Quantum cryptography has become a subject of widespread interest. In particular, quantum key distribution, which provides a secure key agreement by using quantum systems, is believed to be the most important application of quantum cryptography. Quantum key distribution has the potential to achieve the "unconditionally" secure infrastructure. We also have many cryptographic tools that are based on "modern cryptography" at the present time. They are being used in an effort to guarantee secure communication over open networks such as the Internet. Unfortunately, their ultimate efficacy is in doubt. Quantum key distribution systems are believed to be close to practical and commercial use. In this paper, we discuss what we should do to apply quantum cryptography to our communications. We also discuss how quantum key distribution can be combined with or used to replace cryptographic tools based on modern cryptography.

Dr. Kenji Koyama, one of the most respected and prominent Japanese researchers in modern cryptography, passed away on March 27, 2000. He left behind him many outstanding academic achievements in cryptography as well as other areas such as emotion transmission theory, learning and mathematical games. In this manuscript, with our deepest sympathy and greatest appreciation for his contribution to our society, we introduce his major works mainly in cryptography, although his papers in other areas are included in the bibliography list.