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Register a new userCryptanalysis and improvement of Wu-Cai-Wu-Zhangs quantum private comparison protocol
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Guang Ping He
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In a recent paper (Int. J. Quantum Inf. 17 (2019) 1950026), the authors discussed the shortcomings in the security of a quantum private comparison protocol that we previously proposed. They also proposed a new protocol aimed to avoid these problems. Here we analysis the information leaked in their protocol, and find that it is even less secure than our protocol in certain cases. We further propose an improved version which has the following advantages: (1) no entanglement needed, (2) quantum memory is no longer required, and (3) less information leaked. Therefore, better security and great feasibility are both achieved.
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To evade the well-known impossibility of unconditionally secure quantum two-party computations, previous quantum private comparison protocols have to adopt a third party. Here we study how far we can go with two parties only. We propose a very feasible and efficient protocol. Intriguingly, although the average amount of information leaked cannot be made arbitrarily small, we find that it never exceeds 14 bits for any length of the bit-string being compared.
Since unconditionally secure quantum two-party computations are known to be impossible, most existing quantum private comparison (QPC) protocols adopted a third party. Recently, we proposed a QPC protocol which involves two parties only, and showed that although it is not unconditionally secure, it only leaks an extremely small amount of information to the other party. Here we further propose the device-independent version of the protocol, so that it can be more convenient and dependable in practical applications.
Quantum private comparison (QPC) aims to solve Tierce problem based on the laws of quantum mechanics, where the Tierce problem is to determine whether the secret data of two participants are equal without disclosing the data values. In this paper, we study for the fist time the utility of eight-qubit entangled states for QPC by proposing a new protocol. The proposed protocol only adopts necessary quantum technologies such as preparing quantum states and quantum measurements without using any other quantum technologies (e.g., entanglement swapping and unitary operations), which makes the protocol have advantages in quantum device consumption. The measurements adopted only include single-particle measurements, which is easier to implement than entangled-state measurements under the existing technical conditions. The proposed protocol takes advantage of the entanglement characteristics of the eight-qubit entangled state, and uses joint computation, decoy photon technology, the keys generated by quantum key distribution to ensure data privacy.
The participant attack is the most serious threat for quantum secret-sharing protocols. We present a method to analyze the security of quantum secret-sharing protocols against this kind of attack taking the scheme of Hillery, Buzek, and Berthiaume (HBB) [Phys. Rev. A 59 1829 (1999)] as an example. By distinguishing between two mixed states, we derive the necessary and sufficient conditions under which a dishonest participant can attain all the information without introducing any error, which shows that the HBB protocol is insecure against dishonest participants. It is easy to verify that the attack scheme of Karlsson, Koashi, and Imoto [Phys. Rev. A 59, 162 (1999)] is a special example of our results. To demonstrate our results further, we construct an explicit attack scheme according to the necessary and sufficient conditions. Our work completes the security analysis of the HBB protocol, and the method presented may be useful for the analysis of other similar protocols.
In 2013, Gau and Wu introduced a unitary invariant, denoted by $k(A)$, of an $ntimes n$ matrix $A$, which counts the maximal number of orthonormal vectors $textbf x_j$ such that the scalar products $langle Atextbf x_j,textbf x_jrangle$ lie on the boundary of the numerical range $W(A)$. We refer to $k(A)$ as the Gau--Wu number of the matrix $A$. In this paper we take an algebraic geometric approach and consider the effect of the singularities of the base curve, whose dual is the boundary generating curve, to classify $k(A)$. This continues the work of Wang and Wu classifying the Gau-Wu numbers for $3times 3$ matrices. Our focus on singularities is inspired by Chien and Nakazato, who classified $W(A)$ for $4times 4$ unitarily irreducible $A$ with irreducible base curve according to singularities of that curve. When $A$ is a unitarily irreducible $ntimes n$ matrix, we give necessary conditions for $k(A) = 2$, characterize $k(A) = n$, and apply these results to the case of unitarily irreducible $4times 4$ matrices. However, we show that knowledge of the singularities is not sufficient to determine $k(A)$ by giving examples of unitarily irreducible matrices whose base curves have the same types of singularities but different $k(A)$. In addition, we extend Chien and Nakazatos classification to consider unitarily irreducible $A$ with reducible base curve and show that we can find corresponding matrices with identical base curve but different $k(A)$. Finally, we use the recently-proved Lax Conjecture to give a new proof of a theorem of Helton and Spitkovsky, generalizing their result in the process.
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