Salil P. Vadhan
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The Limits of Post-Selection Generalization
The Limits of Post-Selection Generalization Kobbi Nissim∗ Adam Smithy Thomas Steinke Georgetown University Boston University IBM Research – Almaden [email protected] [email protected] [email protected] Uri Stemmerz Jonathan Ullmanx Ben-Gurion University Northeastern University [email protected] [email protected] Abstract While statistics and machine learning offers numerous methods for ensuring gener- alization, these methods often fail in the presence of post selection—the common practice in which the choice of analysis depends on previous interactions with the same dataset. A recent line of work has introduced powerful, general purpose algorithms that ensure a property called post hoc generalization (Cummings et al., COLT’16), which says that no person when given the output of the algorithm should be able to find any statistic for which the data differs significantly from the population it came from. In this work we show several limitations on the power of algorithms satisfying post hoc generalization. First, we show a tight lower bound on the error of any algorithm that satisfies post hoc generalization and answers adaptively chosen statistical queries, showing a strong barrier to progress in post selection data analysis. Second, we show that post hoc generalization is not closed under composition, despite many examples of such algorithms exhibiting strong composition properties. 1 Introduction Consider a dataset X consisting of n independent samples from some unknown population P. How can we ensure that the conclusions drawn from X generalize to the population P? Despite decades of research in statistics and machine learning on methods for ensuring generalization, there is an increased recognition that many scientific findings do not generalize, with some even declaring this to be a “statistical crisis in science” [14]. -
The Computational Complexity of Nash Equilibria in Concisely Represented Games∗
The Computational Complexity of Nash Equilibria in Concisely Represented Games¤ Grant R. Schoenebeck y Salil P. Vadhanz August 26, 2009 Abstract Games may be represented in many di®erent ways, and di®erent representations of games a®ect the complexity of problems associated with games, such as ¯nding a Nash equilibrium. The traditional method of representing a game is to explicitly list all the payo®s, but this incurs an exponential blowup as the number of agents grows. We study two models of concisely represented games: circuit games, where the payo®s are computed by a given boolean circuit, and graph games, where each agent's payo® is a function of only the strategies played by its neighbors in a given graph. For these two models, we study the complexity of four questions: determining if a given strategy is a Nash equilibrium, ¯nding a Nash equilibrium, determining if there exists a pure Nash equilibrium, and determining if there exists a Nash equilibrium in which the payo®s to a player meet some given guarantees. In many cases, we obtain tight results, showing that the problems are complete for various complexity classes. 1 Introduction In recent years, there has been a surge of interest at the interface between computer science and game theory. On one hand, game theory and its notions of equilibria provide a rich framework for modeling the behavior of sel¯sh agents in the kinds of distributed or networked environments that often arise in computer science and o®er mechanisms to achieve e±cient and desirable global outcomes in spite of the sel¯sh behavior. -
Pseudodistributions That Beat All Pseudorandom Generators
Electronic Colloquium on Computational Complexity, Report No. 19 (2021) Pseudodistributions That Beat All Pseudorandom Generators Edward Pyne∗ Salil Vadhany Harvard University Harvard University [email protected] [email protected] February 17, 2021 Abstract A recent paper of Braverman, Cohen, and Garg (STOC 2018) introduced the concept of a pseudorandom pseudodistribution generator (PRPG), which amounts to a pseudorandom gen- erator (PRG) whose outputs are accompanied with real coefficients that scale the acceptance probabilities of any potential distinguisher. They gave an explicit construction of PRPGs for ordered branching programs whose seed length has a better dependence on the error parameter " than the classic PRG construction of Nisan (STOC 1990 and Combinatorica 1992). In this work, we give an explicit construction of PRPGs that achieve parameters that are impossible to achieve by a PRG. In particular, we construct a PRPG for ordered permuta- tion branching programs of unbounded width with a single accept state that has seed length O~(log3=2 n) for error parameter " = 1= poly(n), where n is the input length. In contrast, re- cent work of Hoza et al. (ITCS 2021) shows that any PRG for this model requires seed length Ω(log2 n) to achieve error " = 1= poly(n). As a corollary, we obtain explicit PRPGs with seed length O~(log3=2 n) and error " = 1= poly(n) for ordered permutation branching programs of width w = poly(n) with an arbi- trary number of accept states. Previously, seed length o(log2 n) was only known when both the width and the reciprocal of the error are subpolynomial, i.e. -
Hardness of Non-Interactive Differential Privacy from One-Way
Hardness of Non-Interactive Differential Privacy from One-Way Functions Lucas Kowalczyk* Tal Malkin† Jonathan Ullman‡ Daniel Wichs§ May 30, 2018 Abstract A central challenge in differential privacy is to design computationally efficient non-interactive algorithms that can answer large numbers of statistical queries on a sensitive dataset. That is, we would like to design a differentially private algorithm that takes a dataset D Xn consisting of 2 some small number of elements n from some large data universe X, and efficiently outputs a summary that allows a user to efficiently obtain an answer to any query in some large family Q. Ignoring computational constraints, this problem can be solved even when X and Q are exponentially large and n is just a small polynomial; however, all algorithms with remotely similar guarantees run in exponential time. There have been several results showing that, under the strong assumption of indistinguishability obfuscation (iO), no efficient differentially private algorithm exists when X and Q can be exponentially large. However, there are no strong separations between information-theoretic and computationally efficient differentially private algorithms under any standard complexity assumption. In this work we show that, if one-way functions exist, there is no general purpose differen- tially private algorithm that works when X and Q are exponentially large, and n is an arbitrary polynomial. In fact, we show that this result holds even if X is just subexponentially large (assuming only polynomially-hard one-way functions). This result solves an open problem posed by Vadhan in his recent survey [Vad16]. *Columbia University Department of Computer Science. -
Zero Knowledge and Soundness Are Symmetric∗
Zero Knowledge and Soundness are Symmetric∗ Shien Jin Ong Salil Vadhan Division of Engineering and Applied Sciences Harvard University Cambridge, Massachusetts, USA. E-mail: {shienjin,salil}@eecs.harvard.edu November 14, 2006 Abstract We give a complexity-theoretic characterization of the class of problems in NP having zero- knowledge argument systems that is symmetric in its treatment of the zero knowledge and the soundness conditions. From this, we deduce that the class of problems in NP ∩ coNP having zero-knowledge arguments is closed under complement. Furthermore, we show that a problem in NP has a statistical zero-knowledge argument system if and only if its complement has a computational zero-knowledge proof system. What is novel about these results is that they are unconditional, i.e. do not rely on unproven complexity assumptions such as the existence of one-way functions. Our characterization of zero-knowledge arguments also enables us to prove a variety of other unconditional results about the class of problems in NP having zero-knowledge arguments, such as equivalences between honest-verifier and malicious-verifier zero knowledge, private coins and public coins, inefficient provers and efficient provers, and non-black-box simulation and black-box simulation. Previously, such results were only known unconditionally for zero-knowledge proof systems, or under the assumption that one-way functions exist for zero-knowledge argument systems. Keywords: zero-knowledge argument systems, statistical zero knowledge, complexity classes, clo- sure under complement, distributional one-way functions. ∗Both the authors were supported by NSF grant CNS-0430336 and ONR grant N00014-04-1-0478. -
Pseudorandomness and Combinatorial Constructions
Pseudorandomness and Combinatorial Constructions Luca Trevisan∗ September 20, 2018 Abstract In combinatorics, the probabilistic method is a very powerful tool to prove the existence of combinatorial objects with interesting and useful properties. Explicit constructions of objects with such properties are often very difficult, or unknown. In computer science, probabilistic al- gorithms are sometimes simpler and more efficient than the best known deterministic algorithms for the same problem. Despite this evidence for the power of random choices, the computational theory of pseu- dorandomness shows that, under certain complexity-theoretic assumptions, every probabilistic algorithm has an efficient deterministic simulation and a large class of applications of the the probabilistic method can be converted into explicit constructions. In this survey paper we describe connections between the conditional “derandomization” re- sults of the computational theory of pseudorandomness and unconditional explicit constructions of certain combinatorial objects such as error-correcting codes and “randomness extractors.” 1 Introduction 1.1 The Probabilistic Method in Combinatorics In extremal combinatorics, the probabilistic method is the following approach to proving existence of objects with certain properties: prove that a random object has the property with positive probability. This simple idea has beem amazingly successful, and it gives the best known bounds for arXiv:cs/0601100v1 [cs.CC] 23 Jan 2006 most problems in extremal combinatorics. The idea was introduced (and, later, greatly developed) by Paul Erd˝os [18], who originally applied it to the following question: define R(k, k) to be the minimum value n such that every graph on n vertices has either an independent set of size at least k or a clique of size at least k;1 It was known that R(k, k) is finite and that it is at most 4k, and the question was to prove a lower bound. -
Computational Complexity
Computational Complexity The Harvard community has made this article openly available. Please share how this access benefits you. Your story matters Citation Vadhan, Salil P. 2011. Computational complexity. In Encyclopedia of Cryptography and Security, second edition, ed. Henk C.A. van Tilborg and Sushil Jajodia. New York: Springer. Published Version http://refworks.springer.com/mrw/index.php?id=2703 Citable link http://nrs.harvard.edu/urn-3:HUL.InstRepos:33907951 Terms of Use This article was downloaded from Harvard University’s DASH repository, and is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http:// nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of- use#OAP Computational Complexity Salil Vadhan School of Engineering & Applied Sciences Harvard University Synonyms Complexity theory Related concepts and keywords Exponential time; O-notation; One-way function; Polynomial time; Security (Computational, Unconditional); Sub-exponential time; Definition Computational complexity theory is the study of the minimal resources needed to solve computational problems. In particular, it aims to distinguish be- tween those problems that possess efficient algorithms (the \easy" problems) and those that are inherently intractable (the \hard" problems). Thus com- putational complexity provides a foundation for most of modern cryptogra- phy, where the aim is to design cryptosystems that are \easy to use" but \hard to break". (See security (computational, unconditional).) Theory Running Time. The most basic resource studied in computational com- plexity is running time | the number of basic \steps" taken by an algorithm. (Other resources, such as space (i.e., memory usage), are also studied, but they will not be discussed them here.) To make this precise, one needs to fix a model of computation (such as the Turing machine), but here it suffices to informally think of it as the number of \bit operations" when the input is given as a string of 0's and 1's. -
Pseudorandomness for Regular Branching Programs Via Fourier Analysis
Electronic Colloquium on Computational Complexity, Report No. 86 (2013) Pseudorandomness for Regular Branching Programs via Fourier Analysis Omer Reingold∗ Thomas Steinkey Salil Vadhanz [email protected] [email protected] [email protected] June 2013 Abstract We present an explicit pseudorandom generator for oblivious, read-once, permutation branching pro- grams of constant width that can read their input bits in any order. The seed length is O(log2 n), where n is the length of the branching program. The previous best seed length known for this model was n1=2+o(1), which follows as a special case of a generator due to Impagliazzo, Meka, and Zuckerman (FOCS 2012) (which gives a seed length of s1=2+o(1) for arbitrary branching programs of size s). Our techniques also give seed length n1=2+o(1) for general oblivious, read-once branching programs of width o(1) 2n , which is incomparable to the results of Impagliazzo et al. Our pseudorandom generator is similar to the one used by Gopalan et al. (FOCS 2012) for read-once CNFs, but the analysis is quite different; ours is based on Fourier analysis of branching programs. In particular, we show that an oblivious, read-once, regular branching program of width w has Fourier mass at most (2w)2k at level k, independent of the length of the program. ∗Microsoft Research Silicon Valley, 1065 La Avenida, Mountain View, CA. ySchool of Engineering and Applied Sciences, Harvard University, 33 Oxford Street, Cambridge MA. Work done in part while at Stanford University. Supported by NSF grant CCF-1116616 and the Lord Rutherford Memorial Research Fellowship. -
A Note on Extracting Randomness from Santha-Vazirani Sources
A Note on Extracting Randomness from Santha-Vazirani Sources Omer Reingold∗ Salil Vadhany Incumbent of the Walter and Elise Haas Division of Engineering & Applied Sciences Career Development Chair Harvard University Department of Computer Science Cambridge, MA, USA Weizmann Institute of Science [email protected] Rehovot, Israel [email protected] Avi Wigdersonz School of Mathematics Institute for Advanced Study Princeton, NJ, USA [email protected] October 11, 2004 Abstract Santha and Vazirani [SV86] proposed the notion of a semi-random source of bits (also known as Santha-Vazirani sources), and proved that it is impossible to (deterministically) extract an almost-uniform random bit from such a source (while it is possible given several independent Santha-Vazirani sources). We provide a simpler and more transparent proof of their impossibility result. Our proof has the advantage of applying to a natural strengthening of Santha-Vazirani sources. Moreover, our proof technique has been used in [DOPS04] to obtain impossibility results on doing cryptography with semi-random sources. A source of length n is a random variable X taking values in f0; 1gn. We will denote the individual bits of X by X = X1 ··· Xn. If Y is a random variable taking values in f0; 1g, the bias of Y is j Pr[Y = 0] − Pr[Y = 1]j, i.e. the smallest δ such that (1 − δ)=2 ≤ Pr[Y = 0] ≤ (1 + δ)=2. Definition 1 For δ 2 [0; 1], a source X of length n is a Santha-Vazirani (SV) source (or semi- random source) with bias δ if for every i 2 [n] and every x1; : : : ; xi 2 f0; 1g, the bias of Xi conditioned on X1 = x1;:::;Xi−1 = xi−1 is at most δ. -
Verifiable Random Functions
Verifiable Random Functions y z Silvio Micali Michael Rabin Salil Vadhan Abstract random string of the proper length. The possibility thus ex- ists that, if it so suits him, the party knowing the seed s may We efficiently combine unpredictability and verifiability by declare that the value of his pseudorandom oracle at some x f x extending the Goldreich–Goldwasser–Micali construction point is other than s without fear of being detected. It f s of pseudorandom functions s from a secret seed , so that is for this reason that we refer to these objects as “pseudo- s f knowledge of not only enables one to evaluate s at any random oracles” rather than using the standard terminology f x x NP point , but also to provide an -proof that the value “pseudorandom functions” — the values s come “out f x s is indeed correct without compromising the unpre- of the blue,” as if from an oracle, and the receiver must sim- s f dictability of s at any other point for which no such a proof ply trust that they are computed correctly from the seed . was provided. Therefore, though quite large, the applicability of pseu- dorandom oracles is limited: for instance, to settings in which (1) the “seed owner”, and thus the one evaluating 1Introduction the pseudorandom oracle, is totally trusted; or (2) it is to the seed-owner’s advantage to evaluate his pseudorandom oracle correctly; or (3) there is absolutely nothing for the PSEUDORANDOM ORACLES. Goldreich, Goldwasser, and seed-owner to gain from being dishonest. Micali [GGM86] show how to simulate a random ora- f x One efficient way of enabling anyone to verify that s b cle from a-bit strings to -bit strings by means of a con- f x really is the value of pseudorandom oracle s at point struction using a seed, that is, a secret and short random clearly consists of publicizing the seed s.However,this string. -
Succinct Garbled RAM from Indistinguishablity Obfuscation
Succinct Garbled RAM from Indistinguishablity Obfuscation by Justin Lee Holmgren S.B., Massachusetts Institute of Technology (2013) Submitted to the Department of Electrical Engineering and Computer Science in partial fulfillment of the requirements for the degree of Master of Engineering in Electrical Engineering and Computer Science at the MASSACHUSETTS INSTITUTE OF TECHNOLOGY February 2015 ○c Massachusetts Institute of Technology 2015. All rights reserved. Author................................................................ Department of Electrical Engineering and Computer Science February 1, 2015 Certified by. Shafi Goldwasser RSA Professor of Electrical Engineering and Computer Science Thesis Supervisor Accepted by . Professor Albert R. Meyer Chairman, Masters of Engineering Thesis Committee 2 Succinct Garbled RAM from Indistinguishablity Obfuscation by Justin Lee Holmgren Submitted to the Department of Electrical Engineering and Computer Science on February 1, 2015, in partial fulfillment of the requirements for the degree of Master of Engineering in Electrical Engineering and Computer Science Abstract In this thesis, I give the first construction of a succinct garbling scheme forRAM programs. For a program requiring space S and time T to compute, the size of its garbling is O~(S) instead of poly(T ). This construction relies on the existence of indistinguishability obfuscation, as well as the existence of injective one-way functions. As a building block, I introduce and construct a primitive called asymmetrically constrained encryption (ACE). This primitive is an encryption system for which keys can be punctured on succinctly described sets of plaintexts. For programs acting on ACE-encrypted values, I give a natural and general condition for their obfuscations to be indistinguishable, using the fact that the encryption and decryption keys can be separately punctured. -
Curriculum Vitae
Curriculum Vitae Jens Groth July 11, 2021 1 Contact Information Jens Groth E-mail: [email protected] Homepage: www.cs.ucl.ac.uk/staff/J.Groth 2 Research Interests I am interested in the theory of cryptography and in the practical application of cryptographic techniques. 3 Appointments DFINITY London, UK July 2021 { present. Director of Research Febuary 2021 { June 2021 Team Lead, Research January 2019 { January 2021 Principal Researcher University College London London, UK July 2020 { present. Honorary Professor in the Department of Computer Science. October 2015 { June 2020. Professor of Cryptology in the Department of Computer Science. October 2012 { September 2015. Reader in Cryptology in the Department of Computer Science. October 2010 { September 2012. Senior Lecturer in the Department of Computer Science. September 2007 { September 2010. Lecturer in the Department of Computer Science. University of California, Los Angeles Los Angeles, US February 2005 { August 2007. Postdoctoral Employee at the Computer Science Department. Cryptomathic Arhus,˚ Denmark August 2001 { July 2004. Industrial PhD Student. 1 4 Education Aarhus University Aarhus, Denmark • PhD in Computer Science, December 2004. • Advisor: Professor Ivan Damg˚ard. • Thesis title: Honest Verifier Zero-Knowledge Proofs Applied. Danish Academy of Technical Sciences Aarhus, Denmark • Industrial Research Fellow, October 2004. • Advisor: Senior Systems Engineer, PhD Gorm Salomonsen. Aarhus University Aarhus, Denmark • MSc in Mathematics, April 2001. • Advisor: Professor Ivan Damg˚ard. • Thesis title: Non-malleable Public-Key Encryption Secure against Chosen Ciphertext At- tack based on Trapdoor Permutations. Aarhus University Aarhus, Denmark • Supplement in Philosophy, April 2001. 5 Awards and Distinctions • IACR Test-of-Time Award 2021 for Simulation-sound NIZK proofs for a practical language and constant size group signatures published at ASIACRYPT 2006.