This paper reviews an approach to secret-key distribution based on the bounded observability (BO) model. First, the information-theoretic framework of secret-key agreement from a correlated random source is reviewed. Next, the BO model is introduced. In the context of this model, the BO condition is presented as a necessary and sufficient condition for the possibility of secret-key distribution. This condition describes limits on the information obtained by observation of a random object, and models the practical difficulty of completely observing random physical phenomena. Finally, an implementation of secret-key distribution based on BO in an optical fiber system is described.

Secret key agreement is a procedure for agreeing on a secret key by exchanging messages over a public channel when a sender, a legitimate receiver and an eavesdropper have access to correlated sources. Maurer, IEEE Trans. Inf. Theory 1993 defined secret key capacity, which is the least upper bound of the key generation rate of the secret key agreement, and presented an upper and a lower bound for the secret key capacity. In this paper, the advantage distillation capacity is introduced and it is shown that this quantity equals to the secret key capacity. A naive information theoretical expression of the secret key capacity and the advantage distillation capacity is also presented. An example of correlated sources, for which an analytic expression of the secret key capacity can be obtained, is also presented

The capacity for secret key agreement for permutation-invariant and symmetric sources under a sampling attack is investigated. The supremum of the normalized secret key capacity is introduced, where the supremum is taken over all permutation-invariant sources or all symmetric sources and the normalized secret key capacity is the secret key capacity divided by the description length of the symbol. It is proved that the supremum of the normalized secret key capacity bound under a sampling attack is close to 1/m for permutation-invariant sources and O(1/m) for symmetric sources, where and m is the number of Eve's sources

This paper considers the capacity for secret key agreement under a sampling attack. Given the number of the eavesdropper's sources, we evaluate the secret key capacity bound which is defined as the supremum of the secret key capacity divided by the description length of the alphabet, where the supremum is taken over a set of probability distributions corresponding to the correlated sources. In particular, we consider symmetric sources and permutation-invariant sources. We derive inequalities which show the scaling of the secret key capacity bound with the number of the eavesdropper's sources

Time and Frequency (T/F) transfer systems using space links developed at the Radio Research Laboratories (RRL) are described. The domestic T/F transfer system uses the Japanese communication satellite: CS-2/broadcasting satellite BS-2 and the Global Positioning System (GPS) in addition to the ordinary links of ground TV and Loran-C The Asian area transfer system makes use of the Geostationary Meteorological Satellite (GMS) and a Very Long Baseline Interferometer (VLBI) as well as Loran-C Other systems for international T/F transfers depend on the space techniques of the GPS and the, VLBI. Spread-spectrum equipments for T/F transfers have recently been built to realize both precision and accuracy of about 1 ns via the CS link, together with a 1-m mobile antenna station. Receivers have been developed at RRL to be used routinely for the GPS and GMS T/F transfers on a daily basis with a precision of 10 ns or more and an accuracy of less than 100 ns. T/F transfer experiments using a VLBI system developed at RRL have been conducted on a routine basis about once a month with the U.S. Naval Observatory since April of 1985. The present goal of accuracy is 10 ns with 10-min or longer observations.

Japan has proceeded with the program for millimeter wave satellite communications since 1967, in addition to the programs of preoperational experiment for communications and broadcasting satellites in lower frequencies. The program is composed of two satellites, Engineering Test Satellite Type II (ETS-II) and Experimental Communication Satellite (ECS). ETS-II carries a beacon transmitter with three coherent frequencies including a millimeter wave and was launched in February 1977. ECS has a millimeter wave transponder and will be launched in February 1980. This paper describes the outline of the ETS-II/ECS project, the preliminary results of ETS-II, and the experimental plan of ECS.