  Unambiguous state discrimination (USD) is one of the major obstacles for
practical quantum key distribution (QKD). Often overlooked, it allows efficient
eavesdropping in majority of practical systems, provided the overall channel
loss is above a certain threshold. Thus, to remain secure all such systems must
not only monitor the actual loss, but also possess a comprehensive information
on the safe 'loss vs. BER' levels, which is often well beyond currently known
security analyses. The more advanced the protocol the tougher it becomes to
find and prove corresponding bounds. To get out of this vicious circle and
solve the problem outright, we demonstrate a so called relativistic QKD system,
which uses causality to become inherently immune to USD-based attacks. The
system proves to be practical in metropolitan line-of-sight arrangements. At
the same time it has a very basic structure that allows for a straightforward
and comprehensive security analysis.
