  We calculate the modulation by a magnetic field of the critical current of a
long disordered Josephson junction in the diffusive limit, i.e. when the
dimensions of the junction are larger that the elastic mean free path, and when
the length $L$ is much larger than the width $w$. Due to the averaging of the
gauge invariant phase factor over diffusive trajectories, the well-known
oscillations of the Fraunhofer pattern are smoothed out and replaced by an
exponential decay at large field. The predicted pattern is universal, i.e., it
is independent of the disorder strength. We point out an interesting relation
with the physics of speckle correlations in optics of turbid media.
