  All incoherent as well as 2- and 3-qubit coherent eavesdropping strategies on
the 6 state protocol of quantum cryptography are classified. For a disturbance
of 1/6, the optimal incoherent eavesdropping strategy reduces to the universal
quantum cloning machine. Coherent eavesdropping cannot increase Eve's Shannon
information, neither on the entire string of bits, nor on the set of bits
received undisturbed by Bob. However, coherent eavesdropping can increase as
well Eve's Renyi information as her probability of guessing correctly all bits.
The case that Eve delays the measurement of her probe until after the public
discussion on error correction and privacy amplification is also considered. It
is argued that by doing so, Eve gains only a negligibly small additional
information.
