  Quantum cryptographic protocols solve the longstanding problem of
distributing a shared secret string to two distant users by typically making
use of one-way quantum channel. However, alternative protocols exploiting
two-way quantum channel have been proposed for the same goal and with potential
advantages. Here we overview a security proof for two-way quantum key
distribution protocols, against the most general eavesdropping attack, that
utilize an entropic uncertainty relation. Then, by resorting to the `smooth'
version of involved entropies, we extend such a proof to the case of finite key
size. The results will be compared to those available for one-way protocols
showing some advantages.
