  We consider the two-way quantum cryptographic protocol with coherent states
assuming direct reconciliation. A detailed security analysis is performed
considering a two-mode coherent attack, that represents the residual
eavesdropping once the parties have reduced the general attack by applying
symmetric random permutations. In this context we provide a general analytical
expression for the keyrate, discussing the impact of the residual two-mode
correlations on the security of the scheme. In particular we identify the
optimal eavesdropping against two-way quantum communication, which is given by
a two-mode coherent attack with symmetric and separable correlations.
