A method includes a preparation step and a key agreement step. In the preparation step, a first quantum key distribution (QKD) device at a first location and a second QKD device at a second location distant from the first location together create a quantum secured key according to a QKD protocol, and a first encryption device at the first location and a second encryption device at the second location together create a symmetrically encrypted channel between the first location and the second location using the quantum secured key. In the key agreement step, a first key agreement device at the first location and a second key agreement device at the second location together create an encryption key via the symmetrically encrypted channel.