Severin Winkler

Symmetric private information retrieval is a cryptographic task that allows to query a database and obtain exactly one entry without revealing to the owner of the database which element was accessed. The tasks is a variant of the general two-party protocols called “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, where it is not impossible for dishonest participants to cheat, but they risk detection. We give an explicit attack against any such protocol, showing that quantum protocols do not allow to implement cheat-sensitive symmetric private information retrieval. Our result extends to any form of oblivious transfer and many variants of secure function evaluation.