Phase randomization is a very important assumption in the Bennett-Brassard1984 quantum key distribution (QKD) system with a weak coherent source. Thus anactive phase modulator is needed to randomize the phase of source. However, itis hard to check whether the phase of source is randomized totally or not inpractical QKD systems. In this paper a partially random phase attack isproposed to exploit this imperfection. Our analysis shows that Eve can breakthe security of a two-way QKD system by using our attack, even if an activephase randomization is adopted by Alice. Furthermore, the numerical simulationshows that in some parameter regimes, our attack is immune to theone-decoy-state method.
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