Subliminal Channels in the Private Information Retrieval Protocols

Information-theoretic private information retrieval (PIR) protocols, such as those described by Chor et al. [5], provide a mechanism by which users can retrieve information from a database distributed across multiple servers in such a way that neither the servers nor an outside observer can determine the contents of the data being retrieved. More recent PIR protocols also provide protection against Byzantine servers, such that a user can detect when one or more servers have attempted to tamper with the data he has requested. In some cases (as in the protocols presented by Beimel and Stahl [1]), the user can still recover his data and protect the contents of his query if the number of Byzantine servers is below a certain threshold; this property is referred to as Byzantine-recovery. However, tampering with a user’s data is not the only goal a Byzantine server might have. We present a scenario in which an arbitrarily sized coalition of Byzantine servers transforms the userbase of a PIR network into a signaling framework with varying levels of detectability by means of a subliminal channel [11]. We describe several such subliminal channel techniques, illustrate several use-cases for this subliminal channel, and demonstrate its applicability to a wide variety of PIR protocols.