**1. Introduction**

## **1.1. Introduction of Y-00 (overview of network security)**

Data volume handled by individuals and companies on the internet is significantly increasing at present. The dissemination of cloud computing causes a lot of important information to flow on networks. And such information is stored in data centers and servers. Meanwhile, cyber terrorism and other crimes that aim at such important information are also on the increase and their techniques have been advanced. To respond to these threats, advanced security measures are implemented in Layer 2 (data link layer) and higher layers of the Open System Interconnection (OSI) reference model. However, safety measures of Layer 1 (physical layer) that forms a transmission path have not been established although Layer 1 is an open area. In such a network broadly two issues exist.


For this reason, physical security measures are required in Layer 1 to improve safety. Especially in communication lines requiring high safety, measures for constant monitoring through a dedicated optical fiber path of special line route are presented. But such measures require very high running costs and therefore can be realized only for special purposes. Researches on quantum cryptography for the safety of transmission paths have been made all over the world by above-mentioned background [1]. The quantum cryptography currently studied mainly in Japan and Europe is normally the Quantum Key Distribution

(QKD) system using photon transmission. The mainstream of this system is generally called BB84 that was proposed by C.H.Bennett and G.Brassard in 1984 [2]. On the other hand, Y-00 is a new quantum cipher system published by H.P.Yuen(Professor of Northwestern University in US) in 2000 [3]. BB84 uses photon transmission for the QKD system. But Y-00 is the stream cipher system that uses quantum noise existing in continuous light of a laser diode (LD) to directly encrypt data. Y-00, H.P.Yuen and O.Hirota(Professor of Tamagawa University in Japan) made theoretical verification for safety in reports exceeding the Shannon Limit for cryptography in their respective papers for the final version [4]. However, implementation conditions are limited when practical use and dissemination are considered with the current optical communication technology. As one of the Y-00 features, it has been proved that Y-00 is stronger than the current cipher though the ultimate safety (unconditional safety) cannot be achieved by the current technology [5]. Data cannot be completely deciphered by a ciphertext only attack in the currently installed equipment [6]. Also safety can be ensured by the amount of time up to decryption on the fast correlation attack. Y-00 is a physical cipher system and the safety does not depend on calculation amount like general mathematical cryptography. It information hidden by physical phenomena (quantum effect) must be extracted. No shortcut exists because physical working time for extracting this information becomes the basis of safety. Furthermore, the amount of time can be ensured. Astronomical time of this amount of time can be realized also for the currently installed equipment.

## **1.2. Concept of Y-00**

The QKD system that controls photon and transmits data with key information put on a single photon can theoretically show the safety effect provided that One-Time-Pad can be achieved as described in section 1.1. (The absolute condition required by One Time Pad is key distribution that achieves unconditional safety.) To achieve this unconditional safety by single-photon transmission, ideal devices and transmission path are necessary. And various conditions for implementation are really added. These conditions become a loop hole. Therefore that disables absolutely secure key distribution and deteriorates safety level to allow eavesdropping [7,8]. For this reason, the practical use of the QKD system is difficult while maintaining safety in the current optical communication system. Adaptability to large-capacity optical communication networks handling a great deal of information and construction of a new special transmission system at the time of introduction of a security system require much costs.Therefore, applications that are available be limited. Realizing a system by reducing these limiting conditions as much as possible is also an important task. Research and development for Y-00 have been placing the highest priority on the application to existing optical communication systems and possibility that made based on current optical communication technologies. As a result, the WDM transmission system can be shared and no further special infrastructure needs to be constructed. Because Y-00 effectively uses the existing optical modulation system, it can follow up technology trends of normal optical transceiver modules. And it also enables high-speeding, integration, and power saving of the equipment.
