  To understand the origin of ultra-high energy cosmic rays (UHECRs, defined to
be above 10^18 eV), it is required to model in a realistic way their
propagation in the Universe. UHECRs can interact with low energy radio,
microwave, infrared and optical photons to produce electron/positron pairs or
pions. The latter decay and give rise to neutrinos and electromagnetic cascades
extending down to MeV energies. In addition, deflections in cosmic magnetic
fields can influence the spectrum and sky distribution of primary cosmic rays
and, due to the increased propagation path length, the secondary neutrino and
gamma-ray fluxes. Neutrino, gamma-ray, cosmic ray physics and extra-galactic
magnetic fields are, therefore, strongly linked subjects and should be
considered together in order to extract maximal information from existing and
future data, like the one expected from the Auger Observatory. For that
purpose, we have developed CRPropa, a publicly-available numerical package
which takes into account interactions and deflections of primary UHECRs as well
as propagation of secondary electromagnetic cascades and neutrinos. CRPropa
allows to compute the observable properties of UHECRs and their secondaries in
a variety of models for the sources and propagation of these particles. Here we
present physical processes taken into account as well as benchmark examples; a
detailed documentation of the code can be found on our web site.
