Abstract To save quantum resources and improve efficiency, this article proposes an efficient counterfactual quantum privacy query protocol. Counterfactual quantum privacy query refers to the identification of secure detection and key screening based on the detector's response results without the actual transmission of quantum information between user Bob and database Alice. Alice completes key transmission through a counterfactual device, while Bob measures the photon's polarization with a probability of 1/2 to obtain the encoded classical information, followed by traditional post-processing and privacy query operations. Additionally, to withstand the noise impact during the communication process, a modified counterfactual QKD protocol proposed by Rao (2021) is utilized, where communication parties randomly select a quantum bit flip with a specified probability \textit{f}, and then complete the quantum privacy query.
Abstract To save quantum resources and improve efficiency, this article proposes an efficient counterfactual quantum privacy query protocol. Counterfactual quantum privacy query refers to the identification of secure detection and key screening based on the detector's response results without the actual transmission of quantum information between user Bob and database Alice. Alice completes key transmission through a counterfactual device, while Bob measures the photon's polarization with a probability of 1/2 to obtain the encoded classical information, followed by traditional post-processing and privacy query operations. Additionally, to withstand the noise impact during the communication process, a modified counterfactual QKD protocol proposed by Rao (2021) is utilized, where communication parties randomly select a quantum bit flip with a specified probability \textit{f}, and then complete the quantum privacy query.
Almost all of the previous quantum private query (QPQ) protocols are always based on an ideal environment without any noise. Nevertheless, due to the defects of the channel, channel noise inevitably exists in the actual transmission process, which will give rise to transmission error and reduce the probability of successful quantum transmission. An eavesdropper may disguise her actions as channel noise to avoid being detected by legitimate parties during security checks. To solve this problem, we present a QPQ protocol with perfect performance universally applicable against collective noise. Concretely, the protocol encodes bits in noiseless subspace and thus can function over a quantum channel subjected to an arbitrary degree of collective noise, as occurs, for instance, due to polarization rotation in an optical fiber. Furthermore, Bob cannot get the information about Alice's choice at all by taking fake entangled attack and by taking fake photon attack, the probability Bob can get the information about Alice's choice is only approximately 6.7%, which is far less than the probability of 25% in Yang et al.'s protocol. Moreover, the conclusiveness of the user Alice's measurement results is subject to quantum randomness rather than allowing Alice to choose by herself to obtain a conclusive result in Wei et al.'s protocol, which prevents Alice's intuitive attack and meanwhile makes it arduous for Bob to perform the joint-measurement attack. Our protocol uses only one state, which reduces the communication complexity, and is, therefore, an effective QPQ solution with the high-security level.
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.
This paper proposes two quantum privacy query protocols. In the single vector sub-privacy query of protocol 1, it is proposed for the first time to distinguish different measurement results by publishing 2 bits of classic information. In the protocol, the database Bob selects the Z base and the Bell base to measure the particles sent by Alice according to the key value of the encrypted database and publishes 2 bits of classic information based on the measurement results. The user Alice infers the key value of the encrypted database through these two pieces of classic information. In the two-way quantum privacy query of protocol 2, the database Bob measures the particles sent back by Alice, converts the measurement result into a key value of length 3, and prepares a particle that is the same as the measurement result and sends it back to Alice. Alice returns the measurement of particles can infer a set of data encryption keys, and the length of each key is 3 bits. Users in the two protocols proposed in the article can obtain the query key with a probability of 1/4. As the key length increases, the number of query bits in protocol 2 decreases, but the probability of execution failure increases. But protocol 2 has a higher quantum efficiency. Both protocols can guarantee the privacy of users and databases.
As a kind of practical protocol, quantum-key-distribution (QKD)-based quantum private queries (QPQs) have drawn lots of attention. However, joint-measurement (JM) attack poses a noticeable threat to the database security in such protocols. That is, by JM attack a malicious user can illegally elicit many more items from the database than the average amount an honest one can obtain. Taking Jacobi et al.'s protocol as an example, by JM attack a malicious user can obtain as many as 500 bits, instead of the expected 2.44 bits, from a 104-bit database in one query. It is a noticeable security flaw in theory, and would also arise in application with the development of quantum memories. To solve this problem, we propose a QPQ protocol based on a two-way QKD scheme, which behaves much better in resisting JM attack. Concretely, the user Alice cannot get more database items by conducting JM attack on the qubits because she has to send them back to Bob (the database holder) before knowing which of them should be jointly measured. Furthermore, JM attack by both Alice and Bob would be detected with certain probability, which is quite different from previous protocols. Moreover, our protocol retains the good characters of QKD-based QPQs, e.g., it is loss tolerant and robust against quantum memory attack.
Abstract In order to protect the privacy of query user and database, some QKD-based quantum private query (QPQ) protocols were proposed. Unfortunately some of them cannot resist internal attack from database perfectly; some others can ensure better user privacy but require a reduction of database privacy. In this paper, a novel two-way QPQ protocol is proposed to ensure the privacy of both sides of communication. In our protocol, user makes initial quantum states and derives the key bit by comparing initial quantum state and outcome state returned from database by ctrl or shift mode instead of announcing two non-orthogonal qubits as others which may leak part secret information. In this way, not only the privacy of database be ensured but also user privacy is strengthened. Furthermore, our protocol can also realize the security of loss-tolerance, cheat-sensitive, and resisting JM attack etc.
Quantum private query (QPQ) is a cryptographic application that protects the privacy of both users and databases while querying the database secretly. In most existing QPQ protocols, the protection of user privacy can only be cheat-sensitive. Cheat-sensitive means that Bob will be found later with a certain probability if he tries to get the address queried by Alice. On the premise of cheat-sensitivity, although Alice can discover Bob’s malicious behavior after a query (transaction), the secret information of Alice was leaked in the completed query, which is likely to be a fatal blow to Alice. Or, to prevent Bob’s malicious behavior, Alice executes one or more additional queries to test Bob’s honesty. However, to bypass Alice’s honesty test, Bob can also provide several honest queries before performing dishonest queries. Therefore, cheat-sensitive should not be the ultimate goal of user privacy protection in QPQ. In this paper, we propose a practical QKD-based QPQ protocol with better user privacy protection than cheat-sensitivity based on order rearrangement of qubits. The proposed QPQ protocol can resist the Trojan horse attack even without wavelength filter and photon number splitter (PNS) equipped with auxiliary monitoring detectors.