Quantum digital signatures can be used to authenticate classical messages in an information-theoretically secure way. Previously, a novel quantum digital signature for classical messages has been proposed and gave an experimental demonstration of distributing quantum digital signatures from one sender to two receivers. Some improvement versions were subsequently presented, which made it more feasible with present technology. These proposals for quantum digital signatures are basic building blocks which only deal with the problem of sending single bit messages while no-forging and non-repudiation are guaranteed. For a multi-bit message, it is only mentioned that the basic building blocks must be iterated, but the iteration of the basic building block still does not suffice to define the entire protocol. In this paper, we show that it is necessary to define the entire protocol because some attacks will arise if these building blocks are used in a naive way of iteration. Therefore, we give a way of defining an entire protocol to deal with the problem of sending multi-bit messages based on the basic building blocks and analyse its security.

Most existing quantum private query (QPQ) protocols can hardly work in the presence of noise. The user Alice may obtain a false database item in noisy environments and both participants may cheat under the disguise of noise, so dealing with the noise needs an overall consideration of error correction, user privacy and database security. However, the only two existing protocols aiming to correct errors in QPQ lack such an overall consideration (at least one party's privacy can be revealed), and they did not estimate what extent of errors can be tolerated (actually, noise is seldom discussed in quantum two-party secure computations, and to the best of our knowledge, relevant bounds on tolerable errors remain unattainable so far). To solve this problem, we first exemplify how one participant reveals the other party's privacy in the existing QPQ protocols aiming to correct errors. Then we propose a practical protocol which can really work via noisy channel, that is, the error rate of the retrieved database item is reduced significantly and both parties' privacy are well protected. Besides, we deduce that the final error rate, user privacy and database security are pairwise in a “trade-off” relationship. By balancing them according to the required level of security and reliability, we obtain an upper bound on tolerable errors.

With growing popularity of cloud storage, the number of users of outsourcing data to cloud servers has increased dramatically. On the one hand, the rapidly increasing volume of data in the cloud is accompanied by a lot of data duplication. On the other hand, the cloud server stores only a unique copy of outsourced data in deduplication cloud storage system and the corruption or missing of the unique copy may bring immeasurable loss. Therefore, the file deduplication and integrity auditing are very important and how to securely and efficiently achieve them simultaneously needs to be settled urgently in academia and industry. In this paper, we propose a confidentiality-preserving deduplication cloud storage with public cloud auditing (CPDA). Firstly, our CPDA scheme achieves secure file deduplication on encrypted file, which supports public integrity auditing for the unique copy in the deduplication cloud storage system. Particularly, our CPDA scheme also realizes secure authentication tag deduplication. Secondly, our CPDA scheme utilizes the convergent encryption and random masking techniques to ensure data confidentiality during the file deduplication and integrity auditing process. Thirdly, our scheme not only supports each data owner to independently launch the integrity auditing of their own files, but also supports cloud server to periodically delegate the third party auditor to concurrently handle multiple auditing tasks to ensure the integrity of the outsourced files. Finally, the security of our scheme is formally proved and its performance is confirmed by numerical analyses and simulation experiments.