  We discuss quantum key distribution protocols using quantum continuous
variables. We show that such protocols can be made secure against individual
gaussian attacks regardless the transmission of the optical line between Alice
and Bob. This is achieved by reversing the reconciliation procedure subsequent
to the quantum transmission, that is, using Bob's instead of Alice's data to
build the key. Although squeezing or entanglement may be helpful to improve the
resistance to noise, they are not required for the protocols to remain secure
with high losses. Therefore, these protocols can be implemented very simply by
transmitting coherent states and performing homodyne detection. Here, we show
that entanglement nevertheless plays a crucial role in the security analysis of
coherent state protocols. Every cryptographic protocol based on displaced
gaussian states turns out to be equivalent to an entanglement-based protocol,
even though no entanglement is actually present. This equivalence even holds in
the absence of squeezing, for coherent state protocols. This ``virtual''
entanglement is important to assess the security of these protocols as it
provides an upper bound on the mutual information between Alice and Bob if they
had used entanglement. The resulting security criteria are compared to the
separability criterion for bipartite gaussian variables. It appears that the
security thresholds are well within the entanglement region. This supports the
idea that coherent state quantum cryptography may be unconditionally secure.
