Research On Protocols Of Quantum Secret Sharing: Design And Analysis

Quantum cryptography, the most remarkable application in quantum information, is the synthesis of quantum mechanics with the art of code-making (cryptography). Quantum secret sharing (QSS) is one of the important directions, which provides a new way for managing keys securely.The contributions of this dissertation are mainly on the protocol design and analysis of quantum secret sharing, including design methods, attack strategies, and security analysis. The details are as follows:With respect to the design, we propose two QSS protocols. One is the quantum direct secret sharing protocol with which Bell states are used as quantum information carries. It encodes the message using four local unitary operations, by which the participant can share not only the classical joint key but also the useful message directly. It has the advantage of high intrinsic efficiency. The other is the multiparty quantum secret sharing protocol using the technique of entanglement swapping. It only uses the two-particle entangled states no matter how many participants are involved. This protocol is secure and is closer to practical condition.The dishonest participant may be a stronger attacker than others in quantum secret sharing protocol, and therefore we should pay more attention to the "participant attack". Based on this character of QSS, we design two participant attack strategies. In the first attack scheme, one dishonest agent can distinguish the four encrypted operation using fake signal and entanglement swapping. In the second scheme, the attacker invalidates the random phase shift operations by quantum teleportation. The dishonest agent can restore the secret without being detected in both the two attack. In addition, we give another attack on one QSS protocol, in which an improper detection technique is used. It entangles and then disentangles the ancilla with the carrier using two controlled-not gates, by which not only the dishonest participant but the eavesdropper outside can elicit about half of the key. In our opinion, these ingenious attack strategies are instructive for later work of scheme-designing.As for the security analysis, a general participant attack model for a kind of QSS protocol using quantum information theory is presented. Furthermore, two prominent QSS protocols are analyzed using this model, by which one protocol is shown insecure and the necessary and sufficient condition to attack successfully are deduced, and the other protocol is shown secure and the concrete security is evaluated, i.e. the optimal "information-disturbance" function for an attacker. This analysis can be used to other similar protocols to evaluate their security. Our method may promote the research on the field of formal security analysis of QSS protocols.