  The electrons on the surface of a disordered multi-layer integer quantum Hall
system constitute an unusual chiral metal with ballistic motion transverse to
the field, and diffusive motion parallel to it. We present a non-perturbative
analytic treatment of an appropriate model, consisting of disordered chiral
Fermions in two dimensions. A supersymmetric generating functional is set up
for the correlation functions of this system. The strong disorder limit is
mapped into a supersymmetric spin chain, with ferromagnetic exchange coupling,
reflecting the electron's chiral motion. The ferromagnetic ground state and the
spin wave excitations, corresponding to the diffusion modes of the chiral
metal, are found exactly. The parametric density of states correlator in the
ergodic limit is computed from a Boltzmann-weighted sum over low energy spin
states. The result is of a universal form and coincides with that for a
Hermitian random matrix.
