  Recently, many studies have demonstrated deep neural network (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. However, existing defense techniques
often require modifying the target model or depend on the prior knowledge of
attacks. In this paper, we propose a straightforward method for detecting
adversarial image examples, which can be directly deployed into unmodified
off-the-shelf DNN models. We consider the perturbation to images as a kind of
noise and introduce two classic image processing techniques, scalar
quantization and smoothing spatial filter, to reduce its effect. The image
entropy is employed as a metric to implement an adaptive noise reduction for
different kinds of images. Consequently, the adversarial example can be
effectively detected by comparing the classification results of a given sample
and its denoised version, without referring to any prior knowledge of attacks.
More than 20,000 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 experiments show that our detection method can achieve a
high overall F1 score of 96.39% and certainly raises the bar for defense-aware
attacks.
