  We present a method for determining the presence of an eavesdropper in QKD
systems without using any public bit comparison. Alice and Bob use a duplex QKD
channel and the bit transport technique for relays. The only information made
public is the respective basis choices which must be revealed in standard QKD
systems anyway. We find that every filtered bit can be used to determine the
presence of errors without compromising the security. This is an improvement on
using a random sample in the standard BB84 protocol.
