  Understanding hadronic interactions at high energies represents a very
important ingredient for modeling high energy cosmic ray air showers. In this
paper, we present a new approach to simulate hadronic interactions, including
hadron-hadron, hadron-nucleus, and nucleus-nucleus scattering, in the energy
range roughly between 10^11 and 10^17 eV. Such collisions are very complex,
being composed of many components, and therefore some strategy is needed to
construct a reliable model. The central point of our approach is the
hypothesis, that the behavior of high energy interactions is universal
(universality hypothesis). So, for example, the hadronization of partons in
nuclear interactions follows the same rules as the one in electron-positron
annihilation ; the radiation of off-shell partons in hadronic collisions is
based on the same principles as the one in deep inelastic scattering. We
construct a model for hadronic interactions in a modular fashion. The
individual modules, based on the universality hypothesis, are identified as
building blocks for more elementary interactions (like e^+ e^-, lepton-proton),
and can therefore be studied in a much simpler context. With these building
blocks under control, we can provide a quite reliable model for
nucleus-nucleus, hadron-nucleus and hadron-hadron scattering, providing in
particular very useful tests for the complicated numerical procedures using
Monte Carlo techniques.
