It is shown that it is possible to define quantum field theory of a massless
scalar free field on the Killing horizon of a 2D-Rindler spacetime. Free
quantum field theory on the horizon enjoys diffeomorphism invariance and turns
out to be unitarily and algebraically equivalent to the analogous theory of a
scalar field propagating inside Rindler spacetime, nomatter the value of the
mass of the field in the bulk. More precisely, there exists a unitary
transformation that realizes the bulk-boundary correspondence under an
appropriate choice for Fock representation spaces. Secondly, the found
correspondence is a subcase of an analogous algebraic correspondence described
by injective *-homomorphisms of the abstract algebras of observables generated
by abstract quantum free-field operators. These field operators are smeared
with suitable test functions in the bulk and exact 1-forms on the horizon. In
this sense the correspondence is independent from the chosen vacua. It is
proven that, under that correspondence the ``hidden'' $SL(2,\bR)$ quantum
symmetry found in a previous work gets a clear geometric meaning, it being
associated with a group of diffeomorphisms of the horizon itself.