Impossibility of quantum private queries

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 and is closely related to oblivious transfer. Under the name , 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 [Giovannetti , ]. 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 nontrivial security guarantees for the owner of the database: The user is even able to retrieve the database.