  A simplified model of particle transport at a quasiparallel one-dimensional
collisionless shock is suggested. In this model the MHD-turbulence behind the
shock is dominated by a circularly polarized, large amplitude Alfv\'en wave
originated upstream from the turbulence excited by particles leaking from the
downstream medium. It is argued that such a wave having significantly increased
its magnetic field during the transmission through the shock interface can
effectively trap thermal ions, regulating their leakage upstream. Together with
a background turbulence this wave also plays a fundamental role in
thermalization of the incoming ion flow. The spectrum of leaking particles and
the amplitude of the wave excited by these particles are selfconsistently
calculated. The injection rate into the first order Fermi acceleration based on
this leakage mechanism is obtained and compared with computer simulations. The
related problem of shock energy distribution between thermal and nonthermal
components of the shocked plasma is discussed. The chemical composition of the
leaking particles is studied.
