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Agnostic Setting We focused only on robust PAC learning in the realizable setting, where we assume there is a c ∈ C with zero robust error

# Reducing Adversarially Robust Learning to Non-Robust PAC Learning

NIPS 2020, (2020)

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Abstract

We study the problem of reducing adversarially robust learning to standard PAC learning, i.e. the complexity of learning adversarially robust predictors using access to only a black-box non-robust learner. We give a reduction that can robustly learn any hypothesis class $\mathcal{C}$ using any non-robust learner $\mathcal{A}$ for \$\math...More

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Introduction
• The authors consider the problem of learning predictors that are robust to adversarial examples at test time.
• Neural networks)—that is, whether, if there exists a predictor in C with zero robust risk w.r.t. some unknown distribution D over X ×Y, can the authors find a predictor with robust risk using m i.i.d. samples S = {(xi, that if C is PAC-learnable non-robustly, ythie)}nm i=C1isfroalmsoDa.dRveercseanrtilayl,lyMroonbtuasstsleyr et al [2019] showed learnable
• Their result is not constructive and the robust learning algorithm given is inefficient, complex, and does not directly use a non-robust learner.
• Many systems in practice perform standard learning but with no robustness guarantees, and it would be beneficial to provide wrapper procedures that can guarantee adversarial robustness in a black-box manner without needing to modify current learning systems internally
Highlights
• We consider the problem of learning predictors that are robust to adversarial examples at test time
• Main Results When studying reductions of adversarially robust learning to non-robust learning, an important aspect emerges regarding the form of access that the reduction algorithm has to the adversary U
• How should we model access to the sets of adversarial perturbations represented by U? we explore the setting where the reduction algorithm has explicit knowledge of the adversary U
• Agnostic Setting We focused only on robust PAC learning in the realizable setting, where we assume there is a c ∈ C with zero robust error
• We remark that an agnostic-to-realizable reduction described in Montasser et al [2019, Theorem 6] can be used in our setting, it has runtime that is exponential in vc(A). Another attempt through the agnostic boosting frameworks [e.g. Kalai and Kanade, 2009] requires a non-robust PAC learner A with error ε that scales with |U|2, which results in a sample complexity that depends on |U|, and this is something we would like to avoid
Results
• When studying reductions of adversarially robust learning to non-robust learning, an important aspect emerges regarding the form of access that the reduction algorithm has to the adversary U.
• The authors first show that there is an algorithm that can learn adversarially robust predictors with black-box oracle access to a non-robust algorithm: Theorem 3.1 (Informal).
• For any adversary U, Algorithm 1 robustly learns any target class C using any black-box non-robust PAC learner A for C, with O(log2 |U|) oracle calls to A and sample complexity independent of |U|.
• There exists an adversary U such that for any reduction algorithm B, there exists a target class C and a PAC learner A for C such that Ω(log |U|) oracle queries to A are necessary to robustly learn C
Conclusion
• The main contribution of this paper is in formulating the question of reducing adversarially robust learning to standard non-robust learning and providing answers in some settings.
• The authors remark that an agnostic-to-realizable reduction described in Montasser et al [2019, Theorem 6] can be used in the setting, it has runtime that is exponential in vc(A).
• Another attempt through the agnostic boosting frameworks [e.g.
• What the authors consider in this paper can be viewed as a question of boosting robustness: Can the authors boost non-robust predictors to attain a robust predictor? and can the authors do this efficiently? Another natural question to consider which the authors did not study in this paper is: Can the authors boost weakly robust predictors to attain a robust predictor?
Related work
• Recent work [Mansour et al, 2015, Feige et al, 2015, 2018, Attias et al, 2019]

can be interpreted as giving reduction algorithms for adversarially robust learning. Specifically, Feige et al [2015] gave a reduction algorithm that can robustly learn a finite hypothesis class C using black-box access to an ERM for C. Later, Attias et al [2019] improved this to handle infinite hypothesis classes C. But their complexity and the number of calls to ERM depend super-linearly on the number of possible perturbations |U| = perturbations—we completely avoid a sample csuompxp|lUex(ixty)|d, ewpheincdheinscuenodnes|iUra|b, laenfdorremduocste types of the oracle complexity to at most a poly-logarithmic dependence. Furthermore, their work assumes access specifically to an ERM procedure, which is a very specific type of learner, while we only require access to any method that PAC-learns C and whose image has bounded VC-dimension.

A related goal was explored by Salman et al [2020]: They proposed a method to robustify pretrained predictors. Their method takes as input a black-box predictor (not a learning algorithm) and a point x, and outputs a label prediction y for x and a radius r such that the label y is robust to l2 perturbations of radius r. But this doesn’t guarantee that the predictions y are correct, nor that the radius r would be what we desire, and even if the predictor was returned by a learning algorithm and has a very small non-robust error, we do not end up with any gurantee on the robust risk of the robustified predictor. In this paper, we require black-box access to a learning algorithm (not just to a single predictor), but we output a predictor that is guaranteed to have small robust risk (if one exists in the class, see Definition 2.2). We also provide a general treatment for arbitrary adversaries U, not just lp perturbations. Finally, we note that the approach of Montasser et al [2019] can be interpreted as using black-box access to an oracle RERMC minimizing the robust empirical risk:
Funding
• This work is partially supported by DARPA1 cooperative agreement HR00112020003
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• 2. Query the ERM oracle with a dataset Lt ⊆ X × {0, 1}.
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