According to an embodiment, a communication device is connected with external devices by quantum communication channels and a classical communication channel, and generates an encryption key with photons exchanged over the quantum communication channel. The communication device includes an acquirer, a calculator, a selector, and a communicating unit. The acquirer is configured to acquire a quantum error rate for each quantum communication channel. The calculator is configured to calculate a metric of a first path to each external device based on the quantum error rate. The selector is configured to select a second path in the classical communication channel to a specific one of the external devices based on the metric. The communicating unit is configured to transmit an application key to the specific one of the external devices over the second path.