A simple proof of unconditional security of relativistic quantum cryptography

A simple proof of the unconditional security of a relativistic quantum cryp tosystem based on orthogonal states is given. Limitations imposed by the sp ecial relativity theory allow the proof to be markedly simplified as compar ed to the case of nonrelativistic cryptosystems based on nonorthogonal stat es. An important point in the proposed protocol is a space-time structure o f the quantum states, which is ignored in the non-relativistic protocols us ing only the properties of the space of states of the information carriers. As a consequence, the simplification is related to the inefficacy of using the collective measurements against an eavesdropper, the allowance for whi ch is an especially difficult task in the nonrelativistic case. (C) 2001 MA IK "Nauka/Interperiodica".