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Efficient Non-interactive Proof Systems for Bilinear Groups Jens Groth University College London Amit Sahai University of California Los Angeles TexPoint fonts used in EMF. Read the TexPoint manual before you delete this box.: AAA A A A A A A

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Non-interactive proof Statement: x L Proof Yes dear, x L Why? Witness w (x,w) R L Zero-knowledge: Bob learns nothing about witness Witness-indistinguishable: Bob does not learn which witness Alice has in mind

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A brief history of non-interactive zero- knowledge proofs Blum-Feldman-Micali 88 Damgård 92 Feige-Lapidot-Shamir 99 Kilian-Petrank 98 De Santis-Di Crescenzo-Persiano 02

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Efficiency problems with non-interactive zero-knowledge proofs Non-interactive proofs for general NP-complete language such as Circuit SAT. Any practical statement such as ”the ciphertext c contains a signature on m” must go through a size-increasing NP-reduction. Inefficient non-interactive proofs for Circuit SAT. Use the so-called ”hidden random bits” method.

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Our goal We want non-interactive proofs for statements arising in practice such as ”the ciphertext c contains a signature on m”. No NP-reduction! We want high efficiency. Practical non-interactive proofs!

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A brief history of non-interactive zero- knowledge proofs continued Circuit SATPractical statements Inefficient Efficient Kilian-Petrank 98 Groth-Ostrovsky- Sahai 06 Groth 06 This work

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Bilinear group G 1, G 2, G T finite cyclic groups of order n P 1 generates G 1, P 2 generates G 2 e: G 1 G 2 → G T –e(P 1,P 2 ) generates G T –e(aP 1,bP 2 ) = e(P 1,P 2 ) ab Deciding membership, group operations, bilinear map efficiently computable Prime order or composite order G 1 = G 2 or G 1 ≠ G 2 Many possible assumptions: Subgroup Decision, Symmetric External Diffie-Hellman, Decison Linear,...

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Constructions in bilinear groups a, b Z n, A, C G 1, B, D G 2 t = a+xb T 1 = xY+xA+tC T 2 = B+D+Z t T = e(T 1,B+bT 2 )

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Non-interactive cryptographic proofs for correctness of constructions t = a+xb T 1 = xY+xA+tC T 2 = B+D+Z t T = e(T 1,B+bT 2 ) Are the constructions correct? I do not know your secret x, Y, Z. Proof Yes, here is a proof.

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Cryptographic constructions Constructions can be built from –public exponents and public group elements –secret exponents and secret group elements Using any of the bilinear group operations –Addition and multiplication of exponents –Point addition or scalar multiplication in G 1 or G 2 –Bilinear map e –Multiplication in G T Our result: Non-interactive cryptographic proofs for correctness of a set of bilinear group constructions

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Examples of statements we can prove Here is a ciphertext c and a signature s. They have been constructed such that s is a signature on the secret plaintext. Here are three commitments A,B and C to secret exponents a,b and c. They have been constructed such that c=ab mod n.

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Variables Pairing product equations Multi-scalar multiplication equations in G 1 (or G 2 ) Quadratic equations in Z n Quadratic equations in a bilinear group

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Our contribution Statement S = (eq 1,...,eq N ) bilinear group equations Efficient non-interactive witness-indistinguishable (NIWI) proofs for satisfiability of all equations in S Efficient non-interactive zero-knowledge (NIZK) proofs for satisfiability of all equations in S (all t T =1) Many choices of bilinear groups and cryptographic assumptions Subgroup Decision, Symmetric External Diffie-Hellman, Decision Linear, etc. Common reference string O(1) group elements

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Size of NIWI and NIZK proofs Cost of each variable/equation Subgroup Decision Symmetric External DH Decision Linear Variable in G 1, G 2 or Z n 123 Pairing product (NIZK all t T = 1) 189 Multiscalar mult.169 Quadratic in Z n 146 Each equation constant cost. Cost independent of number of public constants and secret variables. NIWI proofs can have sub-linear size compared with statement!

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Applications of efficient NIWI and NIZK proofs Constant size group signatures Boyen-Waters 07 (independently of our work) Groth 07 Sub-linear size ring signatures Chandran-Groth-Sahai 07 Non-interactive NIZK proof for correctness of shuffle Groth-Lu 07 Non-interactive anonymous credentials Belienky-Chase-Kohlweiss-Lysyanskaya 08 …

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Where does the generality come from? View bilinear groups as special cases of modules with a bilinear map Commutative ring R R-modules A 1, A 2, A T Bilinear map f: A 1 A 2 → A T

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Pairing product equations Use R = Z n, A 1 = G 1, A 2 = G 2, A T = G T, f(X,Y)=e(X,Y) and write A T = G T with additive notation to get

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Multi-scalar multiplication in G 1 Multi-scalar multiplication equations in G 1 Use R = Z n, A 1 = G 1, A 2 = Z n, A T = G 1, f(X,y)=yX

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Quadratic equation in Z n Quadratic equations in Z n Use R = Z n, A 1 = Z n, A 2 = Z n, A T = Z n, f(x,y)=xy

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Generality continued All four types of bilinear group equations can be seen as example of quadratic equations over modules with bilinear map The assumptions Subgroup Decision, Symmetric External Diffie-Hellman, Decision Linear, etc., can be interpreted as assumption in (different) modules with bilinear map as well

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Sketch of NIWI proofs Commit to secret elements in A 1 and A 2 Commitment scheme is homomorphic with respect to addition in A 1, A 2, A T and with respect to bilinear map f Can therefore use homomorphic properties to get commitment c = commit A T (t; r) Reveal commitment randomizer r to verify that equation is satisfied To get witness-indistinguishability first rerandomize commitment c before opening with r´

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Final remarks Summary: Efficient non-interactive cryptographic proofs for use in bilinear groups Open problem: Construct cryptographically useful modules with bilinear map that are not based on bilinear groups Acknowledgment: Thanks to Brent Waters Questions?

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