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175
New and improved constructions of nonmalleable cryptographic protocols
 In 37th Annual ACM Symposium on Theory of Computing
, 2005
"... We present a new constant round protocol for nonmalleable zeroknowledge. Using this protocol as a subroutine, we obtain a new constantround protocol for nonmalleable commitments. Our constructions rely on the existence of (standard) collision resistant hash functions. Previous constructions eith ..."
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Cited by 54 (18 self)
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We present a new constant round protocol for nonmalleable zeroknowledge. Using this protocol as a subroutine, we obtain a new constantround protocol for nonmalleable commitments. Our constructions rely on the existence of (standard) collision resistant hash functions. Previous constructions either relied on the existence of trapdoor permutations and hash functions that are collision resistant against subexponential sized circuits, or required a superconstant number of rounds. Additional results are the first construction of a nonmalleable commitment scheme that is statistically hiding (with respect to opening), and the first nonmalleable commitments that satisfy a strict polynomialtime simulation requirement. Our approach differs from the approaches taken in previous works in that we view nonmalleable zeroknowledge as a buildingblock rather than an end goal. This gives rise to a modular construction of nonmalleable commitments and results in a somewhat simpler analysis.
Universally Composable Security with Global Setup
 In Proceedings of the 4th Theory of Cryptography Conference
, 2007
"... Cryptographic protocols are often designed and analyzed under some trusted setup assumptions, namely in settings where the participants have access to global information that is trusted to have some basic security properties. However, current modeling of security in the presence of such setup falls ..."
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Cited by 53 (5 self)
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Cryptographic protocols are often designed and analyzed under some trusted setup assumptions, namely in settings where the participants have access to global information that is trusted to have some basic security properties. However, current modeling of security in the presence of such setup falls short of providing the expected security guarantees. A quintessential example of this phenomenon is the deniability concern: there exist natural protocols that meet the strongest known composable security notions, and are still vulnerable to bad interactions with rogue protocols that use the same setup. We extend the notion of universally composable (UC) security in a way that reestablishes its original intuitive guarantee even for protocols that use globally available setup. The new formulation prevents bad interactions even with adaptively chosen protocols that use the same setup. In particular, it guarantees deniability. While for protocols that use no setup the proposed requirements are the same as in traditional UC security, for protocols that use global setup the proposed requirements are significantly stronger. In fact, realizing Zero Knowledge or commitment becomes provably impossible, even in the Common Reference String model.
General Composition and Universal Composability in Secure Multiparty Computation
, 2007
"... Concurrent general composition relates to a setting where a secure protocol is run in anetwork concurrently with other, arbitrary protocols. Clearly, security in such a setting is what is desired, or even needed, in modern computer networks where many different protocols areexecuted concurrently. Ca ..."
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Cited by 53 (9 self)
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Concurrent general composition relates to a setting where a secure protocol is run in anetwork concurrently with other, arbitrary protocols. Clearly, security in such a setting is what is desired, or even needed, in modern computer networks where many different protocols areexecuted concurrently. Canetti (FOCS 2001) introduced the notion of universal composability, and showed that security under this definition is sufficient for achieving concurrent generalcomposition. However, it is not known whether or not the opposite direction also holds. Our main result is a proof that security under concurrent general composition, when interpreted in the natural way under the simulation paradigm, is equivalent to a variant of universal composability, where the only difference relates to the order of quantifiers in the definition. (Innewer versions of universal composability, these variants are equivalent.) An important corollary of this theorem is that existing impossibility results for universal composability (for all itsvariants) are inherent for definitions that imply security under concurrent general composition, as formulated here. In particular, there are large classes of twoparty functionalities for whichit is impossible to obtain protocols (in the plain model) that remain secure under concurrent general composition. We stress that the impossibility results obtained are not &quot;blackbox&quot;, andapply even to nonblackbox simulation. Our main result also demonstrates that the definition of universal composability is somewhat&quot;minimal&quot;, in that the composition guarantee provided by universal composability implies the definition itself. This indicates that the security definition of universal composability is notoverly restrictive.
Protocols for BoundedConcurrent Secure TwoParty Computation in the Plain Model
, 2006
"... Until recently, most research on the topic of secure computation focused on the standalonemodel, where a single protocol execution takes place. In this paper, we construct protocols for the setting of boundedconcurrent selfcomposition, where a (single) secure protocol is run manytimes concurrent ..."
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Cited by 48 (7 self)
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Until recently, most research on the topic of secure computation focused on the standalonemodel, where a single protocol execution takes place. In this paper, we construct protocols for the setting of boundedconcurrent selfcomposition, where a (single) secure protocol is run manytimes concurrently, and there is a predetermined bound on the number of concurrent executions. In short, we show that any twoparty functionality can be securely computed under boundedconcurrent selfcomposition, in the
BoundedConcurrent Secure TwoParty Computation in a Constant Number of Rounds
 In 44th FOCS
, 2003
"... We consider the problem of constructing a general protocol for secure twoparty computation in a way that preserves security under concurrent composition. In our treatment, we focus on the case where an apriori bound on the number of concurrent sessions is specified before the protocol is construct ..."
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Cited by 45 (15 self)
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We consider the problem of constructing a general protocol for secure twoparty computation in a way that preserves security under concurrent composition. In our treatment, we focus on the case where an apriori bound on the number of concurrent sessions is specified before the protocol is constructed (a.k.a. bounded concurrency). We make no setup assumptions. Lindell (STOC 2003) has shown that any protocol for boundedconcurrent secure twoparty computation, whose security is established via blackbox simulation, must have round complexity that is strictly larger than the bound on the number of concurrent sessions. In this paper, we construct a (non blackbox) protocol for realizing boundedconcurrent secure twoparty computation in a constant number of rounds. The only previously known protocol for realizing the above task required more rounds than the prespecified bound on the number of sessions (despite usage of non blackbox simulation techniques). Our constructions rely on the existence of enhanced trapdoor permutations, as well as on the existence of hash functions that are collisionresistant against subexponential sized circuits. 1
Concurrent nonmalleable commitments
 In FOCS
, 2005
"... We present a nonmalleable commitment scheme that retains its security properties even when concurrently executed a polynomial number of times. That is, a maninthemiddle adversary who is simultaneously participating in multiple concurrent commitment phases of our scheme, both as a sender and as a ..."
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Cited by 42 (14 self)
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We present a nonmalleable commitment scheme that retains its security properties even when concurrently executed a polynomial number of times. That is, a maninthemiddle adversary who is simultaneously participating in multiple concurrent commitment phases of our scheme, both as a sender and as a receiver, cannot make the values he commits to depend on the values he receives commitments to. Our result is achieved without assuming an apriori bound on the number of executions and without relying on any setup assumptions. Our construction relies on the existence of standard clawfree permutations and only requires a constant number of communication rounds. 1
On SimulationSound Trapdoor Commitments
 In proceedings of EUROCRYPT ’04, LNCS series
, 2003
"... We study the recently introduced notion of a simulationsound trapdoor commitment (SSTC) scheme. In this paper, we present a new, simpler definition for an SSTC scheme that admits more efficient constructions and can be used in a larger set of applications. Specifically, we show how to construct ..."
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Cited by 40 (2 self)
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We study the recently introduced notion of a simulationsound trapdoor commitment (SSTC) scheme. In this paper, we present a new, simpler definition for an SSTC scheme that admits more efficient constructions and can be used in a larger set of applications. Specifically, we show how to construct SSTC schemes from any oneway functions, and how to construct very efficient SSTC schemes based on specific numbertheoretic assumptions. We also show how to construct simulationsound, nonmalleable, and universallycomposable zeroknowledge protocols using SSTC schemes, yielding, for instance, the most efficient universallycomposable zeroknowledge protocols known. Finally, we explore the relation between SSTC schemes and nonmalleable commitment schemes by presenting a sequence of implication and separation results, which in particular imply that SSTC schemes are nonmalleable.
Rationality and adversarial behavior in multiparty computation
 Advances in Cryptology — Crypto 2006
, 2006
"... Abstract. We study multiparty computation in the model where none of n participating parties are honest: they are either rational, acting in their selfish interest to maximize their utility, or adversarial, acting arbitrarily. In this new model, which we call the mixedbehavior model, we define a c ..."
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Cited by 39 (1 self)
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Abstract. We study multiparty computation in the model where none of n participating parties are honest: they are either rational, acting in their selfish interest to maximize their utility, or adversarial, acting arbitrarily. In this new model, which we call the mixedbehavior model, we define a class of functions that can be computed in the presence of an adversary using a trusted mediator. We then give a protocol that allows the rational parties to emulate the mediator and jointly compute the function such that (1) assuming that each rational party prefers that it learns the output while others do not, no rational party has an incentive to deviate from the protocol; and (2) the rational parties are protected from a malicious adversary controlling ⌈ n ⌉ − 2 of the participants: the 2 adversary can only either cause all rational participants to abort (so no one learns the function they are trying to compute), or can only learn whatever information is implied by the output of the function. 1
Strengthening ZeroKnowledge Protocols using Signatures
 IN PROCEEDINGS OF EUROCRYPT ’03, LNCS SERIES
, 2003
"... Recently there has been an interest in zeroknowledge protocols with stronger properties, such as concurrency, unbounded simulation soundness, nonmalleability, and universal composability. In this paper, ..."
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Cited by 39 (8 self)
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Recently there has been an interest in zeroknowledge protocols with stronger properties, such as concurrency, unbounded simulation soundness, nonmalleability, and universal composability. In this paper,
Round Efficiency of MultiParty Computation with a Dishonest Majority
 In Eurocrypt ’03, 2003. LNCS
, 2003
"... Abstract. We consider the round complexity of multiparty computation in the presence of a static adversary who controls a majority of the parties. Here, n players wish to securely compute some functionality and up to n − 1 of these players may be arbitrarily malicious. Previous protocols for this s ..."
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Cited by 35 (7 self)
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Abstract. We consider the round complexity of multiparty computation in the presence of a static adversary who controls a majority of the parties. Here, n players wish to securely compute some functionality and up to n − 1 of these players may be arbitrarily malicious. Previous protocols for this setting (when a broadcast channel is available) require O(n) rounds. We present two protocols with improved round complexity: The first assumes only the existence of trapdoor permutations and dense cryptosystems, and achieves round complexity O(log n) based on a proof scheduling technique of Chor and Rabin [13]; the second requires a stronger hardness assumption (along with the nonblackbox techniques of Barak [2]) and achieves O(1) round complexity. 1