Publications by Year: 2021

2021
Jordan Awan and Salil Vadhan. 8/9/2021. “Canonical Noise Distributions and Private Hypothesis Tests.” In Privacy-Preserving Machine Learning (PPML '21), Privacy in Machine Learning (PriML '21). Publisher's VersionAbstract
f-DP has recently been proposed as a generalization of classical definitions of differential privacy allowing a lossless analysis of composition, post-processing, and privacy amplification via subsampling. In the setting of f-DP, we propose the concept canonical noise distribution (CND) which captures whether an additive privacy mechanism is appropriately tailored for a given f, and give a construction that produces a CND given an arbitrary tradeoff function f. We show that private hypothesis tests are intimately related to CNDs, allowing for the release of private p-values at no additional privacy cost as well as the construction of uniformly most powerful (UMP) tests for binary data.
We apply our techniques to the problem of difference of proportions testing, and construct a UMP unbiased "semi-private" test which upper bounds the performance of any DP test. Using this as a benchmark we propose a private test, based on the inversion of characteristic functions, which allows for optimal inference for the two population parameters and is nearly as powerful as the semi-private UMPU. When specialized to the case of (ϵ,0)-DP, we show empirically that our proposed test is more powerful than any (ϵ/2‾√)-DP test and has more accurate type I errors than the classic normal approximation test.
ARXIV.pdf
Salil Vadhan and Tianhao Wang. 5/2021. “Concurrent Composition of Differential Privacy.” Proceedings of the 19th Theory of Cryptography Conference (TCC '21), 13043. Publisher's VersionAbstract

We initiate a study of the composition properties of interactive dierentially private mechanisms. An interactive dierentially private mechanism is an algorithm that allows an analyst to adaptively ask queries about a sensitive dataset, with the property that an adversarial

analyst's view of the interaction is approximately the same regardless of whether or not any individual's data is in the dataset. Previous studies of composition of dierential privacy have focused on non-interactive algorithms, but interactive mechanisms are needed to capture many of the intended applications of dierential privacy and a number of the important dierentially private primitives.

 

We focus on concurrent composition, where an adversary can arbitrarily interleave its queries to several dierentially private mechanisms, which may be feasible when dierentially private query systems are deployed in practice. We prove that when the interactive mechanisms being composed are pure dierentially private, their concurrent composition achieves privacy parameters (with respect to pure or approximate dierential privacy) that match the (optimal) composition theorem for noninteractive dierential privacy. We also prove a composition theorem for interactive mechanisms that satisfy approximate dierential privacy. That bound is weaker than even the basic (suboptimal) composition theorem for noninteractive dierential privacy, and we leave closing the gap as a direction for future research, along with understanding concurrent composition for other variants of dierential privacy.

ARXIV.pdf TCC 21.pdf
Tyler Piazza. 3/2021. “Differentially Private Ridge Regression”.Abstract

Studying problems of interest, like finding trends in medical data, can require analyzing data which contains sensitive and personally identifying information. As a result, it is often infeasible to release these datasets to researchers or to the general public. In this paper, we study algorithms that are differentially private, where there are theoretical guarantees that the mechanisms studied will reveal only limited amounts of information about individual people while still providing insights about large groups. This thesis discusses various forms of linear and ridge regression in this differentially private setting, with the goal of studying sensitive data to make predictions about future sensitive data. In particular, we will discuss the internal privacy-loss budgeting of the differentially private ridge regression technique adaSSP. This thesis provides 3 contributions. First, we discuss the existing SSP and adaSSP algorithms, and provide detailed proofs that they are each differentially private. Second, we introduce the two new algorithms adaSSPbudget and constSSPfull and prove that these are each differentially private. Third, we conduct experiments using synthetic and real world data to explore whether the precise privacy-loss budgeting used within these algorithms could improve their performance.

These experiments will explore the tradeoff between the accuracy of a hyperparameter and the accuracy of the other releases. Through the experimental results, we find that the performance is often insensitive to the particular privacy-loss budgeting and that for certain datasets, no choice of privacy-loss budget allows for the adaptive adaSSPbudget to outperform the standard SSP algorithm.

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Jörg Drechsler. 2/2021. “Differential Privacy for Government Agencies – Are We There Yet?”. Publisher's VersionAbstract
Government agencies always need to carefully consider potential risks of disclosure whenever they publish statistics based on their data or give external researchers access to the collected data. For this reason, research on disclosure avoiding techniques has a long tradition at statistical agencies. In this context, the promise of formal privacy guarantees offered by concepts such as differential privacy seem to be the panacea enabling the agencies to exactly quantify and control the privacy loss incurred by any data release. Still, despite the excitement in academia and industry, most agencies-with the prominent exception of the U.S. Census Bureau-have been reluctant to even consider the concept for their data release strategy.
This paper aims to shed some light on potential reasons for this. We argue that the requirements when implementing differential privacy approaches at government agencies are often fundamentally different from the requirements in industry. This raises many challenging problems and open questions that still need to be addressed before the concept might be used as an overarching principle when sharing data with the public. The paper will not offer any solutions to these challenges. Instead, we hope to stimulate some collaborative research efforts, as we believe that many of the problems can only be addressed by inter-disciplinary collaborations.
ARXIV.pdf
Kobbi Nissim. 2021. “Legal Theorems: Bridging Computer Science and Privacy Law.” 53rd Annual ACM Symposium on Theory of Computing (STOC 2021).
Kobbi Nissim. 2021. “Privacy: From Database Reconstruction to Legal Theorems.” In 40th ACM SIGMOD-SIGACT-SIGAI Symposium on Principles of Database Systems (PODS 2021).Abstract

 

There are significant gaps between legal and technical thinking around data privacy. Technical standards are described using mathematical language whereas legal standards are not rigorous from a mathematical point of view and often resort to concepts which they only partially define. As a result, arguments about the adequacy of technical privacy measures for satisfying legal privacy often lack rigor, and their conclusions are uncertain. The uncertainty is exacerbated by a litany of successful privacy attacks on privacy measures thought to meet legal expectations but then shown to fall short of doing so.

As computer systems manipulating individual privacy-sensitive data become integrated in almost every aspect of society, and as such systems increasingly make decisions of legal significance, the need to bridge the diverging, and sometimes conflicting legal and technical approaches becomes urgent.

We formulate and prove formal claims – “legal theorems” – addressing legal questions such as whether the use of technological measures satisfies the requirements of a legal privacy standard. In particular, we analyze the notion of singling out from the GDPR and whether technologies such as k-anonymity and differential privacy prevent singling out.

Our long-term goal is to develop concepts which are on one hand technical, so they can be integrated in the design of computer systems, and can be used in legal reasoning and for policymaking on the other hand.

 

PODS.pdf
Victor Balcer, Albert Cheu, Matthew Joseph, and Jieming Mao. 2021. “Connecting Robust Shuffle Privacy and Pan-Privacy.” In In Proceedings of the 2021 ACM-SIAM Symposium on Discrete Algorithms (SODA), Pp. 2384-2403. Publisher's VersionAbstract
In the \emph{shuffle model} of differential privacy, data-holding users send randomized messages to a secure shuffler, the shuffler permutes the messages, and the resulting collection of messages must be differentially private with regard to user data. In the \emph{pan-private} model, an algorithm processes a stream of data while maintaining an internal state that is differentially private with regard to the stream data. We give evidence connecting these two apparently different models.
Our results focus on \emph{robustly} shuffle private protocols, whose privacy guarantees are not greatly affected by malicious users. First, we give robustly shuffle private protocols and upper bounds for counting distinct elements and uniformity testing. Second, we use pan-private lower bounds to prove robustly shuffle private lower bounds for both problems. Focusing on the dependence on the domain size k, we find that robust approximate shuffle privacy and approximate pan-privacy have additive error Θ(k√) for counting distinct elements. For uniformity testing, we give a robust approximate shuffle private protocol with sample complexity Õ (k2/3) and show that an Ω(k2/3) dependence is necessary for any robust pure shuffle private tester. Finally, we show that this connection is useful in both directions: we give a pan-private adaptation of recent work on shuffle private histograms and use it to recover further separations between pan-privacy and interactive local privacy.
ARXIV.pdf
Mark Bun, Marco Gaboardi, and Satchit Sivakumar. 2021. “Multiclass versus Binary Differentially Private PAC Learning.” In Advances in Neural Information Processing Systems 34 (NeurIPS 2021). Publisher's VersionAbstract
We show a generic reduction from multiclass differentially private PAC learning to binary private PAC learning. We apply this transformation to a recently proposed binary private PAC learner to obtain a private multiclass learner with sample complexity that has a polynomial dependence on the multiclass Littlestone dimension and a poly-logarithmic dependence on the number of classes. This yields a doubly exponential improvement in the dependence on both parameters over learners from previous work. Our proof extends the notion of Ψ-dimension defined in work of Ben-David et al. [5] to the online setting and explores its general properties.
NEURIPS.pdf ARXIV.pdf
Marco Gaboardi, Michael Hay, and Salil Vadhan. 2021. “A Programming Framework for OpenDP (extended abstract).” In 6th Workshop on the Theory and Practice of Differential Privacy (TPDP 2020).Abstract

In this paper, we propose a programming framework for the library of dierentially private algorithms that will be at the core of the new OpenDP open-source software project (http://opendp.io/).

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Micah Altman, Aloni Cohen, Kobbi Nissim, and Alexandra Wood. 2021. “What a Hybrid Legal-Technical Analysis Teaches Us About Privacy Regulation: The Case of Singling Out.” BU Journal of Science Technology and Law, Vol 27, 1. Publisher's VersionAbstract
Abstract

This article advocates a hybrid legal-technical approach to the evaluation of technical measures designed to render information anonymous in order to bring it outside the scope of data protection regulation. The article demonstrates how such an approach can be used for instantiating a key anonymization concept appearing in the EU General Data Protection Regulation (GDPR) -- singling out. The analysis identifies and addresses a tension between a common, compelling theory of singling out and a mathematical analysis of this theory, and it demonstrates how to make determinations regarding the sufficiency of specific technologies for satisfying regulatory requirements for anonymization.

Doubts about the feasibility of effective anonymization and de-identification have gained prominence in recent years in response to high-profile privacy breaches enabled by scientific advances in privacy research, improved analytical capabilities, the wider availability of personal data, and the unprecedented richness of available data sources. At the same time, privacy regulations recognize the possibility, at least in principle, of data anonymization that is sufficiently protective so as to free the resulting (anonymized) data from regulation. As a result, practitioners developing privacy enhancing technologies face substantial uncertainty as to the legal standing of these technologies. More fundamentally, it is not clear how to make a determination of compliance even when the tool is fully described and available for examination.

This gap is symptomatic of a more general problem: Legal and technical approaches to data protection have developed in parallel, and their conceptual underpinnings are growing increasingly divergent. When lawmakers rely on purely legal concepts to engage areas that are affected by rapid scientific and technological change, the resulting laws, when applied in practice, frequently create substantial uncertainty for implementation; provide contradictory recommendations in important cases; disagree with current scientific technical understanding; and fail to scale to the rapid pace of technological development. This article argues that new hybrid concepts, created through technical and legal co-design, can inform practices that are practically complete, coherent, and scalable.

As a case study, the article focuses on a key privacy-related concept appearing in Recital 26 of the General Data Protection Regulation (GDPR) called singling out. We identify a compelling theory of singling out that is implicit in the most persuasive guidance available, and demonstrate that the theory is ultimately incomplete. We then use that theory as the basis for a new and mathematically rigorous privacy concept called predicate singling-out. Predicate singling-out sheds light on the notion of singling out in the GDPR, itself inextricably linked to anonymization. We argue that any data protection tool that purports to anonymize arbitrary personal data under the GDPR must prevent predicate singling-out. This enables, for the first time, a legally- and mathematically-grounded analysis of the standing of supposed anonymization technologies like k-anonymity and differential privacy. The analysis in this article is backed by a technical-mathematical analysis previously published by two of the authors.

Conceptually, our analysis demonstrates that a nuanced understanding of baseline risk is unavoidable for a theory of singling out based on current regulatory guidance. Practically, it identifies previously unrecognized failures of anonymization. In particular, it demonstrates that some k-anonymous mechanisms may allow singling out, challenging the prevailing regulatory guidance.

The article concludes with a discussion of specific recommendations for both policymakers and scholars regarding how to conduct a hybrid legal-technical analysis. Rather than formalizing or mathematizing the law, the article provides approaches for wielding formal tools in the service of practical regulation.

 

 
BU JOSTL.pdf