  We address the question of quantifying eavesdropper's information gain in an
individual attack on systems of quantum key distribution. It is connected with
the concept of conclusive eavesdropping introduced by Brandt. Using the BB84
protocol, we examine the problem of estimating a performance of conclusive
entangling probe. The question of interest depends on the choice of a
quantitative measure of eavesdropper's information about the error-free sifted
bits. The Fuchs--Peres--Brandt probe realizes a very powerful individual attack
on the BB84 scheme. In the usual formulation, Eve utilizes the Helstrom scheme
in distinguishing between the two output probe states. In conclusive
eavesdropping, the unambiguous discrimination is used. Comparing these two
versions allows to demonstrate serious distinctions between widely used
quantifiers of mutual information. In particular, the so-called R\'{e}nyi
mutual information does not seem to be a completely legitimate measure of an
amount of mutual information. It is brightly emphasized with the example of
conclusive eavesdropping.
