  We designed high-efficiency error correcting codes allowing to extract an
errorless secret key in a continuous-variable quantum key distribution protocol
using a Gaussian modulation of coherent states and a homodyne detection. These
codes are available for a wide range of signal-to-noise ratios on an AWGN
channel with a binary modulation and can be combined with a multidimensional
reconciliation method proven secure against arbitrary collective attacks. This
improved reconciliation procedure considerably extends the secure range of a
continuous-variable quantum key distribution with a Gaussian modulation, giving
a secret key rate of about 10^{-3} bit per pulse at a distance of 120 km for
reasonable physical parameters.
