  Phase randomization is a very important assumption in the Bennett-Brassard
1984 quantum key distribution (QKD) system with a weak coherent source. Thus an
active phase modulator is needed to randomize the phase of source. However, it
is hard to check whether the phase of source is randomized totally or not in
practical QKD systems. In this paper a partially random phase attack is
proposed to exploit this imperfection. Our analysis shows that Eve can break
the security of a two-way QKD system by using our attack, even if an active
phase randomization is adopted by Alice. Furthermore, the numerical simulation
shows that in some parameter regimes, our attack is immune to the
one-decoy-state method.
