Abstract
We propose a new type of attack for finding adversarial examples for image
classifiers. Our method exploits spanners, i.e. deep neural networks whose
input space is low-dimensional and whose output range approximates the set of
images of interest. Spanners may be generators of GANs or decoders of VAEs. The
key idea in our attack is to search over latent code pairs to find ones that
generate nearby images with different classifier outputs. We argue that our
attack is stronger than searching over perturbations of real images. Moreover,
we show that our stronger attack can be used to reduce the accuracy of
Defense-GAN to 3\%, resolving an open problem from the well-known paper by
Athalye et al. We combine our attack with normal adversarial training to obtain
the most robust known MNIST classifier, significantly improving the state of
the art against PGD attacks. Our formulation involves solving a min-max
problem, where the min player sets the parameters of the classifier and the max
player is running our attack, and is thus searching for adversarial examples in
the low-dimensional input space of the spanner.
All code and models are available at
https://github.com/ajiljalal/manifold-defense.git
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