Federated Learning is a distributed learning technique in which multiple clients cooperate to train a machine learning model. Distributed settings facilitate backdoor attacks by malicious clients, who can embed malicious behaviors into the model during their participation in the training process. These malicious behaviors are activated during inference by a specific trigger. No defense against backdoor attacks has stood the test of time, especially against adaptive attackers, a powerful but not fully explored category of attackers. In this work, we first devise a new adaptive adversary that surpasses existing adversaries in capabilities, yielding attacks that only require one or two malicious clients out of 20 to break existing state-of-the-art defenses. Then, we present Hammer and Anvil, a principled defense approach that combines two defenses orthogonal in their underlying principle to produce a combined defense that, given the right set of parameters, must succeed against any attack. We show that our best combined defense, Krum+, is successful against our new adaptive adversary and state-of-the-art attacks.

In the 1920s, the Soviet scientist Ilya Ivanovich Ivanov used artificial insemination to breed a'humanzee' – a cross between a human and our closest relative species, the chimpanzee. Given the moral quandaries a humanzee might create, we can be thankful that Ivanov failed: when the winds of Soviet scientific preferences changed, he was arrested and exiled. But Ivanov's endeavour points to the persistent, post-Darwinian fear and fascination with the question of whether humans are a creature apart, above all other life, or whether we're just one more animal in a mad scientist's menagerie. Humans have searched and repeatedly failed to rescue ourselves from this disquieting commonality. Numerous dividers between humans and beasts have been proposed: thought and language, tools and rules, culture, imitation, empathy, morality, hate, even a grasp of'folk' physics. But they've all failed, in one way or another. I'd like to put forward a new contender – strangely, the very same tendency that elicits the most dread and excitement among political and economic commentators today.

In the 1920s, the Soviet scientist Ilya Ivanovich Ivanov used artificial insemination to breed a'humanzee' – a cross between a human and our closest relative species, the chimpanzee. Given the moral quandaries a humanzee might create, we can be thankful that Ivanov failed: when the winds of Soviet scientific preferences changed, he was arrested and exiled. But Ivanov's endeavour points to the persistent, post-Darwinian fear and fascination with the question of whether humans are a creature apart, above all other life, or whether we're just one more animal in a mad scientist's menagerie. Humans have searched and repeatedly failed to rescue ourselves from this disquieting commonality. Numerous dividers between humans and beasts have been proposed: thought and language, tools and rules, culture, imitation, empathy, morality, hate, even a grasp of'folk' physics. But they've all failed, in one way or another. I'd like to put forward a new contender – strangely, the very same tendency that elicits the most dread and excitement among political and economic commentators today. We lost our exclusive position in the animal kingdom, not because we overestimated ourselves, but because we underestimated our cousins.