Fully Exploiting Every Real Sample: SuperPixel Sample Gradient Model Stealing
Authors: Yunlong Zhao, Xiaoheng Deng, Yijing Liu, Xinjun Pei, Jiazhi Xia, Wei Chen

Model stealing (MS) involves querying and observing the output of a machine learning model to steal its capabilities. The quality of queried data is crucial yet obtaining a large amount of real data for MS is often challenging. Recent works have reduced reliance on real data by using generative models. However when high-dimensional query data is required these methods are impractical due to the high costs of querying and the risk of model collapse. In this work we propose using sample gradients (SG) to enhance the utility of each real sample as SG provides crucial guidance on the decision boundaries of the victim model. However utilizing SG in the model stealing scenario faces two challenges: 1. Pixel-level gradient estimation requires extensive query volume and is susceptible to defenses. 2. The estimation of sample gradients has a significant variance. This paper proposes Superpixel Sample Gradient stealing (SPSG) for model stealing under the constraint of limited real samples. With the basic idea of imitating the victim model's low-variance patch-level gradients instead of pixel-level gradients SPSG achieves efficient sample gradient estimation through two steps. First we perform patch-wise perturbations on query images to estimate the average gradient in different regions of the image. Then we filter the gradients through a threshold strategy to reduce variance. Exhaustive experiments demonstrate that with the same number of real samples SPSG achieves accuracy agreements and adversarial success rate significantly surpassing the current state-of-the-art MS methods. Codes are available at https://github.com/zyl123456aB/SPSG_attack.