KEY EXCHANGE SYSTEM, QKD APPARATUS, HUB APPARATUS, METHOD, AND PROGRAM

1 . A key exchange system comprising:

a quantum key distribution (QKD) network including

a plurality of QKD apparatuses that performs exchange of a key by using a quantum key distribution protocol, and

a plurality of key management apparatuses that relays the key; and

a plurality of hub apparatuses that performs encrypted communication by using the key received from the key management apparatuses, wherein

one of the QKD apparatuses includes

a first processor, and

a first memory storing program instructions that cause the first processor to,

in a case where the key is transmitted to an unreliable key management apparatus among the plurality of key management apparatuses, use a secret key shared in advance with a hub apparatus that receives the key via the unreliable key management apparatus, and transmit the key subjected to an exclusive OR with the secret key to the unreliable key management apparatus, and the hub apparatus includes

a second processor, and

a second memory storing program instructions that cause the second processor to,

in a case where the key is received from the unreliable key management apparatus among the plurality of key management apparatuses, use the secret key to calculate an exclusive OR of the secret key and the key.

2 . The key exchange system according to claim 1 , wherein the secret key is a key of post-quantum cryptography.

3 . A quantum key distribution (QKD) apparatus in a key exchange system including: a QKD network including a plurality of QKD apparatuses that performs exchange of a key by using a quantum key distribution protocol and a plurality of key management apparatuses that relays the key; and a plurality of hub apparatuses that performs encrypted communication by using the key received from the key management apparatuses, the QKD apparatus comprising:

a processor; and

a memory storing program instructions that cause the processor to,

in a case where the key is transmitted to an unreliable key management apparatus among the plurality of key management apparatuses, use a secret key shared in advance with a hub apparatus that receives the key via the unreliable key management apparatus, and transmit the key subjected to an exclusive OR with the secret key to the unreliable key management apparatus.

4 . A hub apparatus in a key exchange system including: a quantum key distribution (QKD) network including a plurality of QKD apparatuses that performs exchange of a key by using a quantum key distribution protocol and a plurality of key management apparatuses that relays the key; and a plurality of hub apparatuses that performs encrypted communication by using the key received from the key management apparatuses, the hub apparatus comprising:

a processor; and

a memory storing program instructions that cause the processor to,

in a case where the key is received from an unreliable key management apparatus among the plurality of key management apparatuses, use a secret key shared in advance with a QKD apparatus that transmits the key to the unreliable key management apparatus, and calculate an exclusive OR of the secret key and the key.

5 . (canceled)

6 . A non-transitory computer-readable recording medium storing a program for causing a computer to function as the QKD apparatus of claim 3 .

7 . A non-transitory computer-readable recording medium storing a program for causing a computer to function as the hub apparatus of claim 4 .