Research On Protocols Of Quantum Cryptography: Design And Analysis

As we know, the security of most classical cryptosystems is based on computational complexity assumptions. With the fast development of people's ability of computation, especially the presentation of some parallel quantum algorithms, this kind of security is severely challenged. Quantum cryptography is the combination of quantum mechanics and cryptography. Despite of the computational ability of the eavesdropper, the security of quantum cryptosystems is only guaranteed by the fundamental laws of quantum mechanics. Therefore, quantum cryptography has native advantage in the sense of security and it has gradually developed into an important branch of modern cryptography. It is implied by the research both in theory and in experiment that quantum cryptography would grow up in the next generation of technologies in cryptography.The contributions of this dissertation are mainly on the design and analysis of the protocols of quantum cryptography, including quantum key distribution (QKD), quantum identity verification (QIV), quantum secret sharing (QSS), and quantum secure direct communication (QSDC). The details are as follows:With respect to the design, we propose a QKD protocol without alternative measurements and rotations; a QKD protocol by constructing nonorthogonal states with Bell states; a QKD and QIV protocol with entanglement for key bits and nonorthogonal states for detecting eavesdropping and identity verification; and two QSS protocols based on local operations and local measurements respectively. All these protocols are analysed from the perspective of security or proved to be secure in detail.As for the analysis, the securities of some quantum protocols are analysed and several effective attack strategies are presented. Furthermore, some improved versions of these protocols are given. With our attack strategies, a student can copy the answers from the others in a quantum exam; an eavesdropper can elicit about half of the key in QKD and QSS protocols without being detected; and an agent can recover the dealer's secret by himself in a QSS scheme. In addition, two hidden dangers in pursuing high efficiency are pointed out. In our opinion, these simple and effective attack strategies are instructive for later work of scheme designing.Besides, we give a more efficient way to detect eavesdropping in the Bostrom-Felbinger protocol, with which the security of this protocol is improved. We also discuss the essence of two kinds of QKD protocols, i.e., nonorthogonal- and orthogonal-based QKD, and draw a conclusion that these two kinds of protocols, which are totally opposite on the surface, have the same essence, that is, information splitting.