A method of exchanging a combined cryptographic key between a first node and a second node,
the first node and the second node being connected through a first communication and a second communication network, wherein the first communication network is a quantum communication network wherein information is encoded on weak light pulses; and
the first node and the second node being configured to:
exchange one or more first cryptographic keys on the first communication network;
exchange one or more second cryptographic keys using the second communication network; and
form the combined cryptographic key by combining the one or more first cryptographic keys and the one or more second cryptographic keys, such that the first node and the second node share knowledge of the combined cryptographic key.