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143
How to Go Beyond the BlackBox Simulation Barrier
 In 42nd FOCS
, 2001
"... The simulation paradigm is central to cryptography. A simulator is an algorithm that tries to simulate the interaction of the adversary with an honest party, without knowing the private input of this honest party. Almost all known simulators use the adversary’s algorithm as a blackbox. We present t ..."
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Cited by 221 (14 self)
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The simulation paradigm is central to cryptography. A simulator is an algorithm that tries to simulate the interaction of the adversary with an honest party, without knowing the private input of this honest party. Almost all known simulators use the adversary’s algorithm as a blackbox. We present the first constructions of nonblackbox simulators. Using these new nonblackbox techniques we obtain several results that were previously proven to be impossible to obtain using blackbox simulators. Specifically, assuming the existence of collision resistent hash functions, we construct a new zeroknowledge argument system for NP that satisfies the following properties: 1. This system has a constant number of rounds with negligible soundness error. 2. It remains zero knowledge even when composed concurrently n times, where n is the security parameter. Simultaneously obtaining 1 and 2 has been recently proven to be impossible to achieve using blackbox simulators. 3. It is an ArthurMerlin (public coins) protocol. Simultaneously obtaining 1 and 3 was known to be impossible to achieve with a blackbox simulator. 4. It has a simulator that runs in strict polynomial time, rather than in expected polynomial time. All previously known constantround, negligibleerror zeroknowledge arguments utilized expected polynomialtime simulators.
A signature scheme with efficient protocols
 In Proceedings of SCN’02, volume 2576 of LNCS
, 2003
"... Abstract. Digital signature schemes are a fundamental cryptographic primitive, of use both in its own right, and as a building block in cryptographic protocol design. In this paper, we propose a practical and provably secure signature scheme and show protocols (1) for issuing a signature on a commit ..."
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Cited by 159 (20 self)
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Abstract. Digital signature schemes are a fundamental cryptographic primitive, of use both in its own right, and as a building block in cryptographic protocol design. In this paper, we propose a practical and provably secure signature scheme and show protocols (1) for issuing a signature on a committed value (so the signer has no information about the signed value), and (2) for proving knowledge of a signature on a committed value. This signature scheme and corresponding protocols are a building block for the design of anonymityenhancing cryptographic systems, such as electronic cash, group signatures, and anonymous credential systems. The security of our signature scheme and protocols relies on the Strong RSA assumption. These results are a generalization of the anonymous credential system of Camenisch and Lysyanskaya. 1
Provable data possession at untrusted stores. Cryptology ePrint archive
, 2007
"... We introduce a model for provable data possession (PDP) that allows a client that has stored data at an untrusted server to verify that the server possesses the original data without retrieving it. The model generates probabilistic proofs of possession by sampling random sets of blocks from the serv ..."
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Cited by 132 (6 self)
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We introduce a model for provable data possession (PDP) that allows a client that has stored data at an untrusted server to verify that the server possesses the original data without retrieving it. The model generates probabilistic proofs of possession by sampling random sets of blocks from the server, which drastically reduces I/O costs. The client maintains a constant amount of metadata to verify the proof. The challenge/response protocol transmits a small, constant amount of data, which minimizes network communication. Thus, the PDP model for remote data checking supports large data sets in widelydistributed storage systems. We present two provablysecure PDP schemes that are more efficient than previous solutions, even when compared with schemes that achieve weaker guarantees. In particular,
Pors: proofs of retrievability for large files
 In CCS ’07: Proceedings of the 14th ACM conference on Computer and communications security
, 2007
"... Abstract. In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or backup service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient fo ..."
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Cited by 111 (8 self)
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Abstract. In this paper, we define and explore proofs of retrievability (PORs). A POR scheme enables an archive or backup service (prover) to produce a concise proof that a user (verifier) can retrieve a target file F, that is, that the archive retains and reliably transmits file data sufficient for the user to recover F in its entirety. A POR may be viewed as a kind of cryptographic proof of knowledge (POK), but one specially designed to handle a large file (or bitstring) F. We explore POR protocols here in which the communication costs, number of memory accesses for the prover, and storage requirements of the user (verifier) are small parameters essentially independent of the length of F. In addition to proposing new, practical POR constructions, we explore implementation considerations and optimizations that bear on previously explored, related schemes. In a POR, unlike a POK, neither the prover nor the verifier need actually have knowledge of F. PORs give rise to a new and unusual security definition whose formulation is another contribution of our work. We view PORs as an important tool for semitrusted online archives. Existing cryptographic techniques help users ensure the privacy and integrity of files they retrieve. It is also natural, however, for users to want to verify that archives do not delete or modify files prior to retrieval. The goal of a POR is to accomplish these checks without users having to download the files themselves. A POR can also provide qualityofservice guarantees, i.e., show that a file is retrievable within a certain time bound. Key words: storage systems, storage security, proofs of retrievability, proofs of knowledge 1
Efficient Concurrent ZeroKnowledge in the Auxiliary String Model
, 2000
"... We show that if any oneway function exists, then 3round concurrent zeroknowledge arguments for all NP problems can be built in a model where a short auxiliary string with a prescribed distribution is available to the players. We also show that a wide range of known efficient proofs of knowledge ..."
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Cited by 108 (2 self)
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We show that if any oneway function exists, then 3round concurrent zeroknowledge arguments for all NP problems can be built in a model where a short auxiliary string with a prescribed distribution is available to the players. We also show that a wide range of known efficient proofs of knowledge using specialized assumptions can be modified to work in this model with no essential loss of efficiency. We argue that the assumptions of the model will be satisfied in many practical scenarios where public key cryptography is used, in particular our construction works given any secure public key infrastructure. Finally, we point out that in a model with preprocessing (and no auxiliary string) proposed earlier, concurrent zeroknowledge for NP can be based on any oneway function.
COMPUTATIONALLY SOUND PROOFS
, 2000
"... This paper puts forward a new notion of a proof based on computational complexity and explores its implications for computation at large. Computationally sound proofs provide, in a novel and meaningful framework, answers to old and new questions in complexity theory. In particular, given a random o ..."
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Cited by 98 (3 self)
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This paper puts forward a new notion of a proof based on computational complexity and explores its implications for computation at large. Computationally sound proofs provide, in a novel and meaningful framework, answers to old and new questions in complexity theory. In particular, given a random oracle or a new complexity assumption, they enable us to 1. prove that verifying is easier than deciding for all theorems; 2. provide a quite effective way to prove membership in computationally hard languages (such as CoNPcomplete ones); and 3. show that every computation possesses a short certificate vouching its correctness. Finally, if a special type of computationally sound proof exists, we show that Blum’s notion of program checking can be meaningfully broadened so as to prove that NPcomplete languages are checkable.
Parallel CoinTossing and ConstantRound Secure TwoParty Computation
 Journal of Cryptology
, 2001
"... Abstract. In this paper we show that any twoparty functionality can be securely computed in a constant number of rounds, where security is obtained against malicious adversaries that may arbitrarily deviate from the protocol specification. This is in contrast to Yao’s constantround protocol that e ..."
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Cited by 78 (14 self)
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Abstract. In this paper we show that any twoparty functionality can be securely computed in a constant number of rounds, where security is obtained against malicious adversaries that may arbitrarily deviate from the protocol specification. This is in contrast to Yao’s constantround protocol that ensures security only in the face of semihonest adversaries, and to its malicious adversary version that requires a polynomial number of rounds. In order to obtain our result, we present a constantround protocol for secure cointossing of polynomially many coins (in parallel). We then show how this protocol can be used in conjunction with other existing constructions in order to obtain a constantround protocol for securely computing any twoparty functionality. On the subject of cointossing, we also present a constantround perfect cointossing protocol, where by “perfect ” we mean that the resulting coins are guaranteed to be statistically close to uniform (and not just pseudorandom). 1
An Integer Commitment Scheme based on Groups with Hidden Order
, 2001
"... . We present a commitment scheme allowing commitment to arbitrary size integers, based on any Abelian group with certain properties, most importantly that it is hard for the committer to compute its order. Potential examples include RSA and class groups. We also give e#cient zeroknowledge proto ..."
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Cited by 71 (0 self)
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. We present a commitment scheme allowing commitment to arbitrary size integers, based on any Abelian group with certain properties, most importantly that it is hard for the committer to compute its order. Potential examples include RSA and class groups. We also give e#cient zeroknowledge protocols for proving knowledge of the contents of a commitment and for verifying multiplicative relations over the integers on committed values. This means that our scheme can support, for instance, the e#cent interval proofs of Boudot[1]. The scheme can be seen as a modification and a generalization of an earlier scheme of Fujisaki and Okamoto [5], and in particular our results show that we can use a much larger class of RSA moduli than the safe prime products proposed in [5]. Also, we correct some mistakes in the proofs of [5] and give what appears to be the first multiplication protocol for a Fujisaki/Okamotolike scheme with a complete proof of soundness. 1
ConstantRound CoinTossing With a Man in the Middle or Realizing the Shared Random String Model
 In 43rd FOCS
, 2002
"... We construct the first constantround nonmalleable commitment scheme and the first constantround nonmalleable zeroknowledge argument system, as defined by Dolev, Dwork and Naor. Previous constructions either used a nonconstant number of rounds, or were only secure under stronger setup assumption ..."
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Cited by 71 (5 self)
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We construct the first constantround nonmalleable commitment scheme and the first constantround nonmalleable zeroknowledge argument system, as defined by Dolev, Dwork and Naor. Previous constructions either used a nonconstant number of rounds, or were only secure under stronger setup assumptions. An example of such an assumption is the shared random string model where we assume all parties have access to a reference string that was chosen uniformly at random by a trusted dealer. We obtain these results by defining an adequate notion of nonmalleable cointossing, and presenting a constantround protocol that satisfies it. This protocol allows us to transform protocols that are nonmalleable in (a modified notion of) the shared random string model into protocols that are nonmalleable in the plain model (without any trusted dealer or setup assumptions). Observing that known constructions of a noninteractive nonmalleable zeroknowledge argument systems in the shared random string model are in fact nonmalleable in the modified model, and combining them with our cointossing protocol we obtain the results mentioned above. The techniques we use are different from those used in previous constructions of nonmalleable protocols. In particular our protocol uses diagonalization and a nonblackbox proof of security (in a sense similar to Barak’s zeroknowledge argument).