Transparency and explainability are two important aspects to be considered
when employing black-box machine learning models in high-stake applications.
Providing counterfactual explanations is one way of catering this requirement.
However, this also poses a threat to the privacy of the institution that is
providing the explanation, as well as the user who is requesting it. In this
work, we are primarily concerned with the user's privacy who wants to retrieve
a counterfactual instance, without revealing their feature vector to the
institution. Our framework retrieves the exact nearest neighbor counterfactual
explanation from a database of accepted points while achieving perfect,
information-theoretic, privacy for the user. First, we introduce the problem of
private counterfactual retrieval (PCR) and propose a baseline PCR scheme that
keeps the user's feature vector information-theoretically private from the
institution. Building on this, we propose two other schemes that reduce the
amount of information leaked about the institution database to the user,
compared to the baseline scheme. Second, we relax the assumption of mutability
of all features, and consider the setting of immutable PCR (I-PCR). Here, the
user retrieves the nearest counterfactual without altering a private subset of
their features, which constitutes the immutable set, while keeping their
feature vector and immutable set private from the institution. For this, we
propose two schemes that preserve the user's privacy information-theoretically,
but ensure varying degrees of database privacy. Third, we extend our PCR and
I-PCR schemes to incorporate user's preference on transforming their
attributes, so that a more actionable explanation can be received. Finally, we
present numerical results to support our theoretical findings, and compare the
database leakage of the proposed schemes.