1. Introduction
Deep neural networks are vulnerable to adversarial perturbations: “imperceptibly small” (measured via a dissimilarity metric) changes in the model input lead to misclassification [30, 13]. The existence of small norm adversarial attacks can be interpreted to mean that while models generalize well on natural images, and are robust to random perturbations, nevertheless they lose accuracy on worstcase perturbations. The vulnerability of DNNs is a potentially grave security risk. In developing strong perturbations, we hope that better defense mechanisms will be deployed to prevent these attacks from occurring in practice.
Adversarial attacks are often broadly categorized into one of two types: whitebox attacks, where the full structure of the neural network is provided to the attacker, including gradient information, or blackbox attacks, where the attacker is only given the model decision. One of the first proposed adversarial attacks is the Fast Gradient Signed Method (FGSM), which generates an adversarial image with respect to the norm, along with its iterative form, called Iterative FGSM (IFGSM) [13, 17]. A similar iterative attack was also done with respect to the
norm. In their purest form, the above attacks perform gradient ascent on the loss function subject to a norm constraint on the perturbation, either with one step in the case of FGSM and multiple steps in the case of IFGSM, and the
norm equivalent. Apart from loss maximization, attacks have been developed using loss functions that directly measure misclassification [5, 22]. There is also the problem of generating “realistic” attacks, such as through sparse attacks. These include for example, small stickers on a road sign, which may tamper with autonomous vehicles [10]. In the blackbox setting, adversarial examples are generated using only model outputs or model decisions, which is a much more expensive endeavor. However, blackbox methods can sometimes perform better, most notably by avoiding gradient obfuscation, and take advantage of sampling properties near the decision boundary. Notable examples of blackbox (decisionbased) attacks are the Boundary Attack [4] and the recent HopSkipJumpAttack [6].The development of new and improved adversarial attacks has occurred in parallel with various defensive training regiments to provide robustness against adversarial perturbations. The task of training a robust network is twofold: models must be resistant to perturbations of a certain magnitude, while also maintaining classification ability on clean data. It has been argued that these two objectives are inherently “at odds”
[31]. A popular method for training robust networks is adversarial training, where adversarial examples are added to the training data (see for example [19]). While effective, adversarial training has not scaled well with increasing network size. Two recent methods have tackled this problem for large networks on ImageNet1k [29, 11]; this was previously not possible without a massive computational infrastructure [32, 15].Contributions
This paper introduces an attack methodology catered for not just norms, but any adversarial dissimilarity metric with a closed proximal form. This includes, but is not limited to, perturbations and the counting “norm”, i.e. a true measurement of sparseness of the perturbation. Our approach adopts the relaxation structure of the recently proposed LogBarrier attack [12], which required differentiable metrics. We extend this work to include a broad class of nonsmooth (nondifferentiable) metrics. Our algorithm, ProxLogBarrier, uses the proximal gradient method for generating adversarial perturbations. We demonstrate our attack on MNIST, CIFAR10, and ImageNet1k datasets. ProxLogBarrier shows significant improvement over both the LogBarrier attack, and over the other attacks we considered. In particular, in the case, we achieve stateoftheart results with respect to a suite of attacks typically used for this problem class.
2. Problem formulation of adversarial attacks and background
Let be the image space, and be the label space (the unitsimplex for classes). An imagelabel pair is defined by , with the image belonging to one of classes. The trained model is defined by . An adversarial perturbation is supposed to be small with respect to a dissimilarity metric (henceforth simply called a metric) , e.g. . Formally, the optimal adversarial perturbation is the minimizer of the following constrained optimization problem:
(1) 
where is the classification function for the trained network. When using the crossentropy classification function, problem (1) can be written as
(2) 
DNNs might be powerful classifiers, but that does not mean their decision boundaries are wellbehaved. Instead, researchers have popularized using the crossentropy loss as a surrogate for the decision boundary: typically a model is trained until the loss is very low, which is often related to good classification performance. Thus, instead of solving (
1), one can perform Projected Gradient Ascent (PGA) on the crossentropy loss:(3) 
where is typically taken to be either the or
norm, and is defines the perturbation threshold of interest.
Some adversarial attack methods try to solve the problem posed in (1
) without incorporating the loss function used to train the network. For example, Carlini & Wagner attacked the logitlayer of a network and solves a different optimization problem, depending on the choice of norm
[5]. With regards to adversarial defense, they demonstrated how a significant number of adversarial defense methods fail because of “gradient obfuscation”, where gradients are small only locally to the image. However, this is in fact an artifact of the softmax layer
[1]. Another metric of adversarial dissimilarity is the “norm”, which counts the number of total different pixels between the adversary and the clean image [21, 24]. This is of interest because an adversary might have to also budget the number of allowed pixels to perturb, while still remaining “imperceptible” to the human eye. For example, the stickerattack [10] is a practical attack with realworld consequences, and does not interfere with every single part of the image.3. Our method: ProxLogBarrier
We consider the formulation of adversarial attacks as in (2),
(4) 
Here, we abbreviate as the model output before the softmax layer that “projects” onto . This problem is difficult as the constraints have virtually no exploitable structure. The problem can be relaxed using a logarithmic barrier, a technique often used in traditional optimization [23],
(5) 
This objective function now includes the constraint that enforces
misclassification. In [12], (5) was originally
solved via gradient descent, which necessarily assumes that is at least differentiable. Most dissimilarity metrics are not differentiable; for example the norm. In the original LogBarrier paper [12], a smooth approximation of this norm was used to get around this issue.
For brevity, let , then optimization problem (5) becomes
(6) 
This relaxed problem has a composite structure, with being smooth provided . We turn to the proximal gradient method to efficiently solve this problem and outline this method in the following section. Instead of requiring to be differentiable, we only require a closed proximal form. This assumption is satisfied for most of the dissimilarity metrics considered in the adversarial attack literature.
3.1. Proximal gradient method
Proximal algorithms are a driving force for nonsmooth optimization problems, and are receiving more attention in the deep learning community on a myriad of problems
[2, 34, 20, 25]. For a full discussion on this topic, we suggest [3].We consider the following framework for proximal algorithms, namely a composite minimization problem
(7) 
where is a Euclidean space. We make the following assumptions:

is a nondegenerate, closed, and convex function over

is nondegenerate, closed function, with convex, and has Lipschitz gradients over the interior of its domain


the solution set, , is nonempty.
Solving this composite problem with gradient descent is not advisable, since is not necessarily differentiable. The best one can hope for is that has a subgradient at , defined as an element such that
(8) 
The collection of subgradients of is called the subdifferential of , denoted by . When a function is differentiable, the subdifferential is a singleton, namely . One could turn to subgradient descent to solve the composite problem, however a subgradient might not always be helpful. For example, consider the subgradient of for an element in ;
where , and
are the standard basis vectors. At each subgradient step, very little information is obtained.
Since is a nonconvex problem (because is potentially not convex), our goal is to iteratively generate a sequence that converges to , where is a stationary point i.e. . A characterization of these stationary points is the following fixedpoint representation (we take ):
where is defined as the proximal operator of
(9) 
The first line in the equivalence chain uses addition of subdifferentiability, which is guaranteed by our assumptions, and the rest is algebraic manipulation. Thus, to generate a stationary point, it suffices to find a fixed point of the sequence generated in the following manner:
(10) 
where is some step size. The proximal operator exists for any convex
function, but this is not a strict requirement.
Despite not being convex, there are still convergence properties of the sequence of iterates generated in this way. The following theorem is a simplified version of what can be found in [3] (Section 10.3 with proof), and is the main motivation for our proposed method.
3.2. ProxLogBarrier attack algorithm
We iteratively find a minimizer for (6); the attack is outlined in Algorithm 1. Due to the highly nonconvex nature of the decision boundary, we perform a backtracking step to ensure the proposed iterate is in fact adversarial, thus making smooth. We remark that the adversarial attack problem is constrained by the imagespace, and thus requires a further projection step back onto the image space (we consider pixels to be in the range [0,1]). In traditional nonconvex optimization, best practice is to also record the “best iterate”, as valleys are likely pervasive throughout the decision boundary. This way, even at some point our gradient sends the iterate faroff and is unable to return in the remaining iterations, we already have a better candidate.
Proximal operators for metrics of interest
To complete the algorithm, it remains to compute the proximal operator for various choices of . One can turn to [3] for complete derivations of the proximal operators for the adversarial metrics we are considering, namely norms, and the cardinality function. Consider measuring the distance between the clean image and our desired adversarial perturbation:
Due to the Moreau Decomposition Theorem [27], the proximal operator of this function relies on projecting onto the unit ball:
We make use of the algorithm from [9] to perform the projection step, implemented over batches of vectors for efficiency. Similarly, one obtains the proximal operator for and via the same theorem,
where is the wellknown soft thresholding operator. In the case that one wants to minimize the number of perturbed pixels in the adversarial image, one can turn to the counting “norm”, called , which counts the number of nonzero entries in a vector. While this function is nonconvex, the proximal operator still has a closed form:
where is a hardthresholding operator, and acts componentwise in the case of vector arguments.
4. Experimental methodology
Outline
We compare the ProxLogBarrier attack with several other adversarial attacks on MNIST [18], CIFAR10 [16], and ImageNet1k [8] datasets. For MNIST, we use the network described in [24]; on CIFAR10, we use a ResNeXt network [33]; and for ImageNet1k, ResNet50 [14, 7]. We also consider defended models for the aforementioned networks. This is to further benchmark the attack capability of the ProxLogBarrier, and to reaffirm previous work in the area. For defended models, we consider Madrystyle AT for CIFAR10 and MNIST [19]. On ImageNet1k, we use the recently proposed scaleable input gradient regularization for adversarial robustness [11]. We randomly select 1000 (test) images to evaluate performance on MNIST, CIFAR10, and 500 (test) images on ImageNet1k. We consider the same images on their defended counterparts. We note that for ImageNet1k, we consider the problem of Top5 misclassification, where the logrithmic barrier is with respect to the following constraint set
where denotes the largest index.
We compare the ProxLogBarrier attack with a wide range of attack algorithms that are available through the FoolBox adversarial attack library [26]. For perturbations in , we compare against SparseFool [21], Jacobian Saliency Map Attack (JSMA) [24], and Pointwise [28] (this latter attack is blackbox). For attacks, we consider CarliniWagner’s attack (CW) [5], Projected Gradient Ascent (PGA) [17], DeepFool [22], and the original LogBarrier attack [12]. Finally, for norm perturbations, we consider PGA, DeepFool, and LogBarrier. All hyperparameters are left to their implementation defaults, with the exception of SparseFool, where we used the exact parameters indicated in the paper.
Implementation details for our algorithm
When optimizing for based noise, we initialize the adversarial image with sufficiently large Gaussian noise; for and based perturbations, we use uniform noise. For hyperparameter defaults, we recommend , with , which transfer well across all datasets and are the same parameters used in this paper. We observed some computational drawbacks for ImageNet1k: firstly, the proximal operator for the norm is far too strict. We decided to use the norm to induce sparseness in our adversarial perturbation (changing both the prox parameter and the step size to ). Other parameter changes for the ImageNet1k dataset is the proximal parameter in the case, we set and we used 2500 algorithm iterations. Finally, we found that using the softmax layer outputs helps with ImageNet1k attacks against both the defended and undefended network.
Reporting
For perturbations in and , we report the percent misclassification at various threshold levels that are somewhat standard [31]. Our choices for distance thresholds were arbitrary, however we supplement with a median perturbation distances for all attack norms to mitigate cherrypicking. For attacks that were unable to successfully perturb at least half the sampled images, we do not report anything. If the attack was able to perturb more than half but not all, we add an asterisk to the median distance. We denote the defended models by “(D)” (recall that for MNIST and CIFAR10, we are using Madry’s adversarial training, and scaleable inputgradient regularization for Imagenet1k).
4.1. Results
Perturbations in :
Our results for attacks are found in Table 1, with examples available in Figure 1 and Figure 2. Across all datasets considered, ProxLogBarrier outperforms all other attack methods, for both defended and undefended networks. It also seems undeterred from both Madrystyle adversarial training on MNIST and CIFAR10. This is entirely reasonable, for the Madrystyle adversarial training is targeted towards attacks. In contrast, on ImageNet1k, the defended model trained with inputgradient regularization performs significantly better than the undefended model, even though this defence is not aimed towards attacks. Neither JSMA or Pointwise scale to networks on ImageNet1k. Pointwise exceeds at smaller images, since it takes less than 1000 iterations to cycle over every pixel and check if it can be zeroed out. We remark that SparseFool was unable to adversarially attack all images, whereas ProxLogBarrier always succeeded.
Perturbations in :
Results for perturbations are found in Table 2. Our attack stands out on MNIST, in both the defended and undefended case. On CIFAR10, our attack is best on the undefended network, and only slightly worse than PGA when adversarially defended. On ImageNet1k, our method suffers dramatically. This is likely due to very poor decision boundaries on the with respect to this norm , as our method will necessarily be better when the boundaries are not muddled. PGA does not focus on the decision boundaries explicitly, thus has more room to find something adversarial quickly.
Perturbations in :
Results for perturbations measured in Euclidean distance are found in Table 3. For MNIST and ImageNet1k, on both defended and undefended networks, our attack performs better than all other methods, both in median distance and at a given perturbation norm threshold. On CIFAR10, we are best on undefended but lose to CW in the defended case. However, the CW attack did not scale to ImageNet1k using the implementation in the FoolBox attack library.
Algorithm runtime:
We strove to implement ProxLogBarrier so that it could be run in a reasonable amount of time. For that reason, ProxLogBarrier was implemented to work over a batch of images. Our code is publicly available at the following https://github.com/APooladian/ProxLogBarrierAttack. Using one consumer grade GPU, we can comfortably attack several MNIST and CIFAR10 images simultaneously, but only one ImageNet1k image at a given time. We report our algorithm runtimes in Table 4
, that achieve the statistics mentioned in the previous tables. Most of the other algorithms were implemented taken from the FoolBox repository, and were not written to take advantage of the GPU. Hence we omit a comparison with the other attack algorithms based on run time. Heuristically speaking, PGA is one of the faster algorithms, whereas CW, SparseFool, and DeepFool are slower. We are not surprised that our attack in
takes longer than the other norms; this is likely due to the backtracking step to ensure misclassification of the iterate. On ImageNet1k, the ProxLogBarrier attack in the metric is quite slow due to the projection step onto the ball, which is , where is the input dimension size [9].Batch Size  
MNIST  100  8.35  6.91  6.05 
CIFAR10  25  69.07  56.11  30.87 
ImageNet1k  1  35.45  29.47  75.50 
5. Conclusion
We have presented a concise framework for generating adversarial perturbations by incorporating the proximal gradient method. We have expanded upon the LogBarrier attack, which was originally only effective in and norms, by addressing the norm case as well. Thus we have proposed a method unifying all three common perturbation scenarios. Our approach requires fewer hyperparameter tweaks than LogBarrier, and performs significantly better than many attack methods we compared against, both on defended and undefended models, and across all norm choices. We highlight that our method is, to our knowledge, the best choice for perturbations measured in , compared to all other methods available in FoolBox. We also perform better than all other attacks considered on the MNIST network with respect to median distance and commonly reported thresholds. While our paper focuses on adversarial metrics, it is worth noting that the proximal gradient method can open doors to potentially new adversarial metrics, provided they have closed proximal form.
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