“…First, the defense can only be executed at the server side where only local gradients are available. This invalids many backdoor defense methods developed in centralized machine learning, for example, denoising (preprocessing) methods [33], [34], [35], [36], [37], backdoor sample/trigger detection methods [38], [39], [40], [41], [42], [43], robust data augmentations [44], and finetuning methods [44]. Second, the defense method has to be robust to both data poisoning and model poisoning attacks (e.g., Byzantine, backdoor and Sybil attacks).…”