We analyze various eavesdropping strategies on a quantum cryptographic
channel. We present the optimal strategy for an eavesdropper restricted to a
two-dimensional probe, interacting on-line with each transmitted signal. The
link between safety of the transmission and the violation of Bell's inequality
is discussed. We also use a quantum copying machine for eavesdropping and for
broadcasting quantum information.Comment: LaTex, 13 pages, with 6 Postscript figure
