On the Breidbart eavesdropping problem of the extended BB84 QKD protocol

We discuss the Breidbart eavesdropping scheme of the extended BB84 quantum key distribution protocol. Calculation of the effective average Alice/Eve mutual information after performing a standard error-correction under various intercept/resend strategies shows that the Breidbart eavesdropping/Breidbart resend strategy (B/B strategy) is the most effective one. Sine Alice and Bob can test openly whether there is the B/B eavesdropping by making use of the rejected data, we suggest an amendment of the BB84 protocol to reduce the requirements of the privacy amplification algorithm and hence reduce the quantum key loss. Finally, we present a quantum key regeneration method for error-correction which may be more secure than the standard error-correction process.