  We investigate the security against collective attacks of a continuous
variable quantum key distribution scheme in the asymptotic key limit for a
realistic setting. The quantum channel connecting the two honest parties is
assumed to be lossy and imposes Gaussian noise on the observed quadrature
distributions. Secret key rates are given for direct and reverse reconciliation
schemes including postselection in the collective attack scenario. The effect
of a non-ideal error correction and two-way communication in the classical
post-processing step is also taken into account.
