GEAR: A Margin-based Federated Adversarial Training Approach
Previous studies have shown that federated learning (FL) is vulnerable to well-crafted adversarial examples. Some recent efforts tried to combine adversarial training with FL, i.e., federated adversarial training (FAT), in order to achieve adversarial robustness in FL. However, most of the existing FAT works suffer from either low natural accuracy or low robust accuracy. Moreover, none of these works provide a more in-depth understanding of the challenges behind adversarial robustness in FL. To address these issues, we propose a novel marGin-based fEderated Adversarial tRaining Approach called GEAR. It encourages the minority classes to have larger margins by introducing a margin-based cross-entropy loss, and regularizes the decision boundary to be smooth by introducing a regularization loss, thus providing a better decision boundary for the global model. To the best of our knowledge, this work is the first to investigate the impact of decision boundary on FAT and delivers the best natural accuracy and robust accuracy in FL by far. Extensive experiments on multiple datasets across various settings all validate the effectiveness of our proposed method. For example, on SVHN dataset, GEAR can improve the natural accuracy and robust accuracy (against FGSM attack) of the best baseline method (FedTRADES) by 20.17\% and 10.73\%, respectively.
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