  Experimental Quantum Key Distribution (QKD) protocols have to consist of not
only the unconditionally secure quantum transmission, but also a subsequent
classical exchange that enables key reconciliation and error correction. There
is a large body of work examining quantum attacks on the quantum channel, but
here we begin to examine classical attacks to both the classical communication
and the exchange as a whole. Linking together separate secure protocols can
unexpectedly leak information to an eavesdropper, even if the components are
unconditionally secure in isolation. Here we focus specifically on the join
between quantum and classical protocols, finding that in just this crossing of
the quantum-classical boundary, some security is always and unintuitively lost.
This occurs with no communication between the separate parties. While this
particular example applies to only Continuous Variable Quantum Key Distribution
(CVQKD), it highlights the need to re-examine the way all individual protocols
are actually used.
