Quantum Security Theory

Renato Renner

Wang Yun-bing

Recently,many important results on quantum secure communication,especially on quantum key distribution(QKD),have been achieved. QKD,being physically secure in theory,however,is not so easy in realizing an ideal QKD system. How to evaluate the security of QKD system in practice? Can free space QKD network provide secure communication under information war? Can QKD protect information security against quantum computer attack? Studies on all these problems both in theory and practice undoubtedly play an important role in practice.

Wang, Xiang-bin

Osamu Hirota, Masaki Sohma

This paper discusses the foundation of security theory for the Quantum stream cipher based on the Holevo-Yuen theory, which allows the use of "optical amplifiers". This type of cipher is a technology that provides information-theoretic security (ITS) to optical data transmission by randomizing ultrafast optical communication signals with quantum noise. In general, the quantitative security of ITS is evaluated in terms of the unicity distance in Shannon theory. However, the quantum version requires modeling beyond the Shannon model of a random cipher to utilize the characteristics of the physical layer. Therefore, as the first step, one has to develop a generalized unicity distance theory and apply it to the evaluation of security. Although a complete theoretical formulation has not yet been established, this paper explains a primitive structure of a generalization of the Shannon random cipher and shows that the realization of this is the generalized quantum stream cipher. In addition, we present several implementation methods of generalized quantum stream ciphers and their security.

Felix Cheung

Osamu Hirota

It has been pointed out by Yuen that the security theory of quantum key distribution(QKD) guided by Shor-Preskill theory has serious defects, in particular their key rate theory is not correct. Theory groups of QKD tried to improve several defects. Especially, Renner employed trace distance and quantum leftover Hash Lemma. However, the present theory encountered a problem of a quantitative evaluation of security. To cope with it, he uses a wrong interpretation on the trace distance and its level epsilon_{sec}, and justifies the unconditional security of own system when epsilon_{sec} is 10^{-6 } ~ 10^{-20}. In this paper, we discuss the following problems. What is the origin of the misconception of the present theory? How does the present theory lead to the misconception?. To show their process toward the misconception, Koashi-Preskill's theory which has a typical misconception is examined. A main point of our comment is that QKD theory ignores the security requirement against attacker which is necessary to compare whole encryption schemes from classical to quantum. To clarify it, we emphasize that the trace distance itself cannot have any operational meaning such as failure probability, and it is only mathematical tool as a measure of closeness. As a result, it is given that the security with above values derived from their formulation means nothing in the general cryptological sense. In addition, I point out that a comment by Bennett and Riedel on unconditional security of QKD is not correct. Also, I point out that the experimental systems of groups of Los Alamos, Toshiba-UK, NICT, and others cannot have security guarantee even in future.