Cryptanalysis and improvement to the quantum private query protocol for enhancing database privacy

Abstract In order to protect the privacy of the query user and database, some QKD-based quantum private query (QPQ) protocols were proposed. One example is the protocol proposed by Zhou et al , in which the user makes initial quantum states and derives the key bit by comparing the initial quantum state and the outcome state returned from the database by ctrl or shift mode, instead of announcing two non-orthogonal qubits as others which may leak part secret information. To some extent, the security of the database and the privacy of the user are strengthened. Unfortunately, we find that in this protocol, the dishonest user could be obtained, utilizing unambiguous state discrimination, much more database information than that is analyzed in Zhou et al ’s original research. To strengthen the database security, we improved the mentioned protocol by modifying the information returned by the database in various ways. The analysis indicates that the security of the improved protocols is greatly enhanced.