  Basic techniques to prove the unconditional security of quantum cryptography
are described. They are applied to a quantum key distribution protocol proposed
by Bennett and Brassard in 1984. The proof considers a practical variation on
the protocol in which the channel is noisy and photons may be lost during the
transmission. The initial coding into the channel must be perfect (i.e.,
exactly as described in the protocol). No restriction is imposed on the
detector used at the receiving side of the channel, except that whether or not
the received system is detected must be independent of the basis used to
measure this system.
