  Extra dimensions (ED) can provide a useful tool for model-building. In this
paper we introduce a single, flat ED extension of the kinetic-mixing/dark
photon (DP) portal for dark matter (DM) interactions with the Standard Model
(SM) assuming a compactification `radius' of order $R^{-1}\sim10-1000$ MeV and
examine the resulting modifications to and augmentation of the usual DP
phenomenology. In the present scenario, both the DP and DM experience the full
5-D while the SM fields are constrained to lie on a 4-D brane at the boundary
of the ED. Such a setup can naturally yield the observed value of the DM relic
density and explain the required rough degeneracy of the DM and DP masses
needed to obtain it. Gauge symmetry breaking can occur via boundary conditions
without the introduction of an additional singlet Higgs scalar thus avoiding
all constraints associated with the coupling of such a field to the usual SM
Higgs field in 5-D. The self-consistency of the field redefinitions that map
the gauge fields into a canonical basis and thus removing the kinetic mixing
terms is found to lead to a strong model building constraint on the ED setup
involving a brane localized kinetic term for the 5-D gauge field on the SM
brane. Multiple variations of this scenario are found to be possible which are
consistent with all current experimental constraints but which predict very
different phenomenologies. In this paper, after setting up the general model
formalism, we discuss in detail the case of a complex scalar 5-D DM field,
consistent with constraints arising from the CMB, which may or may not obtain a
vacuum expectation value (vev). The resulting Kaluza-Klein (KK) towers of both
the DM and DP fields are found to yield interesting and distinctive signatures.
