  In quantum key distribution(QKD), one can use a classical CSS code to distill
the final key. However, there is a constraint for the two codes in CSS code and
so far it is unknown how to construct a large CSS code efficiently. Here we
show that the BDSW method given by Bennett et al can be modified and the error
correction and privacy amplification can be done separately with two
INDEPENDENT parity matrices. With such a modification, BDSW method can be used
to distill the final key without any classical computational complexity. We
also apply the method to the case of imperfect source where a small fraction of
signals are tagged by Eve.
