  The coherent one-way (COW) quantum key distribution (QKD) is a highly
practical quantum communication protocol that is currently deployed in
off-the-shelves products. However, despite its simplicity and widespread use,
the security of COW-QKD is still an open problem. This is largely due to its
unique security feature based on inter-signal phase distribution, which makes
it very difficult to analyze using standard security proof techniques. Here, to
overcome this problem, we present a simple variant of COW-QKD and prove its
security in the infinite-key limit. The proposed modifications only involve an
additional vacuum tail signal following every encoded signal and a balanced
beam-splitter for passive measurement basis choice. Remarkably, the resulting
key rate of our protocol is comparable with both the existing upper-bound on
COW-QKD key rate and the secure key rate of the coherent-state BB84 protocol.
Our findings therefore suggest that the secured deployment of COW-QKD systems
in high loss optical networks is indeed feasible with minimal adaptations
applied to its hardware and software.
