  The security of quantum key distribution (QKD) relies on the Heisenberg
uncertainty principle, with which legitimate users are able to estimate
information leakage by monitoring the disturbance of the transmitted quantum
signals. Normally, the disturbance is reflected as bit flip errors in the
sifted key; thus, privacy amplification, which removes any leaked information
from the key, generally depends on the bit error rate. Recently, a round-robin
differential-phase-shift QKD protocol for which privacy amplification does not
rely on the bit error rate [Nature 509, 475 (2014)] was proposed. The amount of
leaked information can be bounded by the sender during the state-preparation
stage and hence, is independent of the behaviour of the unreliable quantum
channel. In our work, we apply the tagging technique to the protocol and
present a tight bound on the key rate and employ a decoy-state method. The
effects of background noise and misalignment are taken into account under
practical conditions. Our simulation results show that the protocol can
tolerate channel error rates close to 50% within a typical experiment setting.
That is, there is a negligible restriction on the error rate in practice.
