The study of the security of Quantum Key Distribution (QKD) protocols has undergone rapid evolution during the last ten years. However, some misconceptions are still being published on the topic; in particular, misleading claims have been advanced following the implementation of a particular individual attack, the Slutsky-Brandt attack. This article clarifies why the implementation of such "optimal" attacks is in no contradiction with the established "old-style" theory of BB84 individual attacks, why experiments in general cannot be used to test security, and fills remaining gaps by finally providing tight security bounds for this class of ideal attacks. Extending and systematizing previous ideas, we show that the inference power of the eavesdropper is almost completely determined by the symmetry group of the protocol. Additionally, we emphasize the importance of the specific error reconciliation procedure, which actually determines the protocol. It is shown that leakage of error position is irrelevant (at least for individual attacks). This analysis also brings insight into the relation between the maximal collision probability resulting from an optimal measurement and the fidelity of the final states to be distinguished.

On the optimality of individual entangling-probe attacks against BB84 quantum key distribution, I.M. Herbauts, S. Bettelli, H. Hübel and M. Peev, Eur. Phys. J. D, 46, 395-406 (2008): [Abstract] | [PDF]