  Shor and Preskill have provided a simple proof of security of the standard
quantum key distribution scheme by Bennett and Brassard (BB84) by demonstrating
a connection between key distribution and entanglement purification protocols
with one-way communications. Here we provide proofs of security of standard
quantum key distribution schemes, BB84 and the six-state scheme, against the
most general attack, by using the techniques of *two*-way entanglement
purification. We demonstrate clearly the advantage of classical post-processing
with two-way classical communications over classical post-processing with only
one-way classical communications in QKD. This is done by the explicit
construction of a new protocol for (the error correction/detection and privacy
amplification of) BB84 that can tolerate a bit error rate of up to 18.9%, which
is higher than what any BB84 scheme with only one-way classical communications
can possibly tolerate. Moreover, we demonstrate the advantage of the six-state
scheme over BB84 by showing that the six-state scheme can strictly tolerate a
higher bit error rate than BB84. In particular, our six-state protocol can
tolerate a bit error rate of 26.4%, which is higher than the upper bound of 25%
bit error rate for any secure BB84 protocol. Consequently, our protocols may
allow higher key generation rate and remain secure over longer distances than
previous protocols. Our investigation suggests that two-way entanglement
purification is a useful tool in the study of advantage distillation, error
correction, and privacy amplification protocols.
