  A precise security analysis of practical quantum key distribution (QKD)
systems is an important step for improving their performance. Here we consider
a class of quantum soft filtering operations, which generalizes the unambiguous
state discrimination (USD) technique. These operations can be applied as a
basis for a security analysis of the original coherent one-way (COW) QKD
protocol since their application interpolates between beam-splitting (BS) and
USD attacks. We demonstrate that a zero-error attack based on quantum soft
filtering operations gives a larger amount of the information for Eve at a
given level of losses. We calculate the Eve information as a function of the
channel length. The efficiency of the proposed attack highly depends on the
level of the monitoring under the maintenance of the statistics of control
(decoy) states, and best-case results are achieved in the case of the absence
of maintenance of control state statistics. Our results form additional
requirements for the analysis of practical QKD systems based on the COW QKD
protocol and its variants by providing an upper bound on the security.
