The Caching Broadcast Channel with a Wire and Cache Tapping Adversary of Type II
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This paper introduces the notion of cache-tapping into the information theoretic models of coded caching. In particular, the wiretap II channel model in the presence of multiple receivers equipped with fixed-size cache memories, and an adversary who is able to choose symbols to tap into from cache placement, in addition to or in lieu of, delivery transmission, is introduced. The model is hence termed the caching broadcast channel with a wire and cache tapping adversary of type II. The legitimate parties know neither whether cache placement, delivery, or both phases are tapped, nor the positions in which they are tapped. Only the size of the overall tapped set is known. For the instance of two receivers and two library files, the strong secrecy capacity of the model, i.e., the maximum achievable file rate while keeping the overall library strongly secure, is identified. Lower and upper bounds on the achievable strong secrecy file rate are derived when the library has more than two files. Achievability schemes in this paper rely on a code design which combines wiretap coding, security embedding codes, one-time pad keys, and coded caching. A genie-aided upper bound, in which a genie provides the transmitter with user demands before cache placement, establishes the converse for the two files instance. For the library of more than two files, the upper bound is constructed by three successive channel transformations. Our results establish that strong information theoretic security is possible against a powerful adversary who optimizes its attack over both phases of communication in a cache-aided system.
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