  The SARG04 protocol is one of the most frequently used protocol in commercial
plug-and-play quantum key distribution (QKD) system, where an eavesdropper can
completely control or change the photon number statistics of the QKD source. To
ensure the security of SARG04 protocol in plug-and-play QKD system with an
unknown and untrusted source, the bounds of a few statistical parameters of the
source need to be monitored. An active or a passive source monitor schemes are
proposed to verify these parameters. Furthermore, the practical issues due to
statistical fluctuation and detection noise in the source monitoring process
are quantitatively analyzed. Our simulation results show that the passive
scheme can be efficiently applied to plug-and-play system with SARG04 protocol.
