  Diffusive shock acceleration (DSA) at relativistic shocks is expected to be
an important acceleration mechanism in a variety of astrophysical objects
including extragalactic jets in active galactic nuclei and gamma ray bursts.
These sources remain good candidate sites for the generation of ultra-high
energy cosmic rays. In this paper, key predictions of DSA at relativistic
shocks that are germane to production of relativistic electrons and ions are
outlined. The technique employed to identify these characteristics is a Monte
Carlo simulation of such diffusive acceleration in test-particle, relativistic,
oblique, magnetohydrodynamic (MHD) shocks. Using a compact prescription for
diffusion of charges in MHD turbulence, this approach generates particle
angular and momentum distributions at any position upstream or downstream of
the shock. Simulation output is presented for both small angle and large angle
scattering scenarios, and a variety of shock obliquities including superluminal
regimes when the de Hoffmann-Teller frame does not exist. The distribution
function power-law indices compare favorably with results from other
techniques. They are found to depend sensitively on the mean magnetic field
orientation in the shock, and the nature of MHD turbulence that propagates
along fields in shock environs. An interesting regime of flat spectrum
generation is addressed; we provide evidence for it being due to shock drift
acceleration, a phenomenon well-known in heliospheric shock studies. The impact
of these theoretical results on blazar science is outlined. Specifically,
Fermi-LAT gamma-ray observations of these relativistic jet sources are
providing significant constraints on important environmental quantities for
relativistic shocks, namely the field obliquity, the frequency of scattering
and the level of field turbulence.
