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Ran Canetti, Yael Tauman Kalai, Mayank Varia, Daniel Wichs.

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Presentation on theme: "Ran Canetti, Yael Tauman Kalai, Mayank Varia, Daniel Wichs."— Presentation transcript:

1 Ran Canetti, Yael Tauman Kalai, Mayank Varia, Daniel Wichs

2  An obfuscator takes a program P and outputs an equivalent program P’ = Obf(P) such that the code of P’ is “useless”. “useless”: no more useful than oracle P.  Obfuscation not possible in general. [ BGI + 01 ] P’ P x P(x) Real World Ideal World What’s obfuscation [BGI + 01] ?

3 What’s a point function?  A point function: special point k on which outputs 1, otherwise outputs ?.  A multi-bit point function (MBPF): special point k on which outputs hidden message m, otherwise outputs ?.  Obfuscators of (multi-bit) point functions studied and constructed by [Can97, CMR98, LPS04, Wee05, CD08]. f k (x ) = 1 if x =k ? otherwise f k,m (x ) = m if x =k ? otherwise

4 Relation to Symmetric Encryption  Define: Enc k (m) = Obf(f k,m ) Dec k (c) = c(k)  Is it a good symmetric encryption scheme? Good: ciphertext c only as useful as oracle f k,m ( ¢ ). Good even if k only has entropy, but is not uniform. ○ Cryptography with weak keys, leakage-resilience… Good even if m depends on k. ○ Security with Key Dependent Messages (KDM).

5 Relation to Symmetric Encryption Encryption w. weak keys (leakage-resilience) Encryption w. KDM MBPF Obfuscation

6 Outline  Symmetric Encryption. Weak keys, Leakage-Resilience, KDM  MBPF Obfuscation Definitional variants  Connections between symmetric encryption and MBPF obfuscation.  Implications, new results.

7 Symmetric Key Encryption  Semantic security: one oracle call. CPA: many oracle calls.  Weak Keys: ® -weak keys: key k ~ (adversarial) distribution w. min-entropy ®. Leakage-Resilience: Adversary learn L-bits of information about k. [AGV09, DK09, NS09,…]  Key Dependent Messages: Attacker chooses g() and real oracle outputs Enc k (g(k) ). [BRS02, BHHO08, HH09…] Key k chosen uniformly at random. Attacker chooses messages m Real oracle: outputs Enc k (m ) Fake oracle: outputs Enc k (0 |m| ) Can’t distinguish real and fake oracles. ® –weak key security ) L= |k| - ® Leakage-Resilience.

8 Definition of Obfuscation  A MBPF obfuscator takes (k, m) and creates a program P Ã Obf( f k,m ).  Correctness: For all x, P(x)  f k,m (x)  Polynomial slowdown: P runs in poly-time.  VBB Security ([BGI + 01]): For any PPT A, there exists a PPT S such that, for all k, m | Pr[A(P) = 1] – Pr[S f k,m () = 1] | < negl where P Ã Obf( f k,m ).

9 Weaker Definitions  Alternative: 8 A 9 S 8 distributions {K, M} | Pr[A(P) = 1] – Pr[S f k,m () = 1] | < negl where (k,m) Ã (K, M), P Ã Obf( f k,m )  Weaker definitions place restrictions on {K, M}: ® -entropic security: Require K has min-entropy ¸ ®. Independent messages: Require M independent of K.

10 Composable Obfuscation  VBB does not guarantee security if adversary sees many obfuscations of related functions. [CD08] Problem for application to CPA encryption.  Self-composable: secure if obfuscate many related MBPFs of type: (k, m 1 ), (k, m 2 ), (k, m 3 ). | Pr[A(P 1,P 2,…) = 1] – Pr[S f k,m1 (), f k,m2 (),... = 1] |

11 MBPF Obfuscation ) Encryption  MBPF Obf with entropic sec. ) SS Enc with weak keys + KDM.  Self-Composable MBPF Obf ) CPA Enc …but choice of KDM functions is not adaptive.  MBPF Obf with entropic sec. for indep. msg. ) SS Enc with weak keys.  Self-Composable ) CPA Enc k (m ) = Obf(f k,m )

12 Encryption ) MBPF Obfuscation  Are the connections tight?  Do various strengthened notions of encryption imply restricted notions of MBPF obfuscation?  Yes, but need extra properties from encryption…

13 Key k chosen uniformly at random. Attacker chooses messages m Real oracle: outputs Enc k (m ) Fake oracle: outputs Enc k (0 |m| ) Can’t distinguish real and fake oracles. Extra Properties for Encryption  Need: Encryption hides (distribution of) k. Exists some oracle Fake(). Does not get k,m.  Need: Wrong-Key Detection. For any k  k’,m : Dec k’ ( Enc k (m )) = ? Fake()

14 Encryption, MBPF Obfuscation  MBPF Obf with entropic sec., SS Enc with weak keys + KDM.  Self-Composable, CPA …but choice of KDM functions is not adaptive.  MBPF Obf with entropic sec. for indep. msg., SS Enc with weak keys.  Self-Composable, CPA

15 Implications: Encryption with weak keys  Prior encryption schemes with ® -weak keys allow for ® (n) = n ² for any ²>0. [AGV09, DKL09, NS09]  … BUT the scheme and its efficiency depend on ².  Self-composable MBPF Obfuscators for indep. msg. with VBB security gives us: A single encryption scheme with fixed efficiency. CPA secure if key k ~ any dist with ! (log(n)) entropy. Exact security depends on entropy (graceful degradation).

16 Implications: Encryption with fully weak keys  Self-composable MBPF Obfuscators for indep. msg. with VBB security, constructed by [Can97, CD08].  Require: strengthened DDH assumption: (g, g a, g b, g ab ) ¼ (g, g a, g b, g c ) where a has ! (log(n)) entropy, b, c uniform.  More recently, [GKPV10] construct an encryption scheme with similar “graceful degradation” under standard LWE.

17 Implications: New MBPF Obf Constructions  Use recent leakage-resilience results of [AGV09, NS09, DK09] t o get self-composable MBPF obf. for indep. msg. with ® -entropic security: [AGV09] Under LWE assumption, ® (n) = n ² [DKL09] Under LSN (strengthens LPN) assumption ® (n) = ² n [NS09] Under DDH and K-Linear assumption, ® (n) = n ²

18 Implications: Hardness Results for MBPF Obf.  Result of [HH09] : SS encryption with KDM cannot be BB reduced to any “standard assumption”. Includes e.g. OWF, TDP, DDH, RSA,… Excludes e.g. RO model, KoE, Exponential hardness.  Cannot base MBPF obfuscation (even entropic with uniform k) on “standard assumptions” via BB reductions.

19 THANK YOU! QUESTIONS?


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