  A secret key shared through quantum key distribution between two cooperative
players is secure against any eavesdropping attack allowed by the laws of
physics. Yet, such a key can be established only when the quantum channel error
rate due to eavesdropping or imperfect apparatus is low. Here, I report a
practical quantum key distribution scheme making use of an adaptive privacy
amplification procedure with two-way classical communication. Then, I prove
that the scheme generates a secret key whenever the bit error rate of the
quantum channel is less than $0.5-0.1\sqrt{5} \approx 27.6%$, thereby making it
the most error resistant scheme known to date.
