  Given a boundary of spacetime preserved by a Diff(S^{1}) sub-algebra, we
propose a systematic method to compute the zero mode and the central extension
of the associated Virasoro algebra of charges. Using these values in the Cardy
formula, we may derive an associated statistical entropy to be compared with
the Bekenstein-Hawking result.
  To illustrate our method, we study in detail the BTZ and the rotating
Kerr-adS_{4} black holes (at spatial infinity and on the horizon). In both
cases, we are able to reproduce the area law with the correct factor of 1/4 for
the entropy. We also recover within our framework the first law of black hole
thermodynamics.
  We compare our results with the analogous derivations proposed by Carlip and
others. Although similar, our method differs in the computation of the zero
mode. In particular, the normalization of the ground state is automatically
fixed by our construction.
