  Security of a continuous-variable quantum key distribution protocol based on
noisy coherent states and channel is analyzed. Assuming the noise of coherent
states is induced by Fred, a neutral party relative to others, we prove that
the prepare and measurement scheme and entanglement-based scheme are
equivalent. Then, we show that this protocol is secure against Gaussian
collective attacks even if the channel is lossy and noisy, and further, a lower
bound to the secure key rate is derived.
