Strong Accumulators from CollisionResistant Hashing


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1 Strong Accumulators from CollisionResistant Hashing ISC 2008 Taipei  Taiwan Philippe Camacho (University of Chile Alejandro Hevia (University of Chile Marcos Kiwi (University of Chile Roberto Opazo (CEO Acepta.com
2 Outline Notion of accumulator Motivation einvoice Factoring Our construction Conclusion
3 Notion of accumulator Problem A set X. Given an element x we wish to prove that this element belongs or not to X. Let X={x 1,x 2,,x n }: X will be represented by a short value Acc. Belongs(Acc,x,w = True x belongs to X. Witness
4 Notion of accumulator Accumulator Manager Computes setup values. Computes the accumulated value Acc. Computes the witness w x for a given x. Accumulator Users Check that an element belongs or not to the set, using Acc, w x and x.
5 Applications Timestamping [BeMa94] Certificate Revocation List [LLX07] Anonymous credentials [CamLys02] ECash [AWSM07] Broadcast Encryption [GeRa04]
6 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity Provider (Milk seller Client (Supermarket
7 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity Provider (Milk seller 1 I want (a lot of milk now *. Client (Supermarket (* but I do not want to pay yet.
8 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity Provider (Milk seller 1 I want (a lot of milk now *. 2 Here is your milk. Client (Supermarket (* but I do not want to pay yet.
9 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity 3 Please pay the invoice. Provider (Milk seller 1 I want (a lot of milk now *. 2 Here is your milk. Client (Supermarket (* but I do not want to pay yet.
10 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity 3 Please pay the invoice. 4 Here is your money (**. Provider (Milk seller 1 I want (a lot of milk now *. 2 Here is your milk. Client (Supermarket (* but I do not want to pay yet. (** minus a fee.
11 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity 3 Please pay the invoice. 4 Here is your money (**. 5 It s time to pay. Provider (Milk seller 1 I want (a lot of milk now *. 2 Here is your milk. Client (Supermarket (* but I do not want to pay yet. (** minus a fee.
12 Nothing to see with Number Theory! Factoring Industry in Chile Factoring Entity 3 Please pay the invoice. 4 Here is your money (**. 5 It s time to pay. 6 Here is the money. Provider (Milk seller 1 I want (a lot of milk now *. 2 Here is your milk. Client (Supermarket (* but I do not want to pay yet. (** minus a fee.
13 The Problem A malicious provider could send the same invoice to various Factoring Entities. Then he leaves to a far away country with all the money. Later, several Factoring Entities will try to charge the invoice to the same client. Losts must be shared
14 Solution with Factoring Authority Factoring Authority (4 Is there the invoice? (5 YES / NO FE 1 FE 2 FE i FE n (3 Invoice, Ack Provider (1 Invoice (2 Ack Client
15 Caveat This solution is quite simple. However Trusted Factoring Authority is needed. Can we remove this requirement?
16 Properties Dynamic Allows insertion/deletion of elements. Universal Allows proofs of membership and nonmembership. Strong No need to trust in the Accumulator Manager.
17 Prior work Dynamic Strong Universal Security Efficiency (witness size Note [BeMa94] RSA + RO O(1 First definition [BarPfi97] Strong RSA O(1  [CamLys02] Strong RSA O(1 First dynamic accumulator [LLX07] Strong RSA O(1 First universal accumultor [AWSM07] Pairings O(1 Ecash [WWP08] estrong RSA Paillier O(1 Batch Update
18 Prior work Dynamic Strong Universal Security Efficiency (witness size Note [BeMa94] RSA + RO O(1 First definition [BarPfi97] Strong RSA O(1  [CamLys02] Strong RSA O(1 First dynamic accumulator [LLX07] Strong RSA O(1 First universal accumultor [AWSM07] Pairings O(1 Ecash [WWP08] estrong RSA Paillier O(1 Batch Update [CHKO08] CollisionResistant Hashing O(ln(n Our work
19 Notation H: {0,1}* {0,1} k randomly chosen function from a family of collisionresistant hash functions. x 1,x 2,x 3, є {0,1} k x 1 < x 2 < x 3 < where < is the lexicographic order on binary strings. , Special values such that For all x є {0,1} k :  < x < denotes the concatenation operator.
20 Ideas Merkletrees P=H(Z 1 Z 2 Root value: Z 1 =H(Y 1 Y 2 Z 2 =H(Y 3 Y 4 Represents the set {x 1,,x 8 } Y 1 =H(x 4 x 1 Y 2 =H(x 5 x 6 Y 3 =H(x 2 x 8 Y 4 =H(x 7 x 3 x 4 x 1 x 5 x 6 x 2 x 8 x 7 x 3
21 Ideas Merkletrees P=H(Z 1 Z 2 Root value: Represents the set {x 1,,x 8 } Z 1 =H(Y 1 Y 2 Z 2 =H(Y 3 Y 4 Y 1 =H(x 4 x 1 Y 2 =H(x 5 x 6 Y 3 =H(x 2 x 8 Y 4 =H(x 7 x 3 O(ln(n x 4 x 1 x 5 x 6 x 2 x 8 x 7 x 3
22 Ideas How to prove nonmembership? Kocher s trick [Koch98]: store pair of consecutive values X={1,3,5,6,11} X ={(,1,(1,3,(3,5,(5,6,(6,11,(11, } y=3 belongs to X (1,3 or (,1 belongs to X. y=2 does not belong to X (1,3 belongs to X.
23 Public Data Structure Called Memory. Compute efficiently the accumulated value and the witnesses. In our construction the Memory will be a binary tree.
24 How to insert elements? (, X=Ø, next: x 1
25 How to insert elements? (,x 1 (x 1, X={x 1 }, next: x 2
26 How to insert elements? (,x 1 (x 1, x 2 (x 2, X={x 1,x 2 }, next: x 5
27 How to insert elements? (,x 1 (x 1, x 2 (x 2, x 5 (x 5, X={x 1,x 2,x 5 }, next: x 3
28 How to insert elements? (,x 1 (x 1, x 2 (x 2, x 3 (x 5, (x 3, x 5 X={x 1,x 2,x 3,x 5 }, next: x 4
29 How to insert elements? (,x 1 (x 1, x 2 (x 2, x 3 (x 5, (x 3, x 4 (x 4, x 5 X={x 1,x 2,x 3,x 4,x 5 }, next: x 6
30 How to insert elements? (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, X={x 1,x 2,x 3,x 4,x 5,x 6 }
31 value? (,x 1 How to compute the accumulated (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 9, (x 7, x 9 Proof N =H(Proof left Proof right value Proof Nil = Acc = Proof Root A pair (x i,x j
32 How to update the accumulated value? (Insertion (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 9, (x 7, x 9 Next element to be inserted: x 8 We will need to recompute proof node values.
33 How to update the accumulated value? (Insertion (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 9, (x 7, x 8 (x 8, x 9 New element: x 8. Proof N stored in each node. Dark nodes do not require recomputing Proof N. Only a logarithmic number of values needs recomputation.
34 Security Consistency Difficult to find witnesses that allow to prove inconsistent statements. X={1,2} Update Hard to compute a membership witness for 3. Hard to compute a nonmembership witness for 2. Guarantees that the accumulated value represents the set after insertion/deletion of x.
35 Security Lemma: Given a tree T with accumulated value Acc T, finding a tree T, T T such that Acc T = Acc T is difficult. Proof (Sketch: Proof N = H(Proof left Proof right value (,x 1 (x 1, x 2 (x 2, x 3 (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 5, x 6 (x 3, x 4 (x 4, x 7 (x 6, x 7
36 Security Lemma: Given a tree T with accumulated value Acc T, finding a tree T, T T such that Acc T = Acc T is difficult. Proof (Sketch: Proof N = H(Proof left Proof right value Collision for H (,x 1 (x 1, x 2 (x 2, x 3 (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 5, x 6 (x 3, x 4 (x 4, x 7 (x 6, x 7
37 Security Lemma: Given a tree T with accumulated value Acc T, finding a tree T, T T such that Acc T = Acc T is difficult. Proof (Sketch: Proof N = H(Proof left Proof right value Collision for H (,x 1 (x 1, x 2 (x 2, x 3 (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 5, x 6 (x 3, x 4 (x 4, x 7 (x 6, x 7
38 Security Lemma: Given a tree T with accumulated value Acc T, finding a tree T, T T such that Acc T = Acc T is difficult. Proof (Sketch: Proof N = H(Proof left Proof right value Collision for H (,x 1 (x 1, x 2 (x 2, x 3 (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 5, x 6 (x 3, x 4 (x 4, x 7 (x 6, x 7
39 Security (Consistency (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 9, (x 7, x 9 Witness: blue nodes and the (x 3,x 4 pair, size in O(ln(n Checking that x belongs (or not to X: 1 compute recursively the proof P and verify that P=Acc 2 check that: x=x 3 or x=x 4 (membership x 3 < x < x 4 (nonmembership
40 Security (Update Before After (,x 1 (,x 1 Acc before Acc after (x 1, x 2 (x 2, x 3 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, x 7 (x 9, (x 7, x 9 (x 9, (x 7, x 8 (x 8, x 9 Insertion of x 8
41 Conclusion & Open Problem First dynamic, universal, strong accumulator. Simple. Security Existence of collisionresistant hash functions. Solves the einvoice Factoring Problem. Less efficient than other constructions Size of witness in O(ln(n. Open Problem Is it possible to build a strong,dynamic and universal accumulator with witness size lower than O(ln(n?
42 Thank you!
43 Invoice Factoring using accumulator We need a secure broadcast channel If a message m is published, every participant sees the same m. Depending on the security level required Trusted http of ftp server Bulletin Board [CGS97]
44 Invoice Factoring using accumulator Factoring Authority (4 Is there the invoice? (5 YES / NO We need to see in detail this step FE 1 FE 2 FE i FE n (3 Invoice, Ack Provider (1 Invoice (2 Ack Client
45 Invoice Factoring using accumulator Step 5 (Details FE Factoring Authority Have you got invoice x? YES/NO, w x w x = Witness(m before,x Check(Acc before,w x,x If NO, insert x Acc new,w up,id FE Acc new,w up = Update Add (m before,x CheckUpdate(Acc before,acc after,w x All tests pass => I can buy x.
46 Distributed solutions? Complex to implement Hard to make them robust High bandwith communication Need to be online synchronization problems That s why we focus on a centralized solution.
47 Checking for (nonmembership User Accumulator Manager Does x belong to X? Memory w x = Witness(m,x w x Belongs(Acc,w x = True x є X If w x is not valid Belongs returns.
48 Update of the accumulated value User Accumulator Manager Insert or Delete x m after, Acc after, w up = Update Add/Del (m before,x Acc after, w up CheckUpdate(Acc before,acc after,w up
49 How to delete elements? (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6, X={x 1,x 2,x 3,x 4,x 5,x 6 } element to be deleted: x 2
50 How to delete elements? (x 1,x 3 (,x 1 (x 1, x 2 (x 2, x 3 (x 5, x 6 (x 3, x 4 (x 4, x 5 (x 6,
51 How to delete elements? (,x 1 (x 1, x 3 (x 6, (x 5, x 6 (x 3, x 4 (x 4, x 5
52 Bibliography [BeMa92] Efficient Broadcast TimeStamping Josh Benaloh and Michael de Mare 1992 [BeMa94] Oneway Accumulators: A decentralized Alternative to Digital Signatures Josh Benaloh and Michael de Mare, 1994 [BarPfi97] CollisionFree Accumulators and FailStop Signature Schemes Without Trees Niko Barić and Birgit Pfitzmann 1997 [CGS97] A secure and optimally efficient multiauthority election scheme R. Cramer, R. Gennaro, and B. Schoenmakers 1997 [Koch98] On certificate revocation and validation P.C. Kocher 1998 [CGH98] The random oracle methodoly revisited R. Canetti, O. Goldreich and S. Halevi 1998 [Sand99] Efficient Accumulators Without Trapdoor Tomas Sanders 1999 [GoTa01] An efficient and Distributed Cryptographic Accumulator Michael T. Goodrich and Roberto Tamassia 2001 [CamLys02] Dynamic Accumulators And Application to Efficient Revocation of Anonymous Credentials Jan Camenisch Anna Lysyanskaya 2002 [GeRa04] RSA Accumulator Based Broadcast Encryption Craig Gentry and Zulfikar Ramzan 2004 [LLX07] Universal Accumulators with Efficient Nonmembership Proofs Jiangtao Li, Ninghui Li and Rui Xue 2007 [AWSM07] Compact ECash from Bounded Accumulator Man Ho Au, Qianhong Wu, Willy Susilo and Yi Mu 2007 [WWP08] A new Dynamic Accumulator for Batch Updates Peishun Wang, Huaxiong Wang and Josef Pieprzyk 2008 [CKHO08] Strong Accumulators from CollisionResistant Hashing Philippe Camacho, Alejandro Hevia, Marcos Kiwi, and Roberto Opazo 2008
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