A Novel Scheme with Honest Relay for Secret Key Distribution

Original quantum key distribution (QKD) protocols operate on a peer-to-peer basis between two adjacent nodes, which are directly connected through a quantum channel. Such protocols are severely distance limited. To extend the range, existing schemes use trusted nodes to relay in a hop-by-hop manner, secret key generated at one end node to the other end node. It is difficult to ensure node trustworthiness and more trusted nodes will lead to an enlarged attack surface. This paper proposes a scheme called honest relay key distribution (HRKD) that does not require relays be trusted. Specifically, HRKD removes the trusted node requirement by distilling a secret key directly between end nodes which are separated by multiple hops and imposing strict timing requirements on quantum state bases announcement and key usage. We have analyzed HRKD to show that its key secrecy protection can be as good as BB84 in some cases and is better than trusted node key relaying. We have derived its secret key rate as a function of distance between two end nodes. We have verified the key rate accuracy through extensive simulations. Evaluation results have also confirmed that with an increase in the distance between two end nodes, HRKD requires a maximum of two physical honest relays regardless of the distance while trusted node key relaying requires an increasing number of relays.