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About this paper

Ossi Räisä, Antti Koskela, Antti Honkela

Researchers at CISPA Helmholtz Center and the University of Helsinki proved that pure ex-post differential privacy is post-processing immune while its probabilistic relaxation is not, and introduced "alpha"-ex-post Rényi Differential Privacy (RDP) which guarantees post-processing immunity. The developed framework, including a data-dependent stopping rule, was applied to synthetic data generation and image classifier fine-tuning, demonstrating adaptive privacy budgeting with validated utility.

Key insights

  • Pure *ex-post* privacy holds post-processing immunity, confirming its robustness, while its "delta"-probabilistic relaxation (with "delta" > 0) does not, revealing a critical vulnerability in common relaxed *ex-post* definitions.
  • Making the privacy bound an explicit part of a mechanism's output allows for a rigorous definition and proof of post-processing immunity for *ex-post* differential privacy.
  • The Brownian mechanism, crucial for adaptive noise, satisfies the newly proposed "alpha"-ex-post RDP and its post-processing immunity, providing a robust tool for accuracy-first DP.

Summary by alphaXiv.

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