Development of computer and Internet techniques enables us to duplicate digital files and to exchange data files with somebody else at very high speed. However, the copyrights of digital data, such as movie and music files as well as computer software, must be protected. It is necessary to develop data encryption systems which prevent illegal copies and distributions of digital files. In this laboratory, we verify the strength of image encryption system using DES (Data Encryption Standard) against attacks.
The Internet is a growing network, which consists of vertices (computers or hubs) and edges (links between computers), and randomness is involved in the network stucture. In order to realize an effective packet routing controll, analyzing the distribution of the data flow in a network is necessary. Recenctly, randomly connected networks, called 'scale-free network' or 'small-world network', attracts a lot of attention. In our laboratory, growth process of computer networks is analyzed.
Recently, many things have been electronized and be semi-automated. The sizes of their computer systems become very large. However, once a computer system become hung-up, restoration of the system is often done by human operation. For a huge system, restoration process will take very long time (e.g. the system down of Tokyo Stock Exchange in Nov. 2005). Self-diagonosis is an idea that compters in a network verify their faults by themselves. There are two types of administrations; centralized and distributed (decentralized) ones. We consider that distributed administration system is prefered for comuter networks with self-diagnosis.
The importance of wireless communications is increasing, such as mobile phone, WLAN (wireless local area netowork), and RFID (Radio Frequency Identification). CDMA (code division multiple access) is a key technique to realize flexible multiple access without frequency channel nor time-slot allocations. One of the important issues in CDMA system is the design of pseudorandom codes, assigned to each users, such that cross-correlations between any two codes are small. In our laboratory, the performance of chaos-based codes is examined and is shown better than the conventional codes in asynchronous CDMA systems. We also design chip waveforms for CDMA systems, which is another important problem.
