  We present a complete protocol for BB84 quantum key distribution for a
realistic setting (noise, loss, multi-photon signals of the source) that covers
many of todays experimental implementations. The security of this protocol is
shown against an eavesdropper having unrestricted power to manipulate the
signals coherently on their path from sender to receiver. The protocol and the
security proof take into account the effects concerning the finite size of the
generated key.
