On Coded Caching with Private Demands

Caching is an efficient way to reduce network traffic congestion during peak hours by storing some content at the user's local cache memory without knowledge of later demands. For the shared-link caching model, Maddah-Ali and Niesen (MAN) proposed a two-phase (placement and delivery) coded caching strategy, which is order optimal within a constant factor. However, in the MAN coded caching scheme, each user can obtain the information about the demands of other users, i.e., the MAN coded caching scheme is inherently prone to tampering and spying the activity/demands of other users. In this paper, we formulate an information-theoretic shared-link caching model with private demands, where there are K cache-aided users (which can cache up to M files) connected to a central server with access to N files. Each user requests L files. Our objective is to design a two-phase private caching scheme with minimum load while preserving the information-theoretic privacy of the demands of each user with respect to other users. We propose two novel private coded caching schemes with the general underlying idea, which is to satisfy the users' requests by generating a set of coded multicast messages that is symmetric with respect to the library files. In the first scheme, we introduce a number of virtual users such that each L-subset of files is demanded by K real or virtual (effective) users and use the MAN delivery to generate multicast messages. This scheme incurs in an extremely large sub-packetization. Then, we propose a second scheme based on a novel MDS-coded cache placement. In this case, we generate multicast messages where each multicast message contains one MDS-coded symbol from each file in the library and thus is again symmetric over all the files from the viewpoint of each user. The proposed schemes are generally order optimal except for the case where N > LK and M< N/K.