An arbitrarily varying wiretap channel (AVWTC) with a type constraint on theallowed state sequences is considered, and a single-letter characterization ofits correlated-random (CR) assisted semantic-security (SS) capacity is derived.The allowed state sequences are the ones in a typical set around a singleconstraining type. SS is established by showing that the mutual informationbetween the message and the eavesdropper's observations is negligible even whenmaximized over all message distributions, choices of state sequences andrealizations of the CR-code. Both the achievability and the converse proofs ofthe type constrained coding theorem rely on stronger claims than actuallyrequired. The direct part establishes a novel single-letter lower bound on theCR-assisted SS-capacity of an AVWTC with state sequences constrained by anyconvex and closed set of state probability mass functions. This bound achievesthe best known single-letter secrecy rates for a corresponding compound wiretapchannel over the same constraint set. In contrast to other single-letterresults in the AVWTC literature, this work does not assume the existence of abest channel to the eavesdropper. Instead, SS follows by leveraging theheterogeneous version of the stronger soft-covering lemma and a CR-codereduction argument. Optimality is a consequence of an max-inf upper bound onthe CR-assisted SS-capacity of an AVWTC with state sequences constrained to anycollection of type-classes. When adjusted to the aforementioned compound WTC,the upper bound simplifies to a max-min structure, thus strengthening thepreviously best known single-letter upper bound by Liang et al. that has amin-max form. The proof of the upper bound uses a novel distribution couplingargument.
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