No abstract

Higher security and lower failure probability have always been people's pursuits in quantumoblivious-key-transfer-based private query (QOKT-PQ) protocols since Jacobi et al. [Phys. Rev. A 83, 022301 (2011)] proposed the first protocol of this kind. However, higher database security generally has to be obtained at the cost of a higher failure probability, and vice versa. Recently, based on a round-robin differential-phase-shift quantum key distribution protocol, Liu et al. [Sci. China-Phys. Mech. Astron.58, 100301 (2015)] presented a private query protocol (RRDPS-PQ protocol) utilizing ideal single-photon signal which realizes both ideal database security and zero failure probability. However, ideal single-photon source is not available today, and for large database the required pulse train is too long to implement. Here, we reexamine the security of RRDPS-PQ protocol under imperfect source and present an improved protocol using a special "low-shift and addition" (LSA) technique, which not only can be used to query from large database but also retains the features of "ideal database security" and "zero-failure" even under weak coherent source. Finally, we generalize the LSA technique and establish a generic QOKT-PQ model in which both "ideal database security" and "zero failure" are achieved via acceptable communications.

Until now, the only kind of practical quantum private query (QPQ), quantum-key-distribution (QKD)-based QPQ, focuses on the retrieval of a single bit. In fact, meaningful message is generally composed of multiple adjacent bits (i.e., a multi-bit block). To obtain a message from database, the user Alice has to query l times to get each ai. In this condition, the server Bob could gain Alice's privacy once he obtains the address she queried in any of the l queries, since each ai contributes to the message Alice retrieves. Apparently, the longer the retrieved message is, the worse the user privacy becomes. To solve this problem, via an unbalanced-state technique and based on a variant of multi-level BB84 protocol, we present a protocol for QPQ of blocks, which allows the user to retrieve a multi-bit block from database in one query. Our protocol is somewhat like the high-dimension version of the first QKD-based QPQ protocol proposed by Jacobi et al., but some nontrivial modifications are necessary.

We present a new collusion attack to a kind of quantum secret sharing schemes with entangled states. Using this attack, an unauthorized set of agents can gain access to the shared secret without the others' cooperation. Furthermore, we establish a general model for this kind of quantum secret sharing schemes and then give some necessary conditions to design a secure quantum secret sharing scheme under this model.The concept of secret sharing schemes was firstly introduced by Shamir 1 and Blakely 2 , respectively, in which a secret S is divided into n pieces in such a way that S can be easily reconstructed from any k pieces, but even complete knowledge of k − 1 pieces reveals absolutely no information about S. The unique technique of secret sharing enables the construction of robust key management schemes or any other cryptographic schemes that can function securely and reliably even when misfortunes destroy half the pieces and security breaches expose all but one of the remaining pieces 1 . In contrast to classical secret sharing, the security of quantum secret sharing (QSS) is based on the fundamental principles of quantum physics, which allows agents (holders of the shared secret) to share a secret securely even in the presence of an opponent Eve with unlimited computing ability 3 . Owning to the advantage of unconditional security, QSS has attracted much attention and a lot of schemes have been presented both in theoretical and experimental aspects [4][5][6][7][8][9][10][11][12] .Although an opponent Eve must compromise at least k agents to learn the shared secret, and corrupt more than n − k shares to destroy the information in a (k, n) threshold sharing secret scheme, she has the entire life-time of the secret to mount these attacks. Gradual and instantaneous break-ins into a subset of agents over a long period of time may be feasible for her. Accordingly, the protection provided by traditional secret sharing may be not sufficient. A natural defense is to periodically refresh the secrets, but it is not always possible in some cases such as cryptographic master key and proprietary trade-secret information. As a result, what is actually required to protect the secret of the information is to periodically renew the shares without changing the secret, in such a way that any information learned by Eve about individual shares becomes obsolete after renewing the shares. This is so-called proactive secret sharing, which was firstly introduced by Herzberg et al. 13 So far, many proposals for proactive secret sharing have been given in classical cryptography 14,15 . Based on two-step quantum secure direct communication (QSDC) 16 , a proactive QSS scheme (named QD-scheme hereafter) was proposed recently 17 , in which a dealer Alice prepares Einstein-Podolsky-Rosen (EPR) pairs and then sends all the second particles to every agent in sequence, and the agents code their shares on these particles with four local unitary operations. However, Gao and Wang show that the QD-scheme is not secure in the sense that dishonest ...