Key Superposition Simultaneously Achieves Security and Privacy in Cache-Aided Linear Function Retrieval

This work investigates the problem of cache-aided content Secure and demand Private Linear Function Retrieval (SP-LFR), where three constraints are imposed on the system: (a) each user is interested in retrieving an arbitrary linear combination of the files in the server’s library; (b) the content of the library must be kept secure from a wiretapper who obtains the signal sent by the server; and (c) no subset of colluding users together can obtain information about the demands of the remaining users. A procedure is proposed to derive an SP-LFR scheme from a given Placement Delivery Array (PDA), which is known to give coded caching schemes with low subpacketization for systems with neither security nor privacy constraints. This procedure uses the superposition of security keys and privacy keys, in both the cache placement and transmitted signal, to guarantee content security and demand privacy, respectively. In particular, among all PDA-based SP-LFR schemes, the memory-load pairs achieved by the PDA describing the Maddah-Ali and Niesen’s scheme are Pareto optimal and have the lowest subpacketization. Moreover, the achieved load-memory tradeoff is optimal to within a constant multiplicative gap, except for the small memory regime (i.e., when the cache size is between 1 and 2) and the number of files is smaller than the number of users. Remarkably, the memory-load tradeoff does not worsen compared to the best known schemes that guarantee either only content security in all regimes or only demand privacy in the regime mentioned above.