  A quantum cipher supported by a secret key so called keyed communication in
quantum noise (KCQ) is very attractive in implementing high speed key
generation and secure data transmission. However, Yuen-2000 as a basic model of
KCQ has a difficulty to ensure the quantitative security evaluation because all
physical parameter for the cipher system is finite. Recently, an outline of a
generalized scheme so called coherent pulse position modulation(CPPM) to show
the rigorous security analysis is given, where the parameters are allowed to be
asymptotical. This may open a new way for the quantum key distribution with
coherent states of considerable energy and high speed.
  In this paper, we clarify a generation method of CPPM quantum signal by using
a theory of unitary operator and symplectic transformation, and show an
asymptotic property of security and its numerical examples.
