In this paper, we prove that the unconditionally secure key can be
surprisingly extracted from {\it multi}-photon emission part in the photon
polarization-based QKD. One example is shown by explicitly proving that one can
indeed generate an unconditionally secure key from Alice's two-photon emission
part in ``Quantum cryptography protocols robust against photon number splitting
attacks for weak laser pulses implementations'' proposed by V. Scarani {\it et
al.,} in Phys. Rev. Lett. {\bf 92}, 057901 (2004), which is called SARG04. This
protocol uses the same four states as in BB84 and differs only in the classical
post-processing protocol. It is, thus, interesting to see how the classical
post-processing of quantum key distribution might qualitatively change its
security. We also show that one can generate an unconditionally secure key from
the single to the four-photon part in a generalized SARG04 that uses six
states. Finally, we also compare the bit error rate threshold of these
protocols with the one in BB84 and the original six-state protocol assuming a
depolarizing channel.Comment: The title has changed again. We considerably improved our
presentation, and furthermore we proposed & analyzed a security of a modified
SARG04 protocol, which uses six state