Individual attacks with generalized discrimination and inadequacy of some information measures

We accomplish studies of properly quantifying eavesdropper's information gain in individual attacks on the BB84 system of quantum key distribution. A noticeable sensitivity of conclusions to the choice of information measures is brightly revealed, when generalized state discrimination is used at the last stage. To realize her aims, Eve intrudes into the communication channel with some entangling probe. The Fuchs-Peres-Brandt (FPB) probe is known as a powerful individual attack on the BB84 protocol. In the simplest formulation, Eve uses the Helstrom scheme to distinguish output probe states. In conclusive eavesdropping, the unambiguous discrimination is utilized. In the intermediate scenario, she can apply a generalized discrimination scheme that interpolates between the Helstrom and unambiguous discrimination schemes. We analyze Eve's probe performance from the viewpoint of various measures for quantifying mutual information.