  Coherent-one-way (COW) quantum key distribution (QKD) held the promise of
distributing secret keys over long distances with a simple experimental setup.
Indeed, this scheme is currently used in commercial applications. Surprisingly,
however, it has been recently shown that its secret key rate scales at most
quadratically with the system's transmittance and, thus, it is not appropriate
for long distance QKD transmission. Such pessimistic result was derived by
employing a so-called zero-error attack, in which the eavesdropper does not
introduce any error, but still the legitimate users of the system cannot
distill a secure key. Here, we present a zero-error attack against COW-QKD that
is essentially optimal, in the sense that no other attack can restrict further
its maximum achievable distance in the absence of errors. This translates into
an upper bound on its secret key rate that is more than an order of magnitude
lower than previously known upper bounds.
