Revisiting the Spymasters: The Double Agent Problem

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Delve into the intriguing world of spymaster tactics with a focus on the double agent problem and the feasibility of secure multiparty computation protocols. Explore motivations, related works, and groundbreaking results in this complex domain.


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  1. Best of Both Worlds Revisiting the Spymasters Double Agent Problem Anasuya Acharya Carmit Hazay Oxana Poburinnaya Muthuramakrishnan Venkitasubramaniam Bar-Ilan University Bar-Ilan University Georgetown University

  2. Motivation: Bringing People of Different Beliefs 2

  3. Motivation: Resistance to Future Attacks 3

  4. Motivation: David Versus Many Goliaths [CDG87] 4

  5. When is MPC Feasible? Security from Unbounded Adversaries possible for honest majority Security from Arbitrary Corruption possible for PPT adversaries Together: Impossible in the plain model 5

  6. MPC with Fallback Security One Protocol Secure in the presence of Unbounded adversary for a limited adversary structure Fall-back - PPT adversary corrupting any subset of the parties similar notion considered in [Cha89] 6

  7. Related Work 2PC with one unbounded and one PPT corruption round optimal protocols [KM20, KO04, GMPP16, BPS22] David versus Multiple Goliaths [CDG87] Semi-Honest unconditional security for a designated party and arbitrary PPT corruption The Spymaster s Double Agent Problem [Cha89] Semi-Honest security for <n/2 unbounded corruption and arbitrary PPT corruption Malicious security for <n/3 unbounded corruption and <n/2 PPT corruption 7

  8. Our Results Theorem. Assuming broadcast, any n-party functionality can be realized by a protocol with malicious fallback security for adversary structure Z with semi-honest security for unbounded adversaries. Remark 1. Malicious security for semi-honest adversary structure security with abort, broadcast Remark 2. Black-box in underlying assumptions improves over [Cha89, CDG87] Remark 3. Asymptotically optimal round complexity improves over [CDG87] round complexity 8

  9. Does this Solve our Problems? 9

  10. Semi-Honest Fallback Security 10

  11. David vs Multiple Goliaths TFHE pk, [sk] pk, [sk] P1 Pn Cn C1=Enc(pk,x1) pk, [sk] P2 C2 {C} Any Functionality Semi-Honest Secure Unbounded A : all-but-Pn PPT A : any subset pk, [sk] P3 C3 Ev(f, {C}) pk, [sk] Pn-1 Cn-1 11

  12. David vs Multiple Goliaths Dist. GC r1 P1 Pn G P2 r2 = x ? Gb(f) G Any Functionality Semi-Honest Secure Unbounded A : all-but-Pn PPT A : any subset P3 r3 OT labels X Pn-1 f(x) = Ev(G, X) rn-1 12

  13. Compiling to Fallback Security [ ] [ ] [ ] [ ] n-Party Protocol Compiler Semi-Honest Secure Unbounded A : < n/2 parties PPT A : any subset 13

  14. Malicious Fallback Security 14

  15. Protocol in the Online-Offline Paradigm n-Party Offline Protocol Authenticated Multiplication Triples n-Party Online Protocol Functionality F using Authenticated Triples Maliciously Fallback Secure Inspired by [HVW20] Malicious Unbounded Secure Dishonest Majority in the Fcom-hybrid [DPSZ12] Authenticated Triples n-Party Commitment n-Party Commitment Fcom Maliciously Fallback Semi-Honest Fallback Maliciously Fallback Secure 15

  16. Fcom Fallback Secure Commitment Protocol x x n-Party Commitment Protocol Maliciously Secure Unbounded A : any subset in ZS PPT A : any subset 16

  17. Fallback Secure Commitment Protocol 2-party Statistically Hiding Commitments 2-party Statistically Binding Commitments Hiding Secret Sharing Binding Extractable using Statistical ZK Arguments 2-party Statistically Binding Commitments Proof of Knowledge Extractable using ZK 17

  18. Fallback Secure Commitment Protocol x ComSH(x;r) SZKAoK(x,r; c) ShareZs(x) { [x]i} ComSB([x]i;ri) ZKPoK([x]i,ri; ci) x, r, { [x]i}, ri used in look-ahead trapdoor commitment scheme [PW09] 18

  19. PPT Adversary (Security with Abort) Future Work Best-of-both-worlds for GOD and security with abort [Kat07, IKK+11, PRS20] This Work Unbounded Adversary (Security with Abort) Guaranteed Output Delivery 19

  20. Thank You! https://ia.cr/2023/1013 20

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