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191
Short group signatures
 In proceedings of CRYPTO ’04, LNCS series
, 2004
"... Abstract. We construct a short group signature scheme. Signatures in our scheme are approximately the size of a standard RSA signature with the same security. Security of our group signature is based on the Strong DiffieHellman assumption and a new assumption in bilinear groups called the Decision ..."
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Cited by 273 (19 self)
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Abstract. We construct a short group signature scheme. Signatures in our scheme are approximately the size of a standard RSA signature with the same security. Security of our group signature is based on the Strong DiffieHellman assumption and a new assumption in bilinear groups called the Decision Linear assumption. We prove security of our system, in the random oracle model, using a variant of the security definition for group signatures recently given by Bellare, Micciancio, and Warinschi. 1
Compact ecash
 In EUROCRYPT, volume 3494 of LNCS
, 2005
"... Abstract. This paper presents efficient offline anonymous ecash schemes where a user can withdraw a wallet containing 2 ℓ coins each of which she can spend unlinkably. Our first result is a scheme, secure under the strong RSA and the yDDHI assumptions, where the complexity of the withdrawal and s ..."
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Cited by 91 (18 self)
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Abstract. This paper presents efficient offline anonymous ecash schemes where a user can withdraw a wallet containing 2 ℓ coins each of which she can spend unlinkably. Our first result is a scheme, secure under the strong RSA and the yDDHI assumptions, where the complexity of the withdrawal and spend operations is O(ℓ + k) andtheuser’s wallet can be stored using O(ℓ + k) bits,wherek is a security parameter. The best previously known schemes require at least one of these complexities to be O(2 ℓ · k). In fact, compared to previous ecash schemes, our whole wallet of 2 ℓ coins has about the same size as one coin in these schemes. Our scheme also offers exculpability of users, that is, the bank can prove to third parties that a user has doublespent. We then extend our scheme to our second result, the first ecash scheme that provides traceable coins without a trusted third party. That is, once a user has double spent one of the 2 ℓ coins in her wallet, all her spendings of these coins can be traced. However, the price for this is that the complexity of the spending and of the withdrawal protocols becomes O(ℓ · k) and O(ℓ · k + k 2) bits, respectively, and wallets take O(ℓ · k) bitsofstorage. All our schemes are secure in the random oracle model.
Predicate Encryption Supporting Disjunctions, Polynomial Equations, and Inner Products
"... Abstract. Predicate encryption is a new paradigm generalizing, among other things, identitybased encryption. In a predicate encryption scheme, secret keys correspond to predicates and ciphertexts are associated with attributes; the secret key SKf corresponding to a predicate f can be used to decryp ..."
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Cited by 84 (15 self)
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Abstract. Predicate encryption is a new paradigm generalizing, among other things, identitybased encryption. In a predicate encryption scheme, secret keys correspond to predicates and ciphertexts are associated with attributes; the secret key SKf corresponding to a predicate f can be used to decrypt a ciphertext associated with attribute I if and only if f(I) = 1. Constructions of such schemes are currently known for relatively few classes of predicates. We construct such a scheme for predicates corresponding to the evaluation of inner products over ZN (for some large integer N). This, in turn, enables constructions in which predicates correspond to the evaluation of disjunctions, polynomials, CNF/DNF formulae, or threshold predicates (among others). Besides serving as a significant step forward in the theory of predicate encryption, our results lead to a number of applications that are interesting in their own right. 1
How to win the clonewars: efficient periodic ntimes anonymous authentication
 In ACM Conference on Computer and Communications Security
, 2006
"... We create a credential system that lets a user anonymously authenticate at most n times in a single time period. A user withdraws a dispenser of n etokens. She shows an etoken to a verifier to authenticate herself; each etoken can be used only once, however, the dispenser automatically refreshes ..."
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Cited by 55 (11 self)
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We create a credential system that lets a user anonymously authenticate at most n times in a single time period. A user withdraws a dispenser of n etokens. She shows an etoken to a verifier to authenticate herself; each etoken can be used only once, however, the dispenser automatically refreshes every time period. The only prior solution to this problem, due to Damg˚ard et al. [30], uses protocols that are a factor of k slower for the user and verifier, where k is the security parameter. Damg˚ard et al. also only support one authentication per time period, while we support n. Because our construction is based on ecash, we can use existing techniques to identify a cheating user, trace all of her etokens, and revoke her dispensers. We also offer a new anonymity service: glitch protection for basically honest users who (occasionally) reuse etokens. The verifier can always recognize a reused etoken; however, we preserve the anonymity of users who do not reuse etokens too often. 1
Simulationsound nizk proofs for a practical language and constant size group signatures
, 2006
"... Noninteractive zeroknowledge proofs play an essential role in many cryptographic protocols. We suggest several NIZK proof systems based on prime order groups with a bilinear map. We obtain linear size proofs for relations among group elements without going through an expensive reduction to an NP ..."
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Cited by 48 (9 self)
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Noninteractive zeroknowledge proofs play an essential role in many cryptographic protocols. We suggest several NIZK proof systems based on prime order groups with a bilinear map. We obtain linear size proofs for relations among group elements without going through an expensive reduction to an NPcomplete language such as Circuit Satisfiability. Security of all our constructions is based on the decisional linear assumption. The NIZK proof system is quite general and has many applications such as digital signatures, verifiable encryption and group signatures. We focus on the latter and get the first group signature scheme satisfying the strong security definition of Bellare, Shi and Zhang [7] in the standard model without random oracles where each group signature consists only of a constant number of group elements. We also suggest a simulationsound NIZK proof of knowledge, which is much more efficient than previous constructions in the literature. Caveat: The constants are large, and therefore our schemes are not practical. Nonetheless, we find it very interesting for the first time to have NIZK proofs and group signatures that except for a constant factor are optimal without using the random oracle model to argue security.
Group Signatures: Better Efficiency and New Theoretical Aspects
 In proceedings of SCN ’04, LNCS series
, 2005
"... A group signature scheme allows members of a group to sign messages anonymously. To counter misuse, the socalled group manager can revoke the anonymity. ..."
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Cited by 44 (7 self)
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A group signature scheme allows members of a group to sign messages anonymously. To counter misuse, the socalled group manager can revoke the anonymity.
NonInteractive Anonymous Credentials
 AVAILABLE FROM THE IACR CRYPTOLOGY EPRINT ARCHIVE AS REPORT 2007/384.
, 2008
"... In this paper, we introduce Psignatures. A Psignature scheme consists of a signature scheme, a commitment scheme, and (1) an interactive protocol for obtaining a signature on a committed value; (2) a noninteractive proof system for proving that the contents of a commitment has been signed; (3) a ..."
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Cited by 42 (9 self)
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In this paper, we introduce Psignatures. A Psignature scheme consists of a signature scheme, a commitment scheme, and (1) an interactive protocol for obtaining a signature on a committed value; (2) a noninteractive proof system for proving that the contents of a commitment has been signed; (3) a noninteractive proof system for proving that a pair of commitments are commitments to the same value. We give a definition of security for Psignatures and show how they can be realized under appropriate assumptions about groups with a bilinear map. We make extensive use of the powerful suite of noninteractive proof techniques due to Groth and Sahai. Our Psignatures enable, for the first time, the design of a practical noninteractive anonymous credential system whose security does not rely on the random oracle model. In addition, they may serve as a useful building block for other
Ring signatures: Stronger definitions, and constructions without random oracles. Cryptology ePrint Archive
, 2005
"... Abstract. Ring signatures, first introduced by Rivest, Shamir, and Tauman, enable a user to sign a message so that a ring of possible signers (of which the user is a member) is identified, without revealing exactly which member of that ring actually generated the signature. In contrast to group sign ..."
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Cited by 41 (1 self)
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Abstract. Ring signatures, first introduced by Rivest, Shamir, and Tauman, enable a user to sign a message so that a ring of possible signers (of which the user is a member) is identified, without revealing exactly which member of that ring actually generated the signature. In contrast to group signatures, ring signatures are completely “adhoc ” and do not require any central authority or coordination among the various users (indeed, users do not even need to be aware of each other); furthermore, ring signature schemes grant users finegrained control over the level of anonymity associated with any particular signature. This paper has two main areas of focus. First, we examine previous definitions of security for ring signature schemes and suggest that most of these prior definitions are too weak, in the sense that they do not take into account certain realistic attacks. We propose new definitions of anonymity and unforgeability which address these threats, and then give separation results proving that our new notions are strictly stronger than previous ones. Next, we show two constructions of ring signature schemes in the standard model: one based on generic assumptions which satisfies our strongest definitions of security, and a second, more efficient scheme achieving weaker security guarantees and more limited functionality. These are the first constructions of ring signature schemes that do not rely on random oracles or ideal ciphers. 1
Fulldomain subgroup hiding and constantsize group signatures
 In proceedings of PKC 2007
, 2007
"... We give a short constantsize group signature scheme, which we prove fully secure under reasonable assumptions in bilinear groups, in the standard model. We achieve this result by using a new NIZK proof technique, related to the BGN cryptosystem and the GOS proof system, but that allows us to hide i ..."
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Cited by 41 (0 self)
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We give a short constantsize group signature scheme, which we prove fully secure under reasonable assumptions in bilinear groups, in the standard model. We achieve this result by using a new NIZK proof technique, related to the BGN cryptosystem and the GOS proof system, but that allows us to hide integers from the full domain rather than individual bits. 1
Blacklistable anonymous credentials: Blocking misbehaving users without TTPs
 In ACM Conference on Computer and Communications Security. ACM
, 2007
"... Several credential systems have been proposed in which users can authenticate to services anonymously. Since anonymity can give users the license to misbehave, some variants allow the selective deanonymization (or linking) of misbehaving users upon a complaint to a trusted third party (TTP). The abi ..."
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Cited by 39 (7 self)
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Several credential systems have been proposed in which users can authenticate to services anonymously. Since anonymity can give users the license to misbehave, some variants allow the selective deanonymization (or linking) of misbehaving users upon a complaint to a trusted third party (TTP). The ability of the TTP to revoke a user’s privacy at any time, however, is too strong a punishment for misbehavior. To limit the scope of deanonymization, systems such as “ecash ” have been proposed in which users are deanonymized under only certain types of welldefined misbehavior such as “double spending. ” While useful in some applications, it is not possible to generalize such techniques to more subjective definitions of misbehavior. We present the first anonymous credential system in which services can “blacklist ” misbehaving users without contacting a TTP. Since blacklisted users remain anonymous, misbehaviors can be judged subjectively without users fearing arbitrary deanonymization by a TTP.