Multi-scale Diffusion Denoised Smoothing

Along with recent diffusion models, randomized smoothing has become one of a few tangible approaches that offers adversarial robustness to models at scale, e.g., those of large pre-trained models. Specifically, one can perform randomized smoothing on any classifier via a simple "denoise-and-classify" pipeline, so-called denoised smoothing, given that an accurate denoiser is available - such as diffusion model. In this paper, we investigate the trade-off between accuracy and certified robustness of denoised smoothing: for example, we question on which representation of diffusion model would maximize the certified robustness of denoised smoothing. We consider a new objective that aims collective robustness of smoothed classifiers across multiple noise levels at a shared diffusion model, which also suggests a new way to compensate the cost of accuracy in randomized smoothing for its certified robustness. This objective motivates us to fine-tune diffusion model (a) to perform consistent denoising whenever the original image is recoverable, but (b) to generate rather diverse outputs otherwise. Our experiments show that this fine-tuning scheme of diffusion models combined with the multi-scale smoothing enables a strong certified robustness possible at highest noise level while maintaining the accuracy closer to non-smoothed classifiers.