Two Round Information-Theoretic MPC with Malicious Security Prabhanjan Ananth Arka Rai Choudhuri Aarushi Goel Abhishek Jain TPMPC 2019
Adversarial Model
Adversarial Model Malicious Adversary
Adversarial Model Malicious Adversary Corrupts < "/2 parties (Honest Majority)
Honest Majority MPC
Honest Majority MPC Information-Theoretic security is possible. [Ben-Or, Goldwasser, Widgerson’88] Typically UC secure Simulation proofs are typically straight-line Round complexity lower bounds for dishonest majority do not apply 4 rounds necessary for dishonest majority in the plain model [Garg- Mukherjee-Pandey-Polychroniadou16] Clean Constructions Use lightweight tools such as garbling and secret-sharing
Honest Majority MPC Information-Theoretic security is possible. [Ben-Or, Goldwasser, Widgerson’88] Typically UC secure Simulation proofs are typically straight-line Round complexity lower bounds for dishonest majority do not apply 4 rounds necessary for dishonest majority in the plain model [Garg- Mukherjee-Pandey-Polychroniadou16] Clean Constructions Use lightweight tools such as garbling and secret-sharing
Honest Majority MPC Information-Theoretic security is possible. [Ben-Or, Goldwasser, Widgerson’88] Typically UC secure Simulation proofs are typically straight-line Round complexity lower bounds for dishonest majority do not apply 4 rounds necessary for dishonest majority in the plain model [Garg- Mukherjee-Pandey-Polychroniadou16] Clean Constructions Use lightweight tools such as garbling and secret-sharing
Honest Majority MPC Information-Theoretic security is possible. [Ben-Or, Goldwasser, Widgerson’88] Typically UC secure Simulation proofs are typically straight-line Round complexity lower bounds for dishonest majority do not apply 4 rounds necessary for dishonest majority in the plain model [Garg- Mukherjee-Pandey-Polychroniadou16] Clean Constructions Use lightweight tools such as garbling and secret-sharing
Honest Majority MPC: Applications Useful for constructing efficient ZK-protocols.
Honest Majority MPC: Applications (Courtesy: Sergey Gorbunov’s talk)
History of IT-MPC Round Class of Corruption Adversary Complexity Functions Threshold [BGW’88] > # of P/Poly t<n/2 Malicious multiplications [BB’89, IK’00, constant NC 1 t<n/2 Malicious AIK’06] [IKP’10] 2 NC 1 t<n/3 Malicious Security with selective abort [GIS’18, ABT’18] 2 NC 1 t<n/2 Semi-honest
Our Results Round Complexity Class of Functions Corruption Threshold Adversary 2 NC 1 t<n/2 Malicious Security with Abort over Broadcast + P2P Security with Selective Abort over P2P
Our Results Round Complexity Class of Functions Corruption Threshold Adversary 2 NC 1 t<n/2 Malicious Security with Abort over Concurrent Work [ABT19] Broadcast + P2P Consider security with selective abort. Security with Selective Abort over P2P
This Talk Round Complexity Class of Functions Corruption Threshold Adversary 2 NC 1 t<n/2 Malicious Security with Abort over Broadcast + P2P Security with Selective Abort over P2P
Our Strategy Round 2 Round IT-MPC Compression Constant Round IT-MPC (Privacy with Knowledge of (Security with Abort) Outputs) Broadcast + P2P Broadcast + P2P Security Upgrade 2 Round IT-MPC (Security with Abort) Broadcast + P2P
Security with Abort Party 2 ! Trusted Party Party 1 Party 3
Security with Abort ! 2 ! 1 Party 2 % ! 3 Trusted Party Party 1 Party 3
Security with Abort ! 2 ! 1 Party 2 % = '(! 1, ! 2, ! 3 ) ' ! 3 Trusted Party Party 1 Party 3
Security with Abort ! 2 ! 1 Party 2 % = '(! 1, ! 2, ! 3 ) ' ! 3 %’ = % ,- ⊥ Trusted Party Party 1 Party 3
Security with Abort ! 2 ! 1 %’ Party 2 % = '(! 1, ! 2, ! 3 ) ' ! 3 %’ = % ,- ⊥ Trusted Party Party 1 %’ Party 3
Security with Abort Privacy ! 2 and ! 3 remain hidden $
Security with Abort Privacy ! 2 and ! 3 remain hidden $ Output Correctness Honest Parties either output $ ! % , ! ' , ! ( or ⊥
Privacy with Knowledge of Outputs Privacy ! 2 and ! 3 remain hidden $ Output Correctness Honest Parties either output $ ! % , ! ' , ! ( or ⊥
First Step Round 2 Round IT-MPC Compression Constant Round IT-MPC (Privacy with Knowledge of (Security with Abort) Outputs) Broadcast + P2P Broadcast + P2P Security Upgrade 2 Round IT-MPC (Security with Abort) Broadcast + P2P
Using Signed Outputs [IKP10] " 1 & = ! (" ) , " + , " , ) ! " 2 " 3
Using Signed Outputs [IKP10] * = ! (# ( , # - , # . ) # 1 , &' ( , )' ( (&' (, 0 ( = 1234 (*, )' 1 )) !′ # - , &' - , )' - (&' -, 0 - = 1234 (*, )' - )) # . , &' . , )' . (&' ., 0 . = 1234 (*, )' . ))
Security with abort: Using Signed Outputs ! " , $% " , &% " -./01'(', ( ) , $% ) ) ', ( ) , $% ) , ( " , $% " , (( + , $% + ) -./01'(', ( " , $% " ) 1 ’ -./01'(', ( + , $% + ) Party 2 Trusted Party
Security with abort: Using Signed Outputs ! " , $% " , &% " ./01-'(', ( ) , $% ) ) ', ( ) , $% ) , ( " , $% " , (( + , $% + ) ./01-'(', ( " , $% " ) - ’ ./01-'(', ( + , $% + ) Party 2 Trusted Party Accept if all 3 verify
Security with abort: Using Signed Outputs ! " , $% " , &% " ./01-'(', ( ) , $% ) ) Digital signatures require one-way functions ', ( ) , $% ) , ( " , $% " , (( + , $% + ) ./01-'(', ( " , $% " ) - ’ ./01-'(', ( + , $% + ) Party 2 Trusted Party Accept if all 3 verify
Security with abort: Using Signed Outputs ! " , $% " , &% " ./01-'(', ( ) , $% ) ) Digital signatures require one-way functions ', ( ) , $% ) , ( " , $% " , (( + , $% + ) ./01-'(', ( " , $% " ) MACs are not sufficient - ’ ./01-'(', ( + , $% + ) Party 2 Trusted Party Accept if all 3 verify
Security with abort: Using Signed Outputs ! " , $% " , &% " ./01-'(', ( ) , $% ) ) Digital signatures require one-way functions ', ( ) , $% ) , ( " , $% " , (( + , $% + ) ./01-'(', ( " , $% " ) MACs are not sufficient - ’ ./01-'(', ( + , $% + ) Party 2 Trusted Party How can we do it information theoretically? Accept if all 3 verify
Our Tool: Multi-Key MAC ! " ! # ! $ %
Our Tool: Multi-Key MAC + , + - + . ) ! = #. %&'( ), + , , + - , + .
Our Tool: Multi-Key MAC ! ! ! ) ! = #. %&'( ), + , , + - , + .
Our Tool: Multi-Key MAC !. #$%&'( (*, ,, - . ) , , !. #$%&'( (*, ,, - 0 ) , * !. #$%&'( (*, ,, - 1 ) , = !. 3&45 *, - . , - 0 , - 1
Our Tool: Multi-Key MAC (Correctness) !. #$%&'( (*, ,, - . ) YES , , !. #$%&'( (*, ,, - 0 ) YES , * !. #$%&'( (*, ,, - 1 ) , = !. 3&45 *, - . , - 0 , - 1 YES
Our Tool: Multi-Key MAC (Security) " . & ! , " # , " % & = (. *+,- !, " . , " # , " %
Our Tool: Multi-Key MAC (Security) " . ! / , & ’ & " # ! , " # , " % (. 012+34 (!′, &′, " # ) & = (. *+,- !, " . , " # , " % NO
Our Tool: Multi-Key MAC (Security) An adversary cannot output any valid " , message-signature pair other than the ! - , & ’ & one it received " # ! , " # , " % .. 012)34 (!′, &′, " # ) & = ()*+ !, " , , " # , " % NO
Security with Abort: Using Multi-Key MAC # 1 , & ' ( = ! (# ' , # + , # , ) !′ # + , & + . = /. 1234 ((, & 1 , & 2 , & 3 ) # , , & ,
Security with Abort: Using Multi-Key MAC ! " , $ " '. )*+,-%(%, &, $ " ) %, & - ’ Trusted Party Party 2
Security with abort: Using Multi-Key MAC ( , , * , ( - , * - !, # !, # %′ Honest Party 3 Trusted Party Honest Party 2 IF !, # = %′((( ) * ) ), (( , , * , ), (( - , * - ))
Security with abort: Using Multi-Key MAC 0 - , , - 0 / , , / (, + (, + '′ Honest Party 3 Trusted Party Honest Party 2 IF (, + = '′((0 3 , 3 ), (0 - , , - ), (0 / , , / )) !. #$%&'(((, +, , - ) !. #$%&'(((, +, , / ) YES YES
Security with abort: Using Multi-Key MAC ! " , $ " ! ' , $ ' %, & %, & )′ Honest Party 3 Trusted Party Honest Party 2 IF %, & ≠ )′((! , $ , ), (! " , $ " ), (! ' , $ ' ))
Security with abort: Using Multi-Key MAC ! " , $ " ! ' , $ ' %, & %, & .′ Honest Party 3 Trusted Party Honest Party 2 Honest Party 2 IF %, & ≠ .′((! 3 $ 3 ), (! " , $ " ), (! ' , $ ' )) (. *+,-.%(%, &, $ " ) (. *+,-.%(%, &, $ ' ) NO NO
Recall: Our Strategy Round 2 Round IT-MPC Compression Constant Round IT-MPC (Privacy with Knowledge of (Security with Abort) Outputs) Broadcast + P2P Broadcast + P2P Security Using Multi-Key MAC Upgrade 2 Round IT-MPC (Security with Abort) Broadcast + P2P
Second Step Round 2 Round IT-MPC Compression Constant Round IT-MPC (Privacy with Knowledge of (Security with Abort) Outputs) Broadcast + P2P Broadcast + P2P Security Using Multi-Key MAC Upgrade 2 Round IT-MPC (Security with Abort) Broadcast + P2P
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