Tsuyoshi Nishioka

Quantum key distribution at 1550 nm using a pulse heralded single photon source

Quantum key distribution with pulsed heralded single photon source was performed over 40 km of fiber for the first time to our knowledge. QBER was measured to be 4.23% suggesting security against unconditional attack.

A Hierarchical Non-interactive Key-Sharing Scheme with Low Memory Size and High Resistance against Collusion Attacks

Efficient ID-based key sharing schemes are desired worldwide for secure communications on Internet and other networks. The Key Predistribution Systems (KPSs) are a large class of such key sharing schemes. The remarkable property of KPSs is that in order to share the key, a participant should only input its partner’s identifier to its secret KPS algorithm. Although it has many advantages in terms of efficiency, on the other hand it is vulnerable to certain collusion attacks. While conventional KPSs establish communication links between any pair of entities in a communication system, in many practical communication systems, such as broadcasting, not all links are required. In this paper, we propose a new version of KPS which is called the Hierarchical KPS. In the Hierarchical KPS, simply by removing unnecessary communication links, we can significantly increase the collusion threshold. As an example, for a typical security parameter setting, the collusion threshold of the Hierarchical KPS is 16 times higher than that of the conventional KPS while using the same amount of memory at the KPS center. The memory required by the user is even reduced by a factor 1/16 in comparison with the conventional linear scheme. Hence, Hierarchical KPS provides a more efficient method for secure communication.

How much security does Y-00 protocol provide us?

Abstract New quantum cryptography, often called Y-00 protocol, has much higher performance than the conventional quantum cryptographies. It seems that the conventional quantum cryptographic attacks are inefficient at Y-00 protocol as its security is based on the different grounds from that of the conventional ones. We have, then, tried to cryptoanalyze Y-00 protocol in the view of cryptographic communication system. As a result, it turns out that the security of Y-00 protocol is equivalent to that of classical stream cipher.

An Efficient Hierarchical Identity-Based Key-Sharing Method Resistant against Collusion-Attacks

Efficient ID-based key sharing schemes are desired world-widely for secure communications on Internet and other networks. The Key Predistiribution Systems (KPS) are a large class of such key sharing schemes. The remarkable property of KPS is that in order to share the key, a participant should only input its partner’s identifier to its secret KPS-algorithm. Although it has a lot of advantages in terms of efficiency, on the other hand it is vulnerable by certain collusion attacks. While conventional KPS establishes communication links between any pair of entities in a communication system, in many practical communication systems such as broadcasting, not all links are required. In this article, we propose a new version of KPS which is called Hierarchical KPS. In Hierarchical KPS, simply by removing unnecessary communication links, we can significantly increase the collusion threshold. As an example, for a typical security parameter setting the collusion threshold of the Hierarchical KPS i s 16 times higher than that of the conventional KPS while using the same amount of memory at the KPS center. The memory required by the user is even reduced for a factor 1/16 in comparison with the conventional linear scheme. Hence, Hierarchical KPS provides a more efficient method for secure communication.

Planar Avalanche Photodiode for Long-Haul Single-Photon Optic Fiber Communications

We present a high-performance planar GaInAs/InP avalanche photodiode (APD) for long-haul single-photon optic fiber communications, that is, quantum cryptography. The APDs for single-photon communications require a high single-photon detection efficiency (ηdet) relating to bitrate and a low ratio of dark count probability (Pdc) to ηdet limiting communication distance. We fabricated the APD with the combination of a long multiplication region length and a low carrier sheet density on the basis of numerical analysis. The Pdc/ηdet monotonically decreased with operation temperature lowered, and the ratio reached 5.9 ×10-6 with the ηdet of 13% at 77 K. The Pdc was 7.7 ×10-7, which corresponds to a dark count rate of 0.38 kHz. The useful APD and the effective layer design are reported.

Optimal Construction of Unconditionally Secure ID-Based Key Sharing Scheme for Large-Scale Networks

Efficient ID-based key sharing schemes are desired world-widely for secure communications on the Internet and other networks. The Key Predistiribution Systems (KPS) are a large class of such key sharing schemes. The remarkable property of KPS is that in order to share the key, a participant should only input its partner’s identifier to the secret KPS-algorithm. Although KPS has a lot of advantages in terms of efficiency, to achieve unconditional security a large amount of memory is required. While conventional KPS establishes communication links between any pair of entities in a communication system, in many practical communication systems such as broadcasting, not all links are required. In this article, we show the optimal method to remove the unnecessary communication links. In our scheme, the required memory for each entity is just proportional to the number of its partners, while that in conventional KPS it is proportional to the number of entities in the whole system. E.g., if an entity communicates only with 1/r of others, the required memory is reduced to 1/r of conventional KPS. Furthermore, it is proven that this memory size is optimal. Our scheme provides a more efficient way for secure communication especially in large-scale networks.

Secure quantum key distribution over 40 km of fiber with a pulsed heralded single photon source

Despite technically successful experiments, most of current quantum key distribution (QKD) systems based on the original BB84 protocols are insecure. The reason is the usage of weak coherent pulses (WCP) as pseudo single photon source (SPS), which allows an eavesdropper to get full information on the key in presence of high optical losses on the transmission channel. To overcome this problem, we have developed a pulse heralded single photon source (HSPS), which we have used for QKD over 40 km of fiber.