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Title: Decision-Based Adversarial Attacks: Reliable Attacks Against Black-Box Machine Learning Models

Paper content:
Many machine learning algorithms are vulnerable to almost imperceptible perturbations of their inputs.
So far it was unclear how much risk adversarial perturbations carry for the safety of real-world machine learning applications because most methods used to generate such perturbations rely either on detailed model information (gradient-based attacks) or on confidence scores such as class probabilities (score-based attacks), neither of which are available in most real-world scenarios.
In many such cases one currently needs to retreat to transfer-based attacks which rely on cumbersome substitute models, need access to the training data and can be defended against.
Here we emphasise the importance of attacks which solely rely on the final model decision.
Such decision-based attacks are (1) applicable to real-world black-box models such as autonomous cars, (2) need less knowledge and are easier to apply than transfer-based attacks and (3) are more robust to simple defences than gradient- or score-based attacks.
Previous attacks in this category were limited to simple models or simple datasets.
Here we introduce the Boundary Attack, a decision-based attack that starts from a large adversarial perturbation and then seeks to reduce the perturbation while staying adversarial.
The attack is conceptually simple, requires close to no hyperparameter tuning, does not rely on substitute models and is competitive with the best gradient-based attacks in standard computer vision tasks like ImageNet.
We apply the attack on two black-box algorithms from Clarifai.com.
The Boundary Attack in particular and the class of decision-based attacks in general open new avenues to study the robustness of machine learning models and raise new questions regarding the safety of deployed machine learning systems.
An implementation of the attack is available as part of Foolbox (https://github.com/bethgelab/foolbox).
Many high-performance machine learning algorithms used in computer vision, speech recognition and other areas are susceptible to minimal changes of their inputs BID26 .
As a concrete example, a modern deep neural network like VGG-19 trained on object recognition might perfectly recognize the main object in an image as a tiger cat, but if the pixel values are only slightly perturbed in a specific way then the prediction of the very same network is drastically altered (e.g. to bus).
These so-called adversarial perturbations are ubiquitous in many machine learning models and are often imperceptible to humans.
Algorithms that seek to find such adversarial perturbations are generally denoted as adversarial attacks.Adversarial perturbations have drawn interest from two different sides.
On the one side, they are worrisome for the integrity and security of deployed machine learning algorithms such as autonomous cars or face recognition systems.
Minimal perturbations on street signs (e.g. turning a stop-sign into a 200 km/h speed limit) or street lights (e.g. turning a red into a green light) can have severe consequences.
On the other hand, adversarial perturbations provide an exciting spotlight on the gap between the sensory information processing in humans and machines and thus provide guidance towards more robust, human-like architectures.Adversarial attacks can be roughly divided into three categories: gradient-based, score-based and transfer-based attacks (cp. Figure 1 ).
Gradient-based and score-based attacks are often denoted as white-box and oracle attacks respectively, but we try to be as explicit as possible as to what information is being used in each category 1 .
A severe problem affecting attacks in all of these categories is that they are surprisingly straight-forward to defend against:• Gradient-based attacks.
Most existing attacks rely on detailed model information including the gradient of the loss w.r.t. the input.
Examples are the Fast-Gradient Sign Method (FGSM), the Basic Iterative Method (BIM) BID11 , DeepFool (MoosaviDezfooli et al., 2015) , the Jacobian-based Saliency Map Attack (JSMA) BID20 , Houdini BID5 and the Carlini & Wagner attack BID2 .
Defence: A simple way to defend against gradient-based attacks is to mask the gradients, for example by adding non-differentiable elements either implicitly through means like defensive distillation BID21 or saturated non-linearities BID18 , or explicitly through means like non-differentiable classifiers BID15 ).•
Score-based attacks. A
few attacks are more agnostic and only rely on the predicted scores (e.g. class probabilities or logits) of the model. On
a conceptual level these attacks use the predictions to numerically estimate the gradient. This
includes black-box variants of JSMA BID17 and of the Carlini & Wagner attack BID4 as well as generator networks that predict adversarials BID8 . Defence
: It is straight-forward to severely impede the numerical gradient estimate by adding stochastic elements like dropout into the model. Also,
many robust training methods introduce a sharp-edged plateau around samples BID28 which not only masks gradients themselves but also their numerical estimate.• Transfer-based
attacks. Transfer-based attacks
do not rely on model information but need information about the training data. This data is used to train
a fully observable substitute model from which adversarial perturbations can be synthesized BID22 . They rely on the empirical
observation that adversarial examples often transfer between models. If adversarial examples are
created on an ensemble of substitute models the success rate on the attacked model can reach up to 100% in certain scenarios BID13 . Defence: A recent defence method
against transfer attacks BID28 , which is based on robust training on a dataset augmented by adversarial examples from an ensemble of substitute models, has proven highly successful against basically all attacks in the 2017 Kaggle Competition on Adversarial Attacks 2 .The fact that many attacks can be
easily averted makes it often extremely difficult to assess whether a model is truly robust or whether the attacks are just too weak, which has lead to premature claims of robustness for DNNs BID3 .This motivates us to focus on a category
of adversarial attacks that has so far received fairly little attention:• Decision-based attacks. Direct attacks that solely rely on the final
decision of the model (such as the top-1 class label or the transcribed sentence).The delineation of this category is justified
for the following reasons: First, compared to score-based attacks decision-based attacks are much more relevant in real-world machine learning applications where confidence scores or logits are rarely accessible. At the same time decision-based attacks have
the potential to be much more robust to standard defences like gradient masking, intrinsic stochasticity or robust training than attacks from the other categories. Finally, compared to transferbased attacks they
need much less information about the model (neither architecture nor training data) and are much simpler to apply.There currently exists no effective decision-based attack that scales to natural datasets such as ImageNet and is applicable to deep neural networks (DNNs). The most relevant prior work is a variant of transfer
attacks in which the training set needed to learn the substitute model is replaced by a synthetic dataset (Papernot et al., 2017b) . This synthetic dataset is generated by the adversary
alongside the training of the substitute; the labels for each synthetic sample are drawn from the black-box model. While this approach works well on datasets for which
the intra-class variability is low (such as MNIST) it has yet to be shown that it scales to more complex natural datasets such as CIFAR or ImageNet. Other decision-based attacks are specific to linear
or convex-inducing classifiers BID6 BID14 BID19 and are not applicable to other machine learning models. The work by BID0 basically stands between transfer
attacks and decision-based attacks in that the substitute model is trained on a dataset for which the labels have been observed from the black-box model. This attack still requires knowledge about the data
distribution on which the black-box models was trained on and so we don't consider it a pure decision-based attack. Finally, some naive attacks such as a line-search along
a random direction away from the original sample can qualify as decision-based attacks but they induce large and very visible perturbations that are orders of magnitude larger than typical gradient-based, score-based or transfer-based attacks.Throughout the paper we focus on the threat scenario in which the adversary aims to change the decision of a model (either targeted or untargeted) for a particular input sample by inducing a minimal perturbation to the sample. The adversary can observe the final decision of the model
for arbitrary inputs and it knows at least one perturbation, however large, for which the perturbed sample is adversarial.The contributions of this paper are as follows:• We emphasise decision-based attacks as an important category of adversarial attacks that are highly relevant for real-world applications and important to gauge model robustness.• We introduce the first effective decision-based attack that
scales to complex machine learning models and natural datasets. The Boundary Attack is (1) conceptually surprisingly simple,
(2) extremely flexible, (3) requires little hyperparameter tuning and FORMULA6 is competitive with the best gradient-based attacks in both targeted and untargeted computer vision scenarios.• We show that the Boundary Attack is able to break previously
suggested defence mechanisms like defensive distillation.• We demonstrate the practical applicability of the Boundary Attack
on two black-box machine learning models for brand and celebrity recognition available on Clarifai.com.
In this paper we emphasised the importance of a mostly neglected category of adversarial attacksdecision-based attacks-that can find adversarial examples in models for which only the final decision can be observed.
We argue that this category is important for three reasons: first, attacks in this class are highly relevant for many real-world deployed machine learning systems like autonomous cars for which the internal decision making process is unobservable.
Second, attacks in this class do not rely on substitute models that are trained on similar data as the model to be attacked, thus making real-world applications much more straight-forward.
Third, attacks in this class have the potential to be much more robust against common deceptions like gradient masking, intrinsic stochasticity or robust training.We also introduced the first effective attack in this category that is applicable to general machine learning algorithms and complex natural datasets: the Boundary Attack.
At its core the Boundary Attack follows the decision boundary between adversarial and non-adversarial samples using a very simple rejection sampling algorithm in conjunction with a simple proposal distribution and a dynamic step-size adjustment inspired by Trust Region methods.
Its basic operating principlestarting from a large perturbation and successively reducing it-inverts the logic of essentially all previous adversarial attacks.
Besides being surprisingly simple, the Boundary attack is also extremely flexible in terms of the possible adversarial criteria and performs on par with gradient-based attacks on standard computer vision tasks in terms of the size of minimal perturbations.The mere fact that a simple constrained iid Gaussian distribution can serve as an effective proposal perturbation for each step of the Boundary attack is surprising and sheds light on the brittle information processing of current computer vision architectures.
Nonetheless, there are many ways in which the Boundary attack can be made even more effective, in particular by learning a suitable proposal distribution for a given model or by conditioning the proposal distribution on the recent history of successful and unsuccessful proposals.Decision-based attacks will be highly relevant to assess the robustness of machine learning models and to highlight the security risks of closed-source machine learning systems like autonomous cars.
We hope that the Boundary attack will inspire future work in this area.

assistant: text: A novel adversarial attack that can directly attack real-world black-box machine learning models without transfer.