  We show that the diffeomorphisms, which preserve the null nature for a
generic null metric very near to the null surface, provide {\it noncommutative}
Heisenberg algebra. This is the generalization of the earlier work (Phys. Rev.
D95, 044020 (2017)) \cite{Majhi:2017fua}, done for the Rindler horizon. The
present analysis revels that the algebra is very general as it is obtained for
a generic null surface and is applicable for any spacetime horizon. Finally
using these results, the entropy of the null surface is derived in the form of
the Cardy formula. Our analysis is completely {\it off-shell} as no equation of
motion is used. We believe present discussion can illuminate the paradigm of
`gravity as an emergent phenomenon' and could be a candidate to probe the
origin of gravitational entropy.
