Communication Complexity of Randomness...

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Communication Complexity of Randomness Manipulation

Madhu SudanHarvard University

Joint works with Mitali Bafna (Harvard), Badih Ghazi (Google), and Noah Golowich (Harvard)

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Randomness Processing Industry

Dispersers, Extractors, Merges, Condensers, PRGs …

Interest not in the exact function. (E=RSA most boring.)

Rather in its manipulation of distributions … Distribution of 𝑋𝑋 vs. that of 𝐸𝐸(𝑋𝑋)

Long history … omitted. Key ingredients single processor “𝐸𝐸” unknown source 𝑋𝑋 ∈ 𝒳𝒳

June 03, 2019 IITB: Comm. Comp. Randomness Manipulation 2

𝐸𝐸𝑋𝑋 𝐸𝐸(𝑋𝑋)

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2-player setting:

Is 𝛿𝛿 = 0 possible? If not minimize 𝛿𝛿! Etc. Alice + Bob can use private randomness Zero communication version: “Non-Interactive

Simulation” (NIS).

Distributed Randomness Processing

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Alice 𝑈𝑈

Bob𝑌𝑌 𝑉𝑉

≤ 𝐶𝐶 bits, ≤ 𝑟𝑟 Rounds

𝑋𝑋𝑋𝑋1,𝑋𝑋2, …

𝑌𝑌1,𝑌𝑌2, …

𝑋𝑋,𝑌𝑌 ∼ 𝑃𝑃 Goal: 𝑈𝑈,𝑉𝑉 ≈𝛿𝛿 𝑄𝑄

Infinite sequence

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A classical example

Zero communication 𝑋𝑋,𝑌𝑌 ∼ Unif 0,0 , 0,1 , 1,0 𝑈𝑈,𝑉𝑉 ∼ 𝜌𝜌-correlated bits.

𝑈𝑈,𝑉𝑉 ∼ Unif 0,1 , Pr 𝑈𝑈 = 𝑉𝑉 = 1+𝜌𝜌2

[Witsenhausen ‘70s]: Can’t achieve 𝜌𝜌 = 1. Not even 𝜌𝜌 = .51

(Naïve strategy achieves 𝜌𝜌 = 14)

Can achieve 𝜌𝜌 = 13

by a general method.

Best 𝜌𝜌? Open!!

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Witsenhausen …

Intermediate Problem: Output (𝐴𝐴,𝐵𝐵) w. 𝐴𝐴,𝐵𝐵 ∼ 𝑁𝑁 0,1with maximum correlation.

Definition: Max-Corr 𝜌𝜌 𝑋𝑋,𝑌𝑌 ≜ max𝑓𝑓,𝑔𝑔

𝔼𝔼𝑋𝑋,𝑌𝑌 𝑓𝑓 𝑋𝑋 𝑔𝑔(𝑌𝑌)

Where 𝑓𝑓,𝑔𝑔:Ω → ℝ,𝔼𝔼𝑋𝑋 𝑓𝑓 𝑋𝑋 = 𝔼𝔼𝑌𝑌 𝑔𝑔 𝑌𝑌 = 0𝔼𝔼𝑋𝑋 𝑓𝑓 𝑋𝑋 2 = 𝔼𝔼𝑌𝑌 𝑔𝑔 𝑌𝑌 2 = 1

Thm 1: For any 𝑈𝑈,𝑉𝑉 output, 𝜌𝜌 𝑈𝑈,𝑉𝑉 ≤ 𝜌𝜌(𝑋𝑋,𝑌𝑌) Thm 2: For Gaussian output (𝐴𝐴,𝐵𝐵), can achieve

𝜌𝜌 𝐴𝐴,𝐵𝐵 = 𝜌𝜌(𝑋𝑋,𝑌𝑌) Thm 3: For binary 𝑈𝑈,𝑉𝑉 , can achieve

1 −2 ⋅ cos−1 𝜌𝜌 𝑋𝑋,𝑌𝑌

𝜋𝜋≤ 𝜌𝜌 𝑈𝑈,𝑉𝑉 ≤ 𝜌𝜌 𝑋𝑋,𝑌𝑌

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For Bernoulli/GaussiansMax-Correlation = Correlation

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“Tensorization”

Amortized Problems: Given (1) structure 𝑆𝑆 (graph, distribution, game) (2) product operation 𝑆𝑆⊗𝑡𝑡, (3) amortized measure 𝑀𝑀, compute lim

𝑡𝑡→∞𝑀𝑀 𝑆𝑆⊗𝑡𝑡

Shannon capacity, Compression length, Channel capacity, parallel repetition value of 2-prover game, Direct sum complexity, NIS

“Tensorizing bound”: Typically: 𝑀𝑀 𝑆𝑆⊗𝑡𝑡 ≥ 𝑀𝑀(𝑆𝑆). Find: ℳ(⋅) s.t. 𝑀𝑀 𝑆𝑆 ≤ ℳ(𝑆𝑆) and ℳ 𝑆𝑆⊗𝑡𝑡 = ℳ(𝑆𝑆)

Max-Correlation “Tensorizes” If 𝑋𝑋,𝑌𝑌 ∼ 𝜇𝜇 & 𝑋𝑋𝑡𝑡,𝑌𝑌𝑡𝑡 ∼ 𝜇𝜇⊗𝑡𝑡 then 𝜌𝜌 𝑋𝑋𝑡𝑡 ,𝑌𝑌𝑡𝑡 = 𝜌𝜌(𝑋𝑋,𝑌𝑌) Proof idea: 𝜇𝜇 described by matrix 𝑃𝑃; 𝜌𝜌 related to its

singular values, 𝜇𝜇⊗𝑡𝑡 described by 𝑃𝑃⊗𝑡𝑡

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Problem Single-shot Amortized

Compressibility P P

Channel capacity NP P

Problem Single-shot Amortized

Compressibility P P

Channel capacity NP P

Independent set/Shannon capacity

NP [0,∞]

Value of 2-prover game NP [NP,∞]

Problem Single-shot Amortized

Compressibility P P

Channel capacity NP P

Independent set/Shannon capacity

NP [0,∞]

Value of 2-prover game NP [NP,∞]

Non-interactive-simulation (“correlation”)

NP [0,𝐶𝐶𝐴𝐴]

Communication Complexity [NP,Exp?] 0,𝐶𝐶𝐴𝐴

Problem Single-shot Amortized

Compressibility P P

Channel capacity NP P

Independent set/Shannon capacity

NP [0,∞]

Value of 2-prover game NP [NP,∞]

Non-interactive-simulation (“correlation”)

NP [0,𝐶𝐶𝐴𝐴]

Communication Complexity [NP,Exp?] 0,𝐶𝐶𝐴𝐴

? P [NP,∞]

Amortized Embarassment

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Glossary: 0 ≤ P ≤ 𝑁𝑁𝑃𝑃 ≤ EXP ≤ Computable≤ CA Computably Approximable ≤ ∞

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Complexity of NIS

“Computably approximable” [Ghazi,Kamath,S.], [De,Mossel,Neeman]2,

[Ghazi,Kamath,Raghavendra] There exists a finite time algorithm determining if we can

get 𝜖𝜖-close to (𝑈𝑈,𝑉𝑉) from i.i.d. samples of (𝑋𝑋,𝑌𝑌)

Idea: “Invariance Priniciple” [Mossel] Either there is a method using few samples of (𝑋𝑋,𝑌𝑌) or

many samples 𝑋𝑋1,𝑌𝑌1 , … , 𝑋𝑋𝑛𝑛,𝑌𝑌𝑛𝑛 each with low influence on 𝑈𝑈,𝑉𝑉 .

If latter, can replace 𝑋𝑋,𝑌𝑌 by finite # Gaussians. Gives computable upper bound on #samples needed to

get close to optimal strategy.

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Communication?

Restrict to “Common Randomness Generation” (𝑘𝑘-CRG): Output (𝑈𝑈 = 𝑉𝑉), with 𝑈𝑈 ∼ Unif( 0,1 𝑘𝑘)

First considered ~70 [Ahlswede-Csizar]: “Characterization”: Can communicate 𝑅𝑅 ⋅ 𝑘𝑘 +𝑜𝑜 𝑘𝑘 bits, where 𝑅𝑅 = min

ΠICint(Π)ICext Π

Computable? Computably approximable? For one-way communication: 𝑅𝑅 = 1 − 𝜌𝜌 𝑋𝑋,𝑌𝑌 2

[Zhao-Chia] (see also [Guruswami-Radhakrishnan, Ghazi-Jayram, S.-Tyagi-Watanabe])

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Alice 𝑈𝑈

Bob 𝑉𝑉

≤ 𝐶𝐶 bits, ≤ 𝑟𝑟 Rounds

𝑋𝑋1,𝑋𝑋2, …

𝑌𝑌1,𝑌𝑌2, …

𝑋𝑋,𝑌𝑌 ∼ 𝑃𝑃 Goal: 𝑈𝑈,𝑉𝑉 ≈𝛿𝛿 𝑄𝑄

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This talk: Rounds in CRG

Does increasing #rounds ease CRG? (For some (𝑋𝑋,𝑌𝑌)?) … Is the following true?

Question: ∀𝜖𝜖 > 0 , 𝑟𝑟, ∃(𝑋𝑋,𝑌𝑌) s.t. ∀𝑘𝑘 ∃ 𝑟𝑟, 𝜖𝜖𝑘𝑘 -protocol for 𝑘𝑘-CRG 𝑋𝑋,𝑌𝑌 No 𝑟𝑟 − 1, 1 − 𝜖𝜖 .𝑘𝑘 -protocol for 𝑘𝑘-CRG(𝑋𝑋,𝑌𝑌)

Still don’t know. Partial progress. Thm. [BGGS’19] : ∀𝑛𝑛,𝑘𝑘, 𝑟𝑟 there exists 𝑋𝑋,𝑌𝑌 s.t.

∃ 𝑟𝑟,𝑂𝑂(𝑟𝑟 log 𝑛𝑛 -protocol for 𝑘𝑘-CRG(𝑋𝑋,𝑌𝑌).

No 𝑟𝑟2− 3, min 𝑘𝑘, 𝑛𝑛

polylog 𝑛𝑛-protocol for 𝑘𝑘-CRG(𝑋𝑋,𝑌𝑌)

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Our Source:

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𝑖𝑖0

𝜋𝜋1

𝜋𝜋2

𝜋𝜋3

𝜋𝜋𝑟𝑟

⋮

𝐴𝐴1 𝐴𝐴2 𝐴𝐴3 𝐴𝐴𝑛𝑛 𝐵𝐵1 𝐵𝐵2 𝐵𝐵3 𝐵𝐵𝑛𝑛… …

Alice’s Inputs𝑋𝑋 = (𝜋𝜋1,𝜋𝜋3, … ;𝐴𝐴1, … ,𝐴𝐴𝑛𝑛)

Bob’s Inputs𝑌𝑌 = (𝑖𝑖0;𝜋𝜋2,𝜋𝜋4, … ;𝐵𝐵1, … ,𝐵𝐵𝑛𝑛)

𝑖𝑖0 ∼ [𝑛𝑛]

𝜋𝜋1, … ,𝜋𝜋𝑛𝑛 ∼ 𝑆𝑆𝑛𝑛𝐴𝐴1, … ,𝐴𝐴𝑛𝑛 ∼ 0,1 𝑘𝑘

𝐵𝐵1, … ,𝐵𝐵𝑛𝑛 ∼ 0,1 𝑘𝑘

𝐴𝐴𝑖𝑖𝑟𝑟 = 𝐵𝐵𝑖𝑖𝑟𝑟where 𝑖𝑖𝑗𝑗 = 𝜋𝜋𝑗𝑗 𝑖𝑖𝑗𝑗−1

“Pointer Chasing Source” (PCS)

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CRG from Pointer Chasing Source

Easy direction easy: (𝑟𝑟, 𝑟𝑟 log𝑛𝑛)-protocol for 𝑘𝑘-CRG Hardness?

Pointer chasing is hard: [Duris-Galil-Schnitger, Nisan-Wigderson,…].

CRG from PCS requires pointer chasing? No! Can solve problem without pointers! Small non-deterministic complexity! Hardness needs to use hardness of

disjointness? And of pointer-chasing? How to combine modularly?

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Our solution: Pointer Verification Problem

PVP: Distinguish 𝑋𝑋,𝑌𝑌 ∼ 𝑃𝑃𝑉𝑉YES from 𝑋𝑋,𝑌𝑌 ∼ 𝑃𝑃𝑉𝑉No 𝑃𝑃𝑉𝑉YES:𝑋𝑋 = 𝜋𝜋1,𝜋𝜋3, …𝜋𝜋𝑟𝑟−1 ; 𝑌𝑌 = 𝑖𝑖0, 𝑖𝑖𝑟𝑟; 𝜋𝜋2,𝜋𝜋4, …𝜋𝜋𝑟𝑟 𝑃𝑃𝑉𝑉NO:𝑋𝑋 = 𝜋𝜋1,𝜋𝜋3, …𝜋𝜋𝑟𝑟−1 ; 𝑌𝑌 = 𝑖𝑖0, 𝑖𝑖𝑟𝑟′ ; 𝜋𝜋2,𝜋𝜋4, …𝜋𝜋𝑟𝑟

YES instance satisfy: 𝑖𝑖𝑗𝑗 = 𝜋𝜋𝑗𝑗 𝑖𝑖𝑗𝑗−1 NO instance: 𝑖𝑖𝑟𝑟′ random

Claims: Hardness of (Unique) Set Disjointness ⇒

Protocol for 𝑘𝑘-CRG(PCS) solves PVP.

No 𝑟𝑟2− 𝑂𝑂 1 , 𝑛𝑛

polylog 𝑛𝑛-protocol for PVP.

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PVP ≤ CRG

Let 𝜇𝜇𝑋𝑋𝑌𝑌 denote dist. of PCS source. 𝜇𝜇𝑋𝑋, 𝜇𝜇𝑌𝑌 marginals

CRG Hard ⇐ 𝜇𝜇𝑋𝑋𝑌𝑌 indist. from 𝜇𝜇𝑋𝑋 × 𝜇𝜇𝑌𝑌 (to 𝑟𝑟2

,𝐶𝐶 -protocols)

Intermediate distribution: 𝜇𝜇mid 𝑋𝑋 = (𝜋𝜋odd,𝐴𝐴1, … ,𝐴𝐴𝑛𝑛); 𝑌𝑌 = 𝑖𝑖0,𝜋𝜋even,𝐵𝐵1, … ,𝐵𝐵𝑛𝑛 ,

with 𝐴𝐴𝑗𝑗 = 𝐵𝐵𝑗𝑗 for random 𝑗𝑗. (Correlation exists but pointers don’t point to it.)

Claims: 𝜇𝜇𝑋𝑋𝑌𝑌 indist. from 𝜇𝜇mid ⇐ PVP is hard 𝜇𝜇mid indist. from 𝜇𝜇𝑋𝑋 × 𝜇𝜇𝑌𝑌 ⇐ (Unique) disjointness

hard.

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𝑖𝑖

⋮

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Bulk of paper: PVP is hard

Main idea: “Round elimination” a la NW ’93 No black-box use No simple variant

Induction on #rounds Many invariants … roughly

𝐻𝐻 𝜋𝜋odd,𝜋𝜋even transcript) ≈ 𝑀𝑀𝑀𝑀𝑀𝑀 − 𝑂𝑂 𝐶𝐶 𝐻𝐻 𝑖𝑖𝑡𝑡 transcript) ≈ log𝑛𝑛 − 𝑜𝑜 1 𝑋𝑋 ⫫ 𝑌𝑌 | path, transcript

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Conclusions

Many interesting problems in distributed randomness manipulation.

Complexity of the “Single-letter characterizations”.

Tight characterization of round complexity of CRG.

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Thank You!

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