The present invention relates to a method of performing quantum key distribution across a network, in particular between a first node (202a) and a destination node (202b) connected via at least one intermediate node (204). The method involves the first node (202a) first agreeing a quantum key, i.e. a cryptographic key derived by QKD with the first intermediate node in the path (204) using any known QKD technique. Next the intermediate node (204) exchanges a quantum signal, for instance a series of suitably randomly modulated signal photons, with the next node (202b) in the path - which is termed the targeted node. The intermediate node (204) communicates with the first node (202a) using the previous established quantum key details of the quantum signal sent or received by the intermediate node (204). The first node then performs a key agreement step to agree a quantum key directly with the targeted node. In this way the first node is involved in the authentication step to avoid man in the middle type attacks. Having established a quantum key with the current targeted node the method can be repeated but with the next node in the network path as the targeted node until the destination node is reached. The final quantum key agreed with the destination node can then be used for encrypting communication between those nodes across the network.
