The work in this paper is motivated by a privacy scenario in
which the identity of certain persons (represented as anonymous individuals) should be hidden. We assume that factual information about known
individuals (i.e., individuals whose identity is known) and anonymous
individuals is stored in an ABox and general background information is
expressed in a TBox, where both the TBox and the ABox are publicly
accessible. The identity problem then asks whether one can deduce from
the TBox and the ABox that a given anonymous individual is equal to a
known one. Since this would reveal the identity of the anonymous individual, such a situation needs to be avoided. We first observe that not all
Description Logics (DLs) are able to derive any such equalities between
individuals, and thus the identity problem is trivial in these DLs. We
then consider DLs with nominals, number restrictions, or function dependencies, in which the identity problem is non-trivial. We show that in
these DLs the identity problem has the same complexity as the instance
problem. Finally, we consider an extended scenario in which users with
different rôles can access different parts of the TBox and ABox, and we
want to check whether, by a sequence of rôle changes and queries asked
in each rôle, one can deduce the identity of an anonymous individual.