  Measurement-device-independent quantum key distribution (MDI-QKD) can resist
all attacks on the detection devices, but there are still some security issues
related to the source side. One possible solution is to use the passive
protocol to eliminate the side channels introduced by active modulators at the
source. Recently, a fully passive QKD protocol has been proposed that can
simultaneously achieve passive encoding and passive decoy-state modulation
using linear optics. In this work, we propose a fully passive MDI-QKD scheme
that can protect the system from both side channels of source modulators and
attacks on the measurement devices, which can significantly improve the
implementation security of the QKD systems. We provide a specific passive
encoding strategy and a method for decoy-state analysis, followed by simulation
results for the secure key rate in the asymptotic scenario. Our work offers a
feasible way to improve the implementation security of QKD systems, and serves
as a reference for achieving passive QKD schemes using realistic devices.
