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Title: Training for Faster Adversarial Robustness Verification via Inducing ReLU Stability

Paper content:
We explore the concept of co-design in the context of neural network verification.
Specifically, we aim to train deep neural networks that not only are robust to adversarial perturbations but also whose robustness can be verified more easily.
To this end, we identify two properties of network models - weight sparsity and so-called ReLU stability - that turn out to significantly impact the complexity of the corresponding verification task.
We demonstrate that improving weight sparsity alone already enables us to turn computationally intractable verification problems into tractable ones.
Then, improving ReLU stability leads to an additional 4-13x speedup in verification times.
An important feature of our methodology is its "universality," in the sense that it can be used with a broad range of training procedures and verification approaches.
Deep neural networks (DNNs) have recently achieved widespread success in image classification BID17 , face and speech recognition BID27 , and game playing BID23 BID8 .
This success motivates their application in a broader set of domains, including more safety-critical environments.
This thrust makes understanding the reliability and robustness of the underlying models, let alone their resilience to manipulation by malicious actors, a central question.
However, predictions made by machine learning models are often brittle.
A prominent example is the existence of adversarial examples BID26 : imperceptibly modified inputs that cause state-of-the-art models to misclassify with high confidence.There has been a long line of work on both generating adversarial examples, called attacks BID2 BID13 BID0 BID13 BID9 , and training models robust to adversarial examples, called defenses BID10 BID21 BID19 BID14 .
However, recent research has shown that most defenses are ineffective BID2 BID0 .
Furthermore, even for defenses such as that of BID19 that have seen empirical success against many attacks, we are unable to conclude yet with certainty that they are robust to all attacks that we want these models to be resilient to.This state of affairs gives rise to the need for verification of networks, i.e., the task of formally proving that no small perturbations of a given input can cause it to be misclassified by the network model.
Although many exact verifiers 1 have been designed to solve this problem, the verification process is often intractably slow.
For example, when using the Reluplex verifier of , even verifying a small MNIST network turns out to be computationally infeasible.
Thus, addressing this intractability of exact verification is the primary goal of this work.
In this paper, we use the principle of co-design to develop training methods that emphasize verification as a goal, and we show that they make verifying the trained model much faster.
We first demonstrate that natural regularization methods already make the exact verification problem significantly more tractable.
Subsequently, we introduce the notion of ReLU stability for networks, present a method that improves a network's ReLU stability, and show that this improvement makes verification an additional 4-13x faster.
Our method is universal, as it can be added to any training procedure and should speed up any exact verification procedure, especially MILP-based methods.Prior to our work, exact verification seemed intractable for all but the smallest models.
Thus, our work shows progress toward reliable models that can be proven to be robust, and our techniques can help scale verification to even larger networks.Many of our methods appear to compress our networks into more compact, simpler forms.
We hypothesize that the reason that regularization methods like RS Loss can still achieve very high accuracy is that most models are overparametrized in the first place.
There exist clear parallels between our methods and techniques in model compression BID12 BID6 ) -therefore, we believe that drawing upon additional techniques from model compression can further improve the ease-of-verification of networks.
We also expect that there exist objectives other than weight sparsity and ReLU stability that are important for verification speed.
If so, further exploring the principle of co-design for those objectives is an interesting future direction.
Exact verification and certification are two related approaches to formally verifying properties of neural networks, such as adversarial robustness.
In both cases, the end goal is formal verification.
Certification methods, which solve an easier-to-solve relaxation of the exact verification problem, are important developments because exact verification previously appeared computationally intractable for all but the smallest models.For the case of adversarial robustness, certification methods exploit a trade-off between provable robustness and speed.
They can fail to provide certificates of robustness for some inputs that are actually robust, but they will either find or fail to find certificates of robustness quickly.
On the other hand, exact verifiers will always give the correct answer if given enough time, but exact verifiers can sometimes take many hours to formally verify robustness on even a single input.In general, the process of training a robust neural network and then formally verifying its robustness happens in two steps.•Step
1: Training BID22 and BID20 , propose a method for step 2 (the certification step), and then propose a training objective in step 1 that is directly related to their method for step 2. We call
this paradigm "co-training." In BID22
, they found that using their step 2 on a model trained using Wong and Kolter (2018)'s step 1 resulted in extremely poor provable robustness (less than 10%), and the same was true when using Wong and Kolter (2018)'s step 2 on a model trained using their step 1.We focus on MILP-based exact verification as our step 2, which encompasses the best current exact verification methods. The advantage
of using exact verification for step 2 is that it will be accurate, regardless of what method is used in step 1. The disadvantage
of using exact verification for step 2 is that it could be extremely slow. For our step 1,
we used standard robust adversarial training. In order to significantly
speed up exact verification as step 2, we proposed techniques that could be added to step 1 to induce weight sparsity and ReLU stability.In general, we believe it is important to develop effective methods for step 1, given that step 2 is exact verification. However, ReLU stability can
also be beneficial for tightening the relaxation of certification approaches like that of and , as unstable ReLUs are the primary cause of the overapproximation that occurs in the relaxation step. Thus, our techniques for inducing
ReLU stability can be useful for certification as well.Finally, in recent literature on verification and certification, most works have focused on formally verifying the property of adversarial robustness of neural networks. However, verification of other properties
could be useful, and our techniques to induce weight sparsity and ReLU stability would still be useful for verification of other properties for the exact same reasons that they are useful in the case of adversarial robustness.

assistant: text: The paper presents several ways to regularize plain ReLU networks to opimize the adversarial robustness, provable adversarial robustness, and the verification speed.