Generalized photon-number-splitting attack on a coherent one-way quantum-key-distribution protocol

Quantum key distribution (QKD) offers an unprecedented level of security, which, unlike its classical analog, is not based on computational assumptions. But tight lower and upper security bounds are still not provided for some QKD protocols. Here, we propose an effective attack strategy for coherent one-way (COW) QKD protocol. It is a generalized photon-number splitting (PNS) attack combined with sequential unambiguous state discrimination (USD) attack. We discuss the possibility of a generalized PNS attack because this attack demonstrates efficiency and does not introduce error, while previously the COW protocol was considered secure against PNS attack. To our knowledge, it is the most efficient attack on the COW protocol to date, hence we sharpen the upper security bound for this protocol.