The Caching Broadcast Channel with a Wire and Cache Tapping Adversary of Type II: Multiple Library Files

A wiretap model with two receivers equipped with fixed-size cache memories, and a type II adversary is considered. The adversary in this model chooses a subset of symbols to tap into either from cache placement, delivery transmission, or both phases. The legitimate parties do not know the fractions or the positions of the tapped symbols in either phase. For a library of size three files or more, lower and upper bounds on the strong secrecy capacity, i.e., the maximum achievable file rate while keeping the overall library strongly secure, are derived. The strong secrecy capacity is identified for the instance of large tapped subsets. Achievability is established by wiretap coding, security embedding codes, one-time pad keys, and coded caching techniques. The upper bound is constructed by three successive channel transformations.