According to an embodiment, a communication device is connected to another communication device through a quantum communication channel and a classical communication channel to share an encryption key. The device includes a photon detector, a measurer, a difference generator, and a transmitter. The photon detector is configured to detect photons transmitted from the other communication device through the quantum communication channel. The measurer is configured to measure time when each photon is detected by the photon detector as time stamp information. The difference generator is configured to generate difference time stamp information as a difference between time obtained by adding particular information to the time indicated by the time stamp information previously measured by the measurer and time indicated by the time stamp information subsequently measured by the measurer. The transmitter is configured to transmit the difference time stamp information to the other communication device through the classical communication channel.