  We calculate the energy spectra of cosmic ray (CR) protons and electrons at a
plane shock with quasi-parallel magnetic fields, using time-dependent,
diffusive shock acceleration (DSA) simulations, including energy losses via
synchrotron emission and Inverse Compton (IC) scattering. A thermal leakage
injection model and a Bohm type diffusion coefficient are adopted. The electron
spectrum at the shock becomes steady after the DSA energy gains balance the
synchrotron/IC losses, and it cuts off at the equilibrium momentum p_{eq}. In
the postshock region the cutoff momentum of the electron spectrum decreases
with the distance from the shock due to the energy losses and the thickness of
the spatial distribution of electrons scales as p^{-1}. Thus the slope of the
downstream integrated spectrum steepens by one power of p for p_{br}<p<p_{eq},
where the break momentum decrease with the shock age as p_{br}\propto t^{-1}.
In a CR modified shock, both the proton and electron spectrum exhibit a concave
curvature and deviate from the canonical test-particle power-law, and the
upstream integrated electron spectrum could dominate over the downstream
integrated spectrum near the cutoff momentum. Thus the spectral shape near the
cutoff of X-ray synchrotron emission could reveal a signature of nonlinear DSA.

High Energy Astrophysical Phenomena