  Charge asymmetry in electron (positron) scattering arises from the
interference of the Born amplitude and the box-type amplitude corresponding to
two virtual photons exchange. It can be extracted from electron proton and
positron proton scattering experiments, in the same kinematical conditions.
Considering the virtual photon Compton scattering tensor, which contributes to
the box-type amplitude, we separate proton and inelastic contributions in the
intermediate state and parametrize the proton form-factors as the sum of a pure
QED term and a strong interaction term. Arguments based on analyticity are
given in favor of cancellation of contributions from proton strong interaction
form factors and of inelastic intermediate states in the box type amplitudes.
In frame of this model, with a realistic expression for nucleon form-factors,
numerical estimations are given for moderately high energies.
