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Efficient Semi-Supervised Adversarial Training without Guessing Labels

Adversarial training has been proved to be the most effective defensive strategy to protect models from adversarial attacks. In the practical application scenario of adversarial training, we face not only labeled data, but also an enormous amount of unlabeled data. However, existing adversarial training methods are naturally targeting supervised learning problems. To adapt to semi-supervised learning problems, they need to estimate labels for unlabeled data in advance, which inevitably degenerates the performance of the learned model due to the bias on the estimation of labels for unlabeled data. To mitigate this degeneration, in this paper, we propose a new semi-supervised adversarial training framework via maximizing AUCs which is also a minimax problem but treats the unlabeled samples as both positive and negative ones, so that we do not need to guess the labels for unlabeled data. Unsurprisingly, the minimax problem can be solved via the traditional adversarial training algorithm by extending singly stochastic gradients to triply stochastic gradients, to adapt to the three (i.e. positive, negative, and unlabeled) data sources. To further accelerate the training procedure, we transform the minimax adversarial training problem into an equivalent minimization one based on the kernel perspective. For the minimization problem, we discuss scalable and efficient algorithms not only for deep neural networks but also for kernel support vector machines. Extensive experimental results show that our algorithms not only achieve better generalization performance against various adversarial attacks, but also enjoy efficiency and scalability when considered from the kernel perspective.

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