  A simple proof of the unconditional security of a relativistic quantum
cryptosystem based on orthogonal states is proposed. Restrictions imposed by
special relativity allow to substantially simplify the proof compared with the
non-relativistic cryptosystems involving non-orthogonal states. Important for
the proposed protocol is the spatio-temporal structure of the quantum states
which is actually ignored in the non-relativistic protocols employing only the
structure of the state space of the information carriers. As a consequence, the
simplification arises because of the inefficiency of collective measurements
which constitute the major problem in the non-relativistic case.
