Incoherent and coherent eavesdropping in the six-state protocol of quantumcryptography

All incoherent as well as 2- and 3-qubit coherent eavesdropping strategies on the six-state protocol of quantum cryptography are classified. For a dis turbance of 1/6, the optimal incoherent eavesdropping strategy reduces to t he universal quantum cloning machine. Coherent eavesdropping cannot increas e Eve's Shannon information, neither on the entire string of bits, nor on t he set of bits received undisturbed by Bob. However, coherent eavesdropping can increase as well Eve's Renyi information as her probability of guessin g correctly all bits. The case that Eve delays the measurement of her probe until after the public discussion on error correction and privacy amplific ation is also considered. It is argued that by doing so, Eve gains only neg ligibly small additional information. [S1050-2947(99)08706-5].