Research On Quantum Private Query And Quantum Digital Signature Protocols

Quantum information theory is the combination of quantum mechanics and information theory.As the research on the capability of quantum computers carries on,researchers found that quantum algorithm can speed up the crack of several classical difficult problems,which are the foundation of many classical cryptographic algorithms.These insecure cryptographic schemes are widely-used in every aspects of daily life.Information security is pushed into an unprecedented danger imposed by quantum computers.People have been looking for ways to cope with threatens from quantum computers,with quantum cryptography being one of those.As an indispensable part of modern cryptography,quantum cryptography exploits the characteristics of quantum mechanics to ensure the security of information.Its importance lies in that it can achieve the assumption of several classical cryptography schemes theoretically,and it can construct cryptographic primitives that is unconditional secure.Thus in the coming post-quantum age,quantum cryptography relies on the rules in common with quantum computers to guard security of communication.As a newly-born subject,so far quantum cryptography still carries several problems,such as loopholes,impracticality and unreality.We have researched on several core problems in quantum private query and quantum digital signature,and this thesis makes the following contributions.1.We analyze quantitatively how the proportion of the orthogonal states of the test state influences the security of the protocol,and find that the optimal proportion varies as the probability that a database owner could pass the honesty test implemented by the user.2.A multi-bit quantum private query scheme is proposed,which has higher efficiency than other quantum private query schemes based on quantum key distribution when performing multi-bit queries.The scheme is later applied to block queries,together with an pre-processing on the database to preserve database security.Moreover,two inappropriate post-processing schemes for the quantum private query scheme based on quantum key distribution is analyzed and advice from the perspective of universal security is given.3.We focus on a quantum digital signature protocol which is based on QKD components.By proposing an attack based on entanglement,the tolerable noise of a quantum digital signature is calculated.The signer can repudiate the message without being caught if the noise is higher than the tolerable value.