  The surface charges associated with the symmetries of asymptotically flat
four dimensional spacetimes at null infinity are constructed. They realize the
symmetry algebra in general only up to a field-dependent central extension that
satisfies a suitably generalized cocycle condition. This extension vanishes
when using the globally well defined BMS algebra. For the Kerr black hole and
the enlarged BMS algebra with both supertranslations and superrotations, some
of the supertranslations charges diverge whereas there are no divergences for
the superrotation charges. The central extension is proportional to the
rotation parameter and involves divergent integrals on the sphere.
