Reexamination of the realtime protection for user privacy in practical quantum private query

TL;DR

This paper critically reexamines real-time user privacy protection in quantum private query protocols, revealing vulnerabilities and proposing that existing honesty checks are insufficient, thus urging more careful design in quantum secure computations.

Contribution

The paper identifies flaws in current real-time privacy protection methods in QPQ and provides concrete cheating strategies, emphasizing the need for improved honesty verification techniques.

Findings

Current honesty checks can be bypassed by cheating strategies.

Existing protocols fail to ensure real-time user privacy effectively.

Careful selection of checking qubits is crucial for honest participant verification.

Abstract

Quantum private query (QPQ) is the quantum version for symmetrically private retrieval. However, the user privacy in QPQ is generally guarded in the non-realtime and cheat sensitive way. That is, the dishonest database holder's cheating to elicit user privacy can only be discovered after the protocol is finished (when the user finds some errors in the retrieved database item). Such delayed detection may cause very unpleasant results for the user in real-life applications. Current efforts to protect user privacy in realtime in existing QPQ protocols mainly use two techniques, i.e., adding an honesty checking on the database or allowing the user to reorder the qubits. We reexamine these two kinds of QPQ protocols and find neither of them can work well. We give concrete cheating strategies for both participants and show that honesty checking of inner participant should be dealt more carefully in for example the choosing of checking qubits. We hope such discussion can supply new concerns when detection of dishonest participant is considered in quantum multi-party secure computations.