  We put forward a new approach based on Green's function formalism to evaluate
precisely persistent charge and spin currents in an Aharonov-Bohm ring
subjected to Rashba and Dresselhaus spin-orbit interactions. Unlike
conventional methods our present scheme circumvents direct evaluation of
eigenvalues and eigenstates of the system Hamiltonian to determine persistent
currents which essentially reduces possible numerical errors, especially for
larger rings. The interplay of Aharonov-Bohm flux and spin-orbit interactions
in persistent charge and spin currents of quantum rings is analyzed in detail
and our results lead to a possibility of estimating the strength of any one of
the spin-orbit fields provided the other one is known. All these features are
exactly invariant even in presence of impurities, and therefore, can be
substantiated experimentally.
