Based on the fundamental concept of quantum counterfactuality, we propose a protocol to achieve quantum private database queries, which is a theoretical study of how counterfactuality can be employed beyond counterfactual quantum key distribution (QKD). By adding crucial detecting apparatus to the device of QKD, the privacy of both the distrustful user and the database owner can be guaranteed. Furthermore, the proposed private-database-query protocol makes full use of the low efficiency in the counterfactual QKD, and by adjusting the relevant parameters, the protocol obtains excellent flexibility and extensibility.

Through introducing discrete-time quantum walks on the infinite line and on circles, we present a kind of two-particle interacting quantum walk which has two kinds of interactions. We investigate the characteristics of this kind of quantum walk and the time evolution of the two particles. Then we put forward a kind of quantum Hash scheme based on two-particle interacting quantum walks and discuss their feasibility and security. The security of this kind of quantum Hash scheme relies on the infinite possibilities of the initial state rather than the algorithmic complexity of hard problems, which will greatly enhance the security of the Hash schemes.

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The way to compare the efficiencies of different detect strategies (DSs) in the "ping-pong" protocol is studied. The trade-off between information gain and disturbance is calculated and compared for different DSs. The comparison result primely tallies with our intuitional analysis. It is shown that the analysis of this trade-off is a feasible way to compare the performances of different DSs in theory.quantum key distribution, quantum cryptography, quantum secure direct communicationThe task of cryptography is to make secret messages intelligible only for the two legitimate parties of the secret communication, Alice and Bob, and unreadable for other unauthorized users such as Eve. To this end, Alice and Bob have to encrypt their secret messages using a suitable encryption scheme. In 1926, the one-time pad (OTP) cipher was invented by American AT&T Engineer Vernam [1] . It was later shown, by Shannon [2] , that as long as the key is truly random, has the same length as the message, and is never reused, then OTP is perfectly secure. However, there is a problem, called key distribution. Although the public key cryptography accomplishes this task, it is based on computational security. That is, if and when mathematicians or computer scientists come up with fast and clever procedures for resolving mathematical difficult problems such as factoring large integers, the whole privacy of public-key cryptosystems could vanish overnight. Quantum cryptography, which is based on fundamental physical principles, has been proved to be an effective technique for secure key distribution [3][4][5][6] . It overcomes the drawbacks possessed by conventional cryptography and the public key cryptography, and has the vast developing prospect.Boström and Felbinger [7] presented a "ping-pong" communication protocol which can be used both to distribute a secure key (i.e. quantum dey distribution (QKD)) and to transfer information in a deterministic secure manner (i.e., quantum secure direct communication, or QSDC for short).

Cryptanalysis is an important branch in the study of cryptography, including both the classical cryptography and the quantum one. In this paper we analyze the security of two three-party quantum key distribution protocols (QKDPs) proposed recently, and point out that they are susceptible to a simple and effective attack, i.e. the dense-coding attack. It is shown that the eavesdropper Eve can totally obtain the session key by sending entangled qubits as the fake signal to Alice and performing collective measurements after Alice's encoding. The attack process is just like a dense-coding communication between Eve and Alice, where a special measurement basis is employed. Furthermore, this attack does not introduce any errors to the transmitted information and consequently will not be discovered by Alice and Bob. The attack strategy is described in detail and a proof for its correctness is given. At last, the root of this insecurity and a possible way to improve these protocols are discussed.Comment: 6 pages, 3 figure

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A quantum key distribution protocol based on entanglement swapping is proposed. Through choosing particles by twos from the sequence and performing Bell measurements, two communicators can detect eavesdropping and obtain the secure key. Because the two particles measured together are selected out randomly, we need neither alternative measurements nor rotations of the Bell states to obtain security.

The security of the quantum secure direct communication (QSDC) and authentication protocol based on Bell states is analyzed. It is shown that an eavesdropper can invalidate the authentication function, and implement a successful man-in-the-middle attack, where he/she can obtain or even modify the transmitted secret without introducing any error. The particular attack strategy is demonstrated and an improved protocol is presented.