Athena: Probabilistic Verification of Machine Unlearning

The right to be forgotten, also known as the right to erasure, is the right of individuals to have their data erased from an entity storing it. The status of this long held notion was legally solidified recently by the General Data Protection Regulation (GDPR) in the European Union. As a consequence, there is a need for mechanisms whereby users can verify if service providers comply with their deletion requests. In this work, we take the first step in proposing a formal framework, called Athena, to study the design of such verification mechanisms for data deletion requests – also known as machine unlearning – in the context of systems that provide machine learning as a service (MLaaS). Athena allows the rigorous quantification of any verification mechanism based on hypothesis testing. Furthermore, we propose a novel verification mechanism that leverages backdoors and demonstrate its effectiveness in certifying data deletion with high confidence, thus providing a basis for quantitatively inferring machine unlearning. We evaluate our approach over a range of network architectures such as multi-layer perceptrons (MLP), convolutional neural networks (CNN), residual networks (ResNet), and long short-term memory (LSTM) and over 6 different datasets. We demonstrate that: (1) our approach has minimal effect on the accuracy of the ML service but provides high confidence verification of unlearning, even if multiple users employ our system to ascertain compliance with data deletion requests, and (2) our mechanism is robust against servers deploying state-of-the-art backdoor defense methods. Overall, our approach provides a foundation for a quantitative analysis of verifying machine unlearning, which can provide support for legal and regulatory frameworks pertaining to users’ data deletion requests.