Li et al. propose an adversarial attack motivated by second-order optimization and uses input randomization as defense. Based on a Taylor expansion, the optimal adversarial perturbation should be aligned with the dominant eigenvector of the Hessian matrix of the loss. As the eigenvectors of the Hessian cannot be computed efficiently, the authors propose an approximation; this is mainly based on evaluating the gradient under Gaussian noise. The gradient is then normalized before taking a projected gradient step. As defense, the authors inject random noise on the input (clean example or adversarial example) and compute the average prediction over multiple iterations.
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