Title: A New Wiretap Channel Model and its Strong Secrecy Capacity,
Abstract: In this paper, a new wiretap channel model is proposed, where the legitimate
transmitter and receiver communicate over a discrete memoryless channel. The
wiretapper has perfect access to a fixed-length subset of the transmitted
codeword symbols of her choosing. Additionally, she observes the remainder of
the transmitted symbols through a discrete memoryless channel. This new model
subsumes the classical wiretap channel and wiretap channel II with noisy main
channel as its special cases. The strong secrecy capacity of the proposed
channel model is identified. Achievability is established by solving a dual
secret key agreement problem in the source model, and converting the solution
to the original channel model using probability distribution approximation
arguments. In the dual problem, a source encoder and decoder, who observe
random sequences independent and identically distributed according to the input
and output distributions of the legitimate channel in the original problem,
communicate a confidential key over a public error-free channel using a single
forward transmission, in the presence of a compound wiretapping source who has
perfect access to the public discussion. The security of the key is guaranteed
for the exponentially many possibilities of the subset chosen at wiretapper by
deriving a lemma which provides a doubly-exponential convergence rate for the
probability that, for a fixed choice of the subset, the key is uniform and
independent from the public discussion and the wiretapping source's
observation. The converse is derived by using Sanov's theorem to upper bound
the secrecy capacity of the new wiretap channel model by the secrecy capacity
when the tapped subset is randomly chosen by nature.