  Coherent-one-way quantum key distribution (COW-QKD), which requires a simple
experimental setup and has the ability to withstand photon-number-splitting
attacks, has been not only experimentally implemented but also commercially
applied. However, recent studies have shown that the current COW-QKD system is
insecure and can only distribute secret keys safely within 20 km of the optical
fiber length. In this study, we propose a practical implementation of COW-QKD
by adding a two-pulse vacuum state as a new decoy sequence. This proposal
maintains the original experimental setup as well as the simplicity of its
implementation. Utilizing detailed observations on the monitoring line to
provide an analytical upper bound on the phase error rate, we provide a
high-performance COW-QKD asymptotically secure against coherent attacks. This
ensures the availability of COW-QKD within 100 km and establishes theoretical
foundations for further applications.
