  Deep Neural Networks (DNNs) are widely used for computer vision tasks.
However, it has been shown that deep models are vulnerable to adversarial
attacks, i.e., their performances drop when imperceptible perturbations are
made to the original inputs, which may further degrade the following visual
tasks or introduce new problems such as data and privacy security. Hence,
metrics for evaluating the robustness of deep models against adversarial
attacks are desired. However, previous metrics are mainly proposed for
evaluating the adversarial robustness of shallow networks on the small-scale
datasets. Although the Cross Lipschitz Extreme Value for nEtwork Robustness
(CLEVER) metric has been proposed for large-scale datasets (e.g., the ImageNet
dataset), it is computationally expensive and its performance relies on a
tractable number of samples. In this paper, we propose the Adversarial
Converging Time Score (ACTS), an attack-dependent metric that quantifies the
adversarial robustness of a DNN on a specific input. Our key observation is
that local neighborhoods on a DNN's output surface would have different shapes
given different inputs. Hence, given different inputs, it requires different
time for converging to an adversarial sample. Based on this geometry meaning,
ACTS measures the converging time as an adversarial robustness metric. We
validate the effectiveness and generalization of the proposed ACTS metric
against different adversarial attacks on the large-scale ImageNet dataset using
state-of-the-art deep networks. Extensive experiments show that our ACTS metric
is an efficient and effective adversarial metric over the previous CLEVER
metric.
