  We consider one of the quantum key distribution protocols recently introduced
in Ref. [Pirandola et al., Nature Physics 4, 726 (2008)]. This protocol
consists in a two-way quantum communication between Alice and Bob, where Alice
encodes secret information via a random phase-space displacement of a coherent
state. In particular, we study its security against a specific class of
individual attacks which are based on combinations of Gaussian quantum cloning
machines.
