Impossibility of Quantum Private Queries

TL;DR

This paper proves that in quantum private queries, any protocol secure for the user inherently compromises the database owner’s security, demonstrating an inherent impossibility in achieving full security for both parties.

Contribution

The paper provides a general attack showing that quantum protocols cannot simultaneously guarantee security for both user and database owner in private information retrieval.

Findings

Any cheat-sensitive protocol can be attacked.

Security for the user implies insecurity for the database owner.

Impossibility of fully secure quantum private queries.

Abstract

Symmetric private information retrieval is a cryptographic task allowing a user to query a database and obtain exactly one entry without revealing to the owner of the database which element was accessed. The task is a variant of general two-party protocols called one-sided secure function evaluation and is closely related to oblivious transfer. Under the name quantum private queries, quantum protocols have been proposed to solve this problem in a cheat-sensitive way: In such protocols, it is not impossible for dishonest participants to cheat, but they risk detection [V. Giovannetti, S. Lloyd, and L. Maccone, Phys. Rev. Lett. 100, 230502 (2008)]. We give an explicit attack against any cheat-sensitive symmetric private information retrieval protocol, showing that any protocol that is secure for the user cannot have non-trivial security guarantees for the owner of the database.