On the Utility of Fine-Grained Complexity Theory
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On The Utility of Fine-Grained Complexity Theory Manuel Sabin Electrical Engineering and Computer Sciences University of California at Berkeley Technical Report No. UCB/EECS-2020-165 http://www2.eecs.berkeley.edu/Pubs/TechRpts/2020/EECS-2020-165.html August 14, 2020 Copyright © 2020, by the author(s). All rights reserved. Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission. On The Utility of Fine-Grained Complexity Theory by Manuel Sabin (they/them) A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Computer Science in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Shafi Goldwasser, Chair Associate Professor Prasad Raghavendra Assistant Professor Nikhil Srivastava Summer 2020 On The Utility of Fine-Grained Complexity Theory Copyright 2020 by Manuel Sabin (they/them) 1 Abstract On The Utility of Fine-Grained Complexity Theory by Manuel Sabin (they/them) Doctor of Philosophy in Computer Science University of California, Berkeley Professor Shafi Goldwasser, Chair The nascent field of Fine-Grained Complexity Theory has emerged and grown rapidly in the past decade. By studying \Hardness within P" and the connections of problems computable in, say, n2 time versus n3 time, this field addresses the practical efficiency of problems. However, while this more deeply quantitative approach better addresses practical hardness problem-by-problem, we lose connection to key qualitative claims of classical com- plexity theory such as the general ability to hide secrets (Cryptography), the ability to show that a world where we have access to randomness is no more powerful computationally than a deterministic one (Derandomization), and the ability to take a problem that is hard in the worst-case scenario and turn it into one that is hard almost always (Hardness Amplification). We show that these connections can be recovered and that the problem-specific claims of Fine-Grained Complexity Theory cannot exist in a vacuum without ramifications to clas- sical structural Complexity Theory. Namely, we show that the core worst-case hardness assumptions that define Fine-Grained Complexity Theory yield: Hardness Amplification: We attain Fine-Grained problems that are hard on average (Ball et al., STOC '17). By achieving average-case hardness within the Fine-Grained world, we use this as a stepping stone to achieve both Cryptographic primitives and Derandomization. Cryptography: We obtain the first Proofs of Work (PoWs) from worst-case complexity assumptions, thus finally placing these fundamental primitives on a rigorous theoretical foundation (Ball et al., CRYPTO '18). We further propose the concept of Fine-Grained Cryptography and this call has now been answered in (LaVigne et al., CRYPTO '20) where some progress is made towards achieving Public-Key Fine-Grained Cryptography. Derandomization: We construct complexity-theoretic Pseudorandom Generators (Car- mosino et al., ICALP '18). This both achieves the best known derandomizations from uniform assumptions as well as connects the problem-centric Fine-Grained Complexity to the resource-centric study in Complexity Theory of randomness as a resource. i To my Mom who taught me to breathe, my Dad who taught me to listen to the breath of others, and to the Queer, Trans, and POC community who gave me room to breathe. To all those with just as much potential but not enough room to breathe. ii Contents Contents ii 1 Introduction1 2 The Fine Grained World: Background4 2.1 Fine-Grained Hardness: Hardness Within P .................. 6 2.2 The Main Islands of Fine-Grained Complexity................. 8 2.3 Fine-Grained Easiness: Connecting to Classical Complexity Through Fine- Grained Collapses................................. 10 2.4 Connecting the Fine-Grained and Classical Worlds .............. 11 3 Average-Case Fine-Grained Hardness 13 3.1 Introduction.................................... 14 3.1.1 Our Results................................ 15 3.1.2 Related Work............................... 17 3.1.3 Organization ............................... 19 3.2 Worst-Case Conjectures ............................. 19 3.2.1 Main Islands of Fine-Grained Complexity................ 19 3.2.2 Auxiliary Problems............................ 21 3.3 Average-Case Fine-Grained Hardness...................... 22 3.3.1 Orthogonal Vectors............................ 23 3.3.2 3SUM and All-Pairs Shortest Path ................... 25 3.3.3 SETH, 3SUM, and All-Pairs Shortest Path............... 26 3.4 An Average-Case Time Hierarchy........................ 27 3.5 Towards Fine-Grained Cryptography ...................... 29 3.5.1 Fine-Grained One-Way Functions.................... 29 3.5.2 Barriers and NSETH ........................... 30 3.5.3 A Way Around .............................. 31 3.5.4 An MA Protocol ............................. 32 3.5.5 Proofs of Work .............................. 33 3.5.6 On the Heuristic Falsifiability of Conjectures.............. 35 3.6 Open Questions.................................. 37 iii 4 Proofs of Work From Worst-Case Assumptions 39 4.1 Introduction.................................... 40 4.1.1 On Security From Worst-Case Assumptions .............. 41 4.1.2 Our Results................................ 41 4.1.3 Related Work............................... 43 4.2 Proofs of Work from Worst-Case Assumptions................. 44 4.2.1 Definition ................................. 44 4.2.2 Orthogonal Vectors............................ 45 4.2.3 Preliminaries ............................... 47 4.2.4 The PoW Protocol............................ 49 4.3 Verifying FOVk .................................. 52 4.4 A Direct Sum Theorem for FOV ........................ 54 4.5 Removing Interaction............................... 59 4.6 Zero-Knowledge Proofs of Work......................... 62 5 Fine-Grained Derandomization: From Problem-Centric to Resource- Centric Complexity 66 5.1 Introduction.................................... 67 5.1.1 Our Results................................ 68 5.1.2 Related Work............................... 70 5.2 Preliminaries ................................... 71 5.2.1 Fine-Grained Hardness.......................... 72 5.2.2 Derandomization............................. 75 5.2.3 Uniform Derandomization........................ 76 5.3 Fine-Grained Derandomization ......................... 77 5.3.1 Counting k-OV from Distinguishers................... 78 5.3.2 Printing Distinguishers from Failed Derandomization......... 79 5.4 Open Questions.................................. 85 Bibliography 87 A Appendix for Average-Case Fine-Grained Hardness 99 A.1 Evaluating Low Degree Polynomials....................... 99 A.2 Polynomials Computing Sums.......................... 100 A.3 CONVOLUTION-3SUM............................. 101 A.4 A Tighter Reduction for FOV.......................... 102 A.5 Isolating Orthogonal Vectors........................... 106 B Appendix for Proofs of Work From Worst-Case Assumptions 110 B.1 A Stronger Direct Sum Theorem for FOV . 110 C Appendix for Fine-Grained Derandomization 114 iv C.1 A Polynomial For k-CLIQUE . 114 C.2 Heuristics Imply Separations........................... 116 v Acknowledgments All that you touch You Change. All that you Change Changes you. { Octavia E. Butler, Parable of the Sower After being accepted to UC Berkeley, I went to their Visit Days where, at one point, all of us new admits were gathered into a large auditorium and told all the great things that would await us at Berkeley if we accepted the offer. One thing, that wasn't Berkeley-specific, but excited me immensely was being told \as different as you are now from when you graduated high school, is as different as you will be from now when you graduate from here." Feeling that I had Changed dramatically since my high school years, I couldn't begin imagining how much different I could be finishing my PhD given that I already felt like I knew who I was and what I wanted at that time, and this mystery excited me greatly. I proceeded to fight Change tooth and nail for the next few years. This is likely because I didn't actually have a good sense of who I was or what I wanted. Or maybe they just Changed. In either case, the prediction of massive Change was true and I was very right to be excited for it. This journey has been long and often painful and I have learned and unlearned much about academia and the world. So I'm going to take my time here as I look back on six years of Change and Growth and acknowledge those that helped nurture it: One thing I have learned in graduate school is that research is an emergent process. Course- work and textbooks paint a picture of knowledge that is like building blocks: You start with the fundamentals and then stack concepts like building blocks so that, if you're understanding the material, you can always see the bottom and the clear connections in-between to make a clean, linear, bottom-up view of knowledge. But the social activity of \research" immediately breaks this illusion. In any deep field of study, the myriad towers of building blocks are so high and vast and maze-like that the bottom is always hazy to make out and each person has a different