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the communication channel and is retransmitted by recov-ering it in the channel cannot be detected but a QKD device can detect such a sophisticated intermediate attack.Moreover, any eavesdropping attack that leads to leak-age of information from an optical communication channel can be detected even if such eavesdropping attack becomes more sophisticated in the future. is is a merit that con-ventional cryptography technology did not have and is a very important feature for coping with the realization of eavesdropping of optical communication infrastructure. On the other hand, in order to guarantee unconditional safety, some communication performance such as distance and speed may be sacriced. e performance of the QKD link has a cryptography generation rate of 200,000-300,000 bits (200-300 kbps) through eld installed ber of 50 km.at means that the speed for cryptography of a one-time pad in real time is capable of an MPEG-4 video at most. On the contrary, the performance of commercial devices by Europe, the US and China is worse than that, about 1 kbps in an urban area.3QKD platformAlthough there are still upper limits in the distance and speed of direct transmission of QKD, “key capsule relay (key relay)” via “reliable node (trusted node)” enables a wide eld and secure key exchange by networking QKD. e system connecting multiple QKD links for networking and implementing key management function necessary for key capsule relay is generally called a “QKD network.”e cost to construct a QKD network is still high, but a generated cryptography key can be distributed to various communication devices and control apparatuses to strengthen security by controlling and managing the key properly. Also, if a large-size key is prepared, cryptography can be largely simplied because encoding is only a “simple” logical sum of the plaintext and key. Hence, the problem of delay of processing is almost resolved and a cryptography method for communication devices becomes easy to unite. So, it will be possible to ensure compatibility in cryptog-raphy between organizations of dierent specications or methods of security systems by managing key ID properly and relaying key data. Actually, in a special and important communication purpose, an exclusive cryptography net-work system independent from the open internet is used and its specications for cryptography are mostly closed. So, it is forbidden to connect between concerned organiza-tions and dicult to interact with each other, which is a basic problem. e introduction of QKD networks will solve such problems and it is expected to be eective to improve interconnectivity.e system of network solution that users can use as a black box, implementing ecient key management func-tion necessary for realization of such new added value and an interface to support various applications on a QKD network is called a “QKD platform” here. As shown in Fig. 5, it consists of three layers of a quantum layer, a key management layer, and a key supply layer.In the quantum layer, the cryptographic key is distrib-uted by QKD. QKD itself is done one-by-one via an optical communication channel such as optical ber or optical space communication.A network is realized by setting reliable nodes and connecting terminals of QKD links to make a capsule of the cryptographic key from a QKD link by the other cryptographic key of the other link (exclusive logical sum of bits of the key) and relaying them one by one. is key relay is done in the key management layer. at is, the cryptography key generated in each QKD link is managed and operated in the key management layer. In the key management layer, there is a key management agent (KMA) device in each node. e device realizes safe key relay by combining authorization technology not to relay the cryp-tographic key to anyone other than the legitimate user. e Wegman-Carter authentication method based on informa-tion theoretical safety, not on computation amount, is used [26].e key management server (KMS) also manages the condition of creation of cryptographic keys, the condition of consumption, and the existence of eavesdropping inten-sively and switches the route in the case of an eavesdrop-ping attack.Generally, the specications for requirement and ac-ceptance work are dierent from the application or de-vices for installation. In order to supply cryptographic keys to various applications freely, a key supply agent (KSA) is dened in the key management agent, and the necessary application interface is implemented. is layer of a key supplying agent is called the key supply layer. By dening the key supplying layer, the work of interface design by a key supply vendor and a key receiving client and the re-sponsibility of each work can be claried. Physically, both the KMA and KSA are implemented in the same device (such as a PC), so the key management layer and key sup-ply layer are degenerated.us, the QKD platform consists of a quantum layer 3 Quantum Key Distribution Network18　　　Journal of the National Institute of Information and Communications Technology Vol. 64 No. 1 (2017)