  In this paper a higher-order mixed finite element method for elastoplasticity
with linear kinematic hardening is analyzed. Thereby, the non-differentiability
of the involved plasticity functional is resolved by a Lagrange multiplier
leading to a three field formulation. The finite element discretization is
conforming in the displacement field and the plastic strain but potentially
non-conforming in the Lagrange multiplier as its Frobenius norm is only
constrained in a certain set of Gauss quadrature points. A discrete inf-sup
condition with constant 1 and the well posedness of the discrete mixed problem
are shown. Moreover, convergence and guaranteed convergence rates are proved
with respect to the mesh size and the polynomial degree, which are optimal for
the lowest order case. Numerical experiments underline the theoretical results.
