  We consider a subcarrier wave quantum key distribution (QKD) system, where
the quantum en- coding is carried by weak sidebands generated to a coherent
optical beam by means of an electrooptic phase modulation. We study the
security of two protocols, B92 and BB84, against one of the most powerful
attacks on the systems of this class: the collective beam splitting attack. We
show that a subcarrier wave QKD system with realistic parameters is capable of
distributing a cryptographic key over large distances. We show also that a
modification of the BB84 protocol, with discrimina- tion of only one state in
each basis, performs not worse than the original BB84 protocol for this class
of QKD systems, which brings a significant simplification to the development of
cryprographic networks on the basis of considered technique.
