Inducing Uncertainty for Test-Time Privacy

Unlearning is the predominant method for removing the influence of data in machine learning models. However, even after unlearning, models often continue to produce the same predictions on the unlearned data with high confidence. This persistent behavior can be exploited by adversaries using confident model predictions on incorrect or obsolete data to harm users. We call this threat model, which unlearning fails to protect against, *test-time privacy*. In particular, an adversary with full model access can bypass any naive defenses which ensure test-time privacy. To address this threat, we introduce an algorithm which perturbs model weights to induce maximal uncertainty on protected instances while preserving accuracy on the rest of the instances. Our core algorithm is based on finetuning with a Pareto optimal objective that explicitly balances test-time privacy against utility. We also provide a certifiable approximation algorithm which achieves $(\varepsilon, \delta)$ guarantees without convexity assumptions. We then prove a tight, non-vacuous bound that characterizes the privacy-utility tradeoff that our algorithms incur. Empirically, our method obtains $>3\times$ stronger uncertainty than pretraining with $<0.2\%$ drops in accuracy on various image recognition benchmarks. Altogether, this framework provides a tool to guarantee additional protection to end users.