Publications

2025

1. arXiv

   

   Unified Privacy Guarantees for Decentralized Learning via Matrix Factorization

   Aurélien Bellet, Edwige Cyffers, Davide Frey, Romaric Gaudel, Dimitri Lerévérend, and François Taı̈ani

   arXiv preprint arXiv:2510.17480, 2025

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   Decentralized Learning (DL) enables users to collaboratively train models without sharing raw data by iteratively averaging local updates with neighbors in a network graph. This setting is increasingly popular for its scalability and its ability to keep data local under user control. Strong privacy guarantees in DL are typically achieved through Differential Privacy (DP), with results showing that DL can even amplify privacy by disseminating noise across peer-to-peer communications. Yet in practice, the observed privacy-utility trade-off often appears worse than in centralized training, which may be due to limitations in current DP accounting methods for DL. In this paper, we show that recent advances in centralized DP accounting based on Matrix Factorization (MF) for analyzing temporal noise correlations can also be leveraged in DL. By generalizing existing MF results, we show how to cast both standard DL algorithms and common trust models into a unified formulation. This yields tighter privacy accounting for existing DP-DL algorithms and provides a principled way to develop new ones. To demonstrate the approach, we introduce MAFALDA-SGD, a gossip-based DL algorithm with user-level correlated noise that outperforms existing methods on synthetic and real-world graphs.

   @article{bellet2025unified,
     title = {Unified Privacy Guarantees for Decentralized Learning via Matrix Factorization},
     author = {Bellet, Aur{\'e}lien and Cyffers, Edwige and Frey, Davide and Gaudel, Romaric and Ler{\'e}v{\'e}rend, Dimitri and Ta{\"\i}ani, Fran{\c{c}}ois},
     journal = {arXiv preprint arXiv:2510.17480},
     year = {2025},
     keywords = {Decentralized learning, Differential privacy, Correlated noises},
     url = {https://arxiv.org/abs/2510.17480},
   }

2. PETS

   

   Low-Cost Privacy-Preserving Decentralized Learning

   Sayan Biswas, Davide Frey, Romaric Gaudel, Anne-Marie Kermarrec, Dimitri Lerévérend, Rafael Pires, Rishi Sharma, and François Taïani

   Proceedings on Privacy Enhancing Technologies, 2025

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   This paper introduces ZIP-DL, a novel privacy-aware decentralized learning (DL) algorithm that relies on adding correlated noise to each model update during the model training process. This technique ensures that the added noise almost neutralizes itself during the aggregation process due to its correlation, thus minimizing the impact on model accuracy. In addition, ZIP-DL does not require multiple communication rounds for noise cancellation, addressing the common trade-off between privacy protection and communication overhead. We provide theoretical guarantees for both convergence speed and privacy guarantees, thereby making ZIP-DL applicable to practical scenarios. Our extensive experimental study shows that ZIP-DL achieves the best trade-off between vulnerability and accuracy. In particular, ZIP-DL (i) reduces the effectiveness of a linkability attack by up to 52 points compared to baseline DL, and (ii) achieves up to 37 more accuracy points for the same vulnerability under membership inference attacks against a privacy-preserving competitor

   @article{biswasLowCostPrivacyPreservingDecentralized2025,
     title = {Low-{{Cost Privacy-Preserving Decentralized Learning}}},
     author = {Biswas, Sayan and Frey, Davide and Gaudel, Romaric and Kermarrec, Anne-Marie and Ler{\'e}v{\'e}rend, Dimitri and Pires, Rafael and Sharma, Rishi and Ta{\"i}ani, Fran{\c c}ois},
     year = {2025},
     journal = {Proceedings on Privacy Enhancing Technologies},
     volume = {2025},
     number = {3},
     pages = {451--474},
     issn = {2299-0984},
     doi = {10.56553/popets-2025-0108},
     urldate = {2025-05-20},
     url = {https://petsymposium.org/popets/2025/popets-2025-0108.php},
     keywords = {Decentralized learning, Differential privacy, Correlated noises},
   }