  Quantum key distribution protocols typically make use of a one-way quantum
channel to distribute a shared secret string to two distant users. However,
protocols exploiting a two-way quantum channel have been proposed as an
alternative route to the same goal, with the potential advantage of
outperforming one-way protocols. Here we provide a strategy to prove security
for two-way quantum key distribution protocols against the most general quantum
attack possible by an eavesdropper. We utilize an entropic uncertainty
relation, and only a few assumptions need to be made about the devices used in
the protocol. We also show that a two-way protocol can outperform comparable
one-way protocols.
