Interactive Proofs Lecture 19 And Beyond 1 So far 2 So far IP - - PowerPoint PPT Presentation

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Interactive Proofs Lecture 19 And Beyond 1 So far 2 So far IP - - PowerPoint PPT Presentation

Interactive Proofs Lecture 19 And Beyond 1 So far 2 So far IP = PSPACE = AM[poly] 2 So far IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] 2 So far IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly]


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SLIDE 1

Interactive Proofs

Lecture 19 And Beyond

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SLIDE 2

So far

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SLIDE 3

So far

IP = PSPACE = AM[poly]

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SLIDE 4

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes]

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SLIDE 5

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization

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SLIDE 6

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n)

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SLIDE 7

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n) Using a public-coin set lower-bound proof

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SLIDE 8

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n) Using a public-coin set lower-bound proof AM[k] = AM for constant k ! 2

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SLIDE 9

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n) Using a public-coin set lower-bound proof AM[k] = AM for constant k ! 2 Using MA ⊆ AM and alternate characterization in terms of pairs of complementary ATTMs

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SLIDE 10

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n) Using a public-coin set lower-bound proof AM[k] = AM for constant k ! 2 Using MA ⊆ AM and alternate characterization in terms of pairs of complementary ATTMs Perfect completeness: One-sided-error-AM = AM

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SLIDE 11

So far

IP = PSPACE = AM[poly] PSPACE enough to calculate max Pr[yes] AM[poly] protocol for TQBF using arithmetization In fact IP[k] ⊆ AM[k+2] for all k(n) Using a public-coin set lower-bound proof AM[k] = AM for constant k ! 2 Using MA ⊆ AM and alternate characterization in terms of pairs of complementary ATTMs Perfect completeness: One-sided-error-AM = AM Similar to BPP ⊆ Σ2P (yields MAM protocol; MAM=AM)

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AM ⊆ Π2P

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AM ⊆ Π2P

Consider any L with an AM protocol

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AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness:

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AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness: x∈L ⇒ ∀ yArthur ∃ zMerlin R(x,yArthur,zMerlin) = 1

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SLIDE 16

AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness: x∈L ⇒ ∀ yArthur ∃ zMerlin R(x,yArthur,zMerlin) = 1 And by (any positive) soundness:

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SLIDE 17

AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness: x∈L ⇒ ∀ yArthur ∃ zMerlin R(x,yArthur,zMerlin) = 1 And by (any positive) soundness: x∉L ⇒ ∃ yArthur ∀ zMerlin R(x,yArthur,zMerlin) = 0

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SLIDE 18

AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness: x∈L ⇒ ∀ yArthur ∃ zMerlin R(x,yArthur,zMerlin) = 1 And by (any positive) soundness: x∉L ⇒ ∃ yArthur ∀ zMerlin R(x,yArthur,zMerlin) = 0 i.e., x∈L ⇔ ∀y ∃z R(x,y,z) = 1

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SLIDE 19

AM ⊆ Π2P

Consider any L with an AM protocol By perfect completeness: x∈L ⇒ ∀ yArthur ∃ zMerlin R(x,yArthur,zMerlin) = 1 And by (any positive) soundness: x∉L ⇒ ∃ yArthur ∀ zMerlin R(x,yArthur,zMerlin) = 0 i.e., x∈L ⇔ ∀y ∃z R(x,y,z) = 1 Similarly, MA ⊆ Σ2P

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AM and coNP

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP

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SLIDE 24

AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP MAM protocol for L: Merlin sends y, and then they run an AM protocol for (x,y) ∈ L ’

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP MAM protocol for L: Merlin sends y, and then they run an AM protocol for (x,y) ∈ L ’ But MAM = AM

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP MAM protocol for L: Merlin sends y, and then they run an AM protocol for (x,y) ∈ L ’ But MAM = AM Corollary: If GI is NP-complete, PH collapses (recall GNI ∈ AM)

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP MAM protocol for L: Merlin sends y, and then they run an AM protocol for (x,y) ∈ L ’ But MAM = AM Corollary: If GI is NP-complete, PH collapses (recall GNI ∈ AM)

P BPP coNP NP

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AM and coNP

If coNP ⊆ AM, then PH collapses to level 2 Will show coNP ⊆ AM ⇒ Σ2P ⊆ AM ⊆ Π2P L ∈ Σ2P: { x| ∃y (x,y) ∈ L ’} where L ’ ∈ coNP MAM protocol for L: Merlin sends y, and then they run an AM protocol for (x,y) ∈ L ’ But MAM = AM Corollary: If GI is NP-complete, PH collapses (recall GNI ∈ AM)

P BPP coNP NP AM

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BPP RP

Zoo

P

PSPACE

EXP NP NEXP L Σ2P MA AM Π2P IP

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Program Checking

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Program Checking

Suppose a special computer (using nano-bio-quantum technology!) is being sold for solving Graph Non-Isomorphism (GNI) efficiently

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Program Checking

Suppose a special computer (using nano-bio-quantum technology!) is being sold for solving Graph Non-Isomorphism (GNI) efficiently How do we trust this?

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Program Checking

Suppose a special computer (using nano-bio-quantum technology!) is being sold for solving Graph Non-Isomorphism (GNI) efficiently How do we trust this? Vendor: Trust me, this always works

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Program Checking

Suppose a special computer (using nano-bio-quantum technology!) is being sold for solving Graph Non-Isomorphism (GNI) efficiently How do we trust this? Vendor: Trust me, this always works User: In fact I just care if it works correctly on the inputs I want to solve. Maybe for each input I have, your machine could prove correctness using an IP protocol?

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Program Checking

Suppose a special computer (using nano-bio-quantum technology!) is being sold for solving Graph Non-Isomorphism (GNI) efficiently How do we trust this? Vendor: Trust me, this always works User: In fact I just care if it works correctly on the inputs I want to solve. Maybe for each input I have, your machine could prove correctness using an IP protocol? Vendor: But I don’ t have a (nano-bio-quantum) implementation of the prover’ s program...

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Program Checking

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Program Checking

Program checker

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Program Checking

Program checker

User

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Program Checking

Program checker

User

P

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Program Checking

Program checker

User checker

P

x

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Program Checking

Program checker

User checker

P

x

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Program Checking

Program checker

User checker

P

x

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Program Checking

Program checker

User checker

P

x f(x) or P!f

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Program Checking

Program checker On each input, either ensures (w.h.p) that P’ s output is correct,

  • r finds out that P"f, efficiently

User checker

P

x f(x) or P!f

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Program Checking

Program checker On each input, either ensures (w.h.p) that P’ s output is correct,

  • r finds out that P"f, efficiently

Completeness: Vendor need not fear being falsely accused

User checker

P

x f(x) or P!f

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SLIDE 46

Program Checking

Program checker On each input, either ensures (w.h.p) that P’ s output is correct,

  • r finds out that P"f, efficiently

Completeness: Vendor need not fear being falsely accused Soundness: User need not fear using a wrong value as f(x)

User checker

P

x f(x) or P!f

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SLIDE 47

Program Checking

Program checker On each input, either ensures (w.h.p) that P’ s output is correct,

  • r finds out that P"f, efficiently

Completeness: Vendor need not fear being falsely accused Soundness: User need not fear using a wrong value as f(x) Will consider boolean f (i.e., a language L)

User checker

P

x f(x) or P!f

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Program Checking and IP

User f(x) or P!f

P

x

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Program Checking and IP

PC for L from IP protocols (for L and Lc)

User f(x) or P!f

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc)

User f(x) or P!f Verifier

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x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be

User f(x) or P!f Verifier

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x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC!

User f(x) or P!f Verifier

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x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC!

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC!

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC!

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC!

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC! Retains completeness and soundness

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC! Retains completeness and soundness e.g. For PSPACE-complete L (why?)

User f(x) or P!f

L

Verifier

P

x

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Prover

Program Checking and IP

PC for L from IP protocols (for L and Lc) PC must be efficient. Provers may not be If provers (for L and Lc) are efficient given L-oracle, can construct PC! Retains completeness and soundness e.g. For PSPACE-complete L (why?) How about Graph Isomorphism?

User f(x) or P!f

L

Verifier

P

x

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Program Checking for GI

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism Pick node v1 in G0. For each node u in G1 attach a marker (say a large clique) to u and v1 and ask if the new graphs G0’ and G1’ are isomorphic.

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism Pick node v1 in G0. For each node u in G1 attach a marker (say a large clique) to u and v1 and ask if the new graphs G0’ and G1’ are isomorphic. If P says no for all u in G1, report “P bad”

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism Pick node v1 in G0. For each node u in G1 attach a marker (say a large clique) to u and v1 and ask if the new graphs G0’ and G1’ are isomorphic. If P says no for all u in G1, report “P bad” Else remember v1u, and continue with v2; keep old markers and use new larger markers to get G0’’ and G1’’

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism Pick node v1 in G0. For each node u in G1 attach a marker (say a large clique) to u and v1 and ask if the new graphs G0’ and G1’ are isomorphic. If P says no for all u in G1, report “P bad” Else remember v1u, and continue with v2; keep old markers and use new larger markers to get G0’’ and G1’’ On finding isomorphism, verify and output G0 ≡ G1

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Program Checking for GI

If P(G0,G1) says G0 ≡ G1, try to extract the isomorphism Pick node v1 in G0. For each node u in G1 attach a marker (say a large clique) to u and v1 and ask if the new graphs G0’ and G1’ are isomorphic. If P says no for all u in G1, report “P bad” Else remember v1u, and continue with v2; keep old markers and use new larger markers to get G0’’ and G1’’ On finding isomorphism, verify and output G0 ≡ G1 Note: An IP protocol (i.e., NP proof) for GI, where prover is in PGI

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Program Checking for GI

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle)

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle) Let H = π(Gb) where π is a random permutation and b = 0 or 1 at random

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle) Let H = π(Gb) where π is a random permutation and b = 0 or 1 at random Run P(G0,H) with many such H

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle) Let H = π(Gb) where π is a random permutation and b = 0 or 1 at random Run P(G0,H) with many such H If P says G0 ≡ H exactly whenever b=0, output G0 ≢ G1

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle) Let H = π(Gb) where π is a random permutation and b = 0 or 1 at random Run P(G0,H) with many such H If P says G0 ≡ H exactly whenever b=0, output G0 ≢ G1 Else output “Bad P”

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Program Checking for GI

If P(G0,G1) says G0 ≢ G1, test P similar to in IP protocol for GNI (coke from can/bottle) Let H = π(Gb) where π is a random permutation and b = 0 or 1 at random Run P(G0,H) with many such H If P says G0 ≡ H exactly whenever b=0, output G0 ≢ G1 Else output “Bad P” Note: Prover in the IP protocol for GNI is in PGI

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Multi-Prover Interactive Proofs

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately Provers can’ t talk to each other during the interrogation (but can agree on a strategy a priori)

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately Provers can’ t talk to each other during the interrogation (but can agree on a strategy a priori) Verifier cross-checks answers from the provers

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately Provers can’ t talk to each other during the interrogation (but can agree on a strategy a priori) Verifier cross-checks answers from the provers 2 provers as good as k provers

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately Provers can’ t talk to each other during the interrogation (but can agree on a strategy a priori) Verifier cross-checks answers from the provers 2 provers as good as k provers MIP = NEXP

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Multi-Prover Interactive Proofs

Interrogate multiple provers separately Provers can’ t talk to each other during the interrogation (but can agree on a strategy a priori) Verifier cross-checks answers from the provers 2 provers as good as k provers MIP = NEXP Parallel repetition theorem highly non-trivial!

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Probabilistically Checkable Proofs (PCPs)

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof Verifier reads some positions (probabilistically chosen) from the proof and decides to accept or reject

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof Verifier reads some positions (probabilistically chosen) from the proof and decides to accept or reject PCP[r,q]: length of proof 2r, number of queries q

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof Verifier reads some positions (probabilistically chosen) from the proof and decides to accept or reject PCP[r,q]: length of proof 2r, number of queries q Intuitively, in MIP, the provers cannot change their strategy (because one does not know what the other sees), so must stick to a prior agreed up on strategy

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof Verifier reads some positions (probabilistically chosen) from the proof and decides to accept or reject PCP[r,q]: length of proof 2r, number of queries q Intuitively, in MIP, the provers cannot change their strategy (because one does not know what the other sees), so must stick to a prior agreed up on strategy Which will be the written proof

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Probabilistically Checkable Proofs (PCPs)

Prover submits a (very long) written proof Verifier reads some positions (probabilistically chosen) from the proof and decides to accept or reject PCP[r,q]: length of proof 2r, number of queries q Intuitively, in MIP, the provers cannot change their strategy (because one does not know what the other sees), so must stick to a prior agreed up on strategy Which will be the written proof PCP[poly,poly] = MIP = NEXP

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PCP Theorem

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PCP Theorem

NP = PCP[log,const]

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PCP Theorem

NP = PCP[log,const] PCP is only poly long (just like usual NP certificate)

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PCP Theorem

NP = PCP[log,const] PCP is only poly long (just like usual NP certificate) But verifier reads only constantly many bits!

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PCP Theorem

NP = PCP[log,const] PCP is only poly long (just like usual NP certificate) But verifier reads only constantly many bits! Extensively useful in proving “hardness of approximation” results for optimization problems

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PCP Theorem

NP = PCP[log,const] PCP is only poly long (just like usual NP certificate) But verifier reads only constantly many bits! Extensively useful in proving “hardness of approximation” results for optimization problems Also useful in certain cryptographic protocols

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Zero-Knowledge Proofs

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Zero-Knowledge Proofs

Interactive Proof for membership in L

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L

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SLIDE 98

Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L

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SLIDE 99

Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L

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SLIDE 100

Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L

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SLIDE 101

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L

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SLIDE 102

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated”

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SLIDE 103

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated”

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SLIDE 104

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated”

14

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SLIDE 105

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated” x i n L

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SLIDE 106

Ah, got it!

42

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated” x i n L

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SLIDE 107

Ah, got it!

42

Ah, got it!

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Zero-Knowledge Proofs

Interactive Proof for membership in L Complete and Sound ZK Property: Verifier “learns nothing” except that x is in L Verifier’ s view could have been “simulated” For every adversarial strategy, there exists a simulation strategy x i n L

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Summary

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Summary

Interactive Protocols

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Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness

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Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness Zoo: MA and AM, between 1st and 2nd levels of PH

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Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness Zoo: MA and AM, between 1st and 2nd levels of PH Other related concepts

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SLIDE 113

Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness Zoo: MA and AM, between 1st and 2nd levels of PH Other related concepts MIP, PCP, ZK proofs

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SLIDE 114

Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness Zoo: MA and AM, between 1st and 2nd levels of PH Other related concepts MIP, PCP, ZK proofs Understanding power of interaction/non-determinism and randomness

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SLIDE 115

Summary

Interactive Protocols Public coins, ATTMs, collapse of AM[k], arithmetization, set lower-bound, perfect completeness Zoo: MA and AM, between 1st and 2nd levels of PH Other related concepts MIP, PCP, ZK proofs Understanding power of interaction/non-determinism and randomness Useful in “hardness of approximation”, in cryptography, ...

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