop the Quantum Quasi-Trusted Relay (QQTR) model to show that we could securely distribute QKD keys over arbitrarily long distances. The QQTR model requires EPR pair sources, but does not use entanglement swapping and entanglement purification schemes. Secondly, we show that our quasi-trusted models could be improved to become untrusted models in which the security is not compromised even though attackers have full controls over some relaying nodes. We call our two improved models the Quantum Untrusted Bridge (QUB) and Quantum Untrusted Relay (QUR) ones. The QUB model works on single photons and allows securely extend up to two times the limited QKD range. The QUR model works on entangled photons but does not use entanglement swapping and entanglement purification operations. This model allows securely transmit shared keys over arbitrarily long distances without dramatically decreasing the key rate of the original QKD schemes.
Since the range of QKD is limited, QKD relaying methods are necessary. This becomes indispensable when one aims at building QKD networks as in the last recent years. All QKD relaying methods so far introduce some undesirable drawbacks. The most practical method is based on trusted model. This method has been applied in two famous QKD networks, DAPRA and SECOCQ @cite_20 @cite_14 @cite_15 @cite_10 . In this method, all the relaying nodes must be assumed perfectly secured. Such an assumption is critical since passive attacks or eavesdropping on intermediate nodes are very difficult to detect. A few number of intermediate nodes could lead to a great vulnerability in practice. Consequently, one wants to limit the number of trusted nodes in QKD networks.
@cite_15 @cite_14 @cite_10 @cite_20
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The performances of Quantum Key Distribution (QKD) systems have notably progressed since the early experimental demonstrations and several recent works indicate that the pace of this progression is very likely to be maintained -if not increased- in the future years. In parallel to this fast progression of QKD techniques, commercial products are also being developed, making QKD deployment for securization of some specific ?real? data networks more and more likely to occur. It is the goal to the European project Secoqc to deploy a secure long-distance network based on quantum cryptography. It implies the conception of a specific architecture able to connect multiple users that may possibly be very far away from each other while QKD links are currently?point-to-point only? and intrinsically limited in distance. QKD networks are of much interest due to their capacity of providing extremely high security keys to network participants. Most QKD network studies so far focus on trusted models where all the network nodes are assumed to be perfectly secured. This restricts QKD networks to be small. In this paper, we first develop a novel model dedicated to large-scale QKD networks, some of whose nodes could be eavesdropped secretely. Then, we investigate the key transmission problem in the new model by an approach based on percolation theory and stochastic routing. Analyses show that under computable conditions large-scale QKD networks could protect secret keys with an extremely high probability. Simulations validate our results. We show how quantum key distribution (QKD) techniques can be employed within realistic, highly secure communications systems, using the internet architecture for a specific example. We also discuss how certain drawbacks in existing QKD point-to-point links can be mitigated by building QKD networks, where such networks can be composed of trusted relays or untrusted photonic switches.