Detecting Adversarial Examples in Deep Networks with Adaptive Noise Reduction

Deep neural networks (DNNs) play a key role in many applications. Unsurprisingly, they also became a potential attack target of adversaries. Some studies have demonstrated DNN classifiers can be fooled by the adversarial example, which is crafted via introducing some perturbations into an original sample. Accordingly, some powerful defense techniques were proposed against adversarial examples. However, existing defense techniques require modifying the target model or depend on the prior knowledge of attack techniques to different degrees. In this paper, we propose a straightforward method for detecting adversarial image examples. It doesn't require any prior knowledge of attack techniques and can be directly deployed into unmodified off-the-shelf DNN models. Specifically, we consider the perturbation to images as a kind of noise and introduce two classical image processing techniques, scalar quantization and smoothing spatial filter, to reduce its effect. The image two-dimensional entropy is employed as a metric to implement an adaptive noise reduction for different kinds of images. As a result, the adversarial example can be effectively detected by comparing the classification results of a given sample and its denoised version. Thousands of adversarial examples against some state-of-the-art DNN models are used to evaluate the proposed method, which are crafted with different attack techniques. The experiment shows that our detection method can achieve an overall recall of 93.73% and an overall precision of 95.45% without referring to any prior knowledge of attack techniques.