MimicDiffusion: Purifying Adversarial Perturbation via Mimicking Clean Diffusion Model
Authors: Kaiyu Song, Hanjiang Lai, Yan Pan, Jian Yin

Deep neural networks (DNNs) are vulnerable to adversarial perturbation where an imperceptible perturbation is added to the image that can fool the DNNs. Diffusion-based adversarial purification uses the diffusion model to generate a clean image against such adversarial attacks. Unfortunately the generative process of the diffusion model is also inevitably affected by adversarial perturbation since the diffusion model is also a deep neural network where its input has adversarial perturbation. In this work we propose MimicDiffusion a new diffusion-based adversarial purification technique that directly approximates the generative process of the diffusion model with the clean image as input. Concretely we analyze the differences between the guided terms using the clean image and the adversarial sample. After that we first implement MimicDiffusion based on Manhattan distance. Then we propose two guidance to purify the adversarial perturbation and approximate the clean diffusion model. Extensive experiments on three image datasets including CIFAR-10 CIFAR-100 and ImageNet with three classifier backbones including WideResNet-70-16 WideResNet-28-10 and ResNet-50 demonstrate that MimicDiffusion significantly performs better than the state-of-the-art baselines. On CIFAR-10 CIFAR-100 and ImageNet it achieves 92.67% 61.35% and 61.53% average robust accuracy which are 18.49% 13.23% and 17.64% higher respectively. The code is available at https://github.com/psky1111/MimicDiffusion.