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NonMalleable Cryptography
 SIAM Journal on Computing
, 2000
"... The notion of nonmalleable cryptography, an extension of semantically secure cryptography, is defined. Informally, in the context of encryption the additional requirement is that given the ciphertext it is impossible to generate a different ciphertext so that the respective plaintexts are related. ..."
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Cited by 454 (22 self)
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The notion of nonmalleable cryptography, an extension of semantically secure cryptography, is defined. Informally, in the context of encryption the additional requirement is that given the ciphertext it is impossible to generate a different ciphertext so that the respective plaintexts are related. The same concept makes sense in the contexts of string commitment and zeroknowledge proofs of possession of knowledge. Nonmalleable schemes for each of these three problems are presented. The schemes do not assume a trusted center; a user need not know anything about the number or identity of other system users. Our cryptosystem is the first proven to be secure against a strong type of chosen ciphertext attack proposed by Rackoff and Simon, in which the attacker knows the ciphertext she wishes to break and can query the decryption oracle on any ciphertext other than the target.
The exact security of digital signatures: How to sign with RSA and Rabin
, 1996
"... We describe an RSAbased signing scheme called PSS which combines essentially optimal efficiency with attractive security properties. Signing takes one RSA decryption plus some hashing, verification takes one RSA encryption plus some hashing, and the size of the signature is the size of the modulus. ..."
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Cited by 336 (14 self)
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We describe an RSAbased signing scheme called PSS which combines essentially optimal efficiency with attractive security properties. Signing takes one RSA decryption plus some hashing, verification takes one RSA encryption plus some hashing, and the size of the signature is the size of the modulus. Assuming the underlying hash functions are ideal, our schemes are not only provably secure, but are so in a tight way — an ability to forge signatures with a certain amount of computational resources implies the ability to invert RSA (on the same size modulus) with about the same computational effort. Furthermore, we provide a second scheme which maintains all of the above features and in addition provides message recovery. These ideas extend to provide schemes for Rabin signatures with analogous properties; in particular their security can be tightly related to the hardness of factoring.
Concurrent ZeroKnowledge
 IN 30TH STOC
, 1999
"... Concurrent executions of a zeroknowledge protocol by a single prover (with one or more verifiers) may leak information and may not be zeroknowledge in toto. In this paper, we study the problem of maintaining zeroknowledge We introduce the notion of an (; ) timing constraint: for any two proces ..."
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Cited by 162 (19 self)
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Concurrent executions of a zeroknowledge protocol by a single prover (with one or more verifiers) may leak information and may not be zeroknowledge in toto. In this paper, we study the problem of maintaining zeroknowledge We introduce the notion of an (; ) timing constraint: for any two processors P1 and P2 , if P1 measures elapsed time on its local clock and P2 measures elapsed time on its local clock, and P2 starts after P1 does, then P2 will finish after P1 does. We show that if the adversary is constrained by an (; ) assumption then there exist fourround almost concurrent zeroknowledge interactive proofs and perfect concurrent zeroknowledge arguments for every language in NP . We also address the more specific problem of Deniable Authentication, for which we propose several particularly efficient solutions. Deniable Authentication is of independent interest, even in the sequential case; our concurrent solutions yield sequential solutions without recourse to timing, i.e., in the standard model.
On MemoryBound Functions for Fighting Spam
 In Crypto
, 2002
"... In 1992, Dwork and Naor proposed that email messages be accompanied by easytocheck proofs of computational effort in order to discourage junk email, now known as spam. They proposed specific CPUbound functions for this purpose. Burrows suggested that, since memory access speeds vary across ma ..."
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Cited by 82 (2 self)
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In 1992, Dwork and Naor proposed that email messages be accompanied by easytocheck proofs of computational effort in order to discourage junk email, now known as spam. They proposed specific CPUbound functions for this purpose. Burrows suggested that, since memory access speeds vary across machines much less than do CPU speeds, memorybound functions may behave more equitably than CPUbound functions; this approach was first explored by Abadi, Burrows, Manasse, and Wobber [8].
Access control and signatures via quorum secret sharing
 IEEE TRANSACTIONS ON PARALLEL AND DISTRIBUTED SYSTEMS
, 1998
"... We suggest a method of controlling the access to a secure database via quorum systems. A quorum system is a collection of sets (quorums) every two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We propose a separ ..."
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Cited by 35 (13 self)
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We suggest a method of controlling the access to a secure database via quorum systems. A quorum system is a collection of sets (quorums) every two of which have a nonempty intersection. Quorum systems have been used for a number of applications in the area of distributed systems. We propose a separation between access servers, which are protected and trustworthy, but may be outdated, and the data servers, which may all be compromised. The main paradigm is that only the servers in a complete quorum can collectively grant (or revoke) access permission. The method we suggest ensures that, after authorization is revoked, a cheating user Alice will not be able to access the data even if many access servers still consider her authorized and even if the complete raw database is available to her. The method has a low overhead in terms of communication and computation. It can also be converted into a distributed system for issuing secure signatures. An important building block in our method is the use of secret sharing schemes that realize the access structures of quorum systems. We provide several efficient constructions of such schemes which may be of interest in their own right.
Identification protocols secure against reset attacks
 Adv. in Cryptology — Eurocrypt 2001, LNCS
, 2001
"... Abstract. We provide identi£cation protocols that are secure even when the adversary can reset the internal state and/or randomization source of the user identifying itself, and when executed in an asynchronous environment like the Internet that gives the adversary concurrent access to instances of ..."
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Cited by 30 (4 self)
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Abstract. We provide identi£cation protocols that are secure even when the adversary can reset the internal state and/or randomization source of the user identifying itself, and when executed in an asynchronous environment like the Internet that gives the adversary concurrent access to instances of the user. These protocols are suitable for use by devices (like smartcards) which when under adversary control may not be able to reliably maintain their internal state between invocations. 1
Securely Combining PublicKey Cryptosystems
 Proceedings of the ACM Computer and Security Conference
, 2001
"... It is a maxim of sound computersecurity practice that a cryptographic key should have only a single use. For example, an RSA key pair should be used only for publickey encryption or only for digital signatures, and not for both. In this paper we show that in many cases, the simultaneous use of ..."
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Cited by 17 (1 self)
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It is a maxim of sound computersecurity practice that a cryptographic key should have only a single use. For example, an RSA key pair should be used only for publickey encryption or only for digital signatures, and not for both. In this paper we show that in many cases, the simultaneous use of related keys for two cryptosystems, e.g. for a publickey encryption system and for a publickey signature system, does not compromise their security. We demonstrate this for a variety of publickey encryption schemes that are secure against chosenciphertext attacks, and for a variety of digital signature schemes that are secure against forgery under chosenmessage attacks. The precise form of the statement of security that we are able to prove depends on the particular cryptographic schemes in question and on the cryptographic assumptions needed for their proofs of security; but in every case, our proof of security does not require any additional cryptographic assumptions. Among the cryptosystems that we analyze in this manner are the publickey encryption schemes of Cramer and Shoup, Naor and Yung, and Dolev, Dwork, and Naor, which are all defined in the standard model, while in the randomoracle model we analyze plaintextaware encryption schemes (as defined by Bellare and Rogaway) and in particular the OAEP+ cryptosystem. Among publickey signature schemes, we analyze those of Cramer and Shoup and of Gennaro, Halevi, and Rabin in the standard model, while in the randomoracle model we analyze the RSA PSS scheme as well as variants of the El Gamal and Schnorr schemes. (See references within.) 1
A Practical and Tightly Secure Signature Scheme Without Hash Function
, 2007
"... In 1999, two signature schemes based on the flexible RSA problem (a.k.a. strong RSA problem) were independently introduced: the GennaroHaleviRabin (GHR) signature scheme and the CramerShoup (CS) signature scheme. Remarkably, these schemes meet the highest security notion in the standard model. T ..."
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Cited by 3 (0 self)
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In 1999, two signature schemes based on the flexible RSA problem (a.k.a. strong RSA problem) were independently introduced: the GennaroHaleviRabin (GHR) signature scheme and the CramerShoup (CS) signature scheme. Remarkably, these schemes meet the highest security notion in the standard model. They however differ in their implementation. The CS scheme and its subsequent variants and extensions proposed so far feature a loose security reduction, which, in turn, implies larger security parameters. The security of the GHR scheme and of its twinningbased variant are shown to be tightly based on the flexible RSA problem but additionally (i) either assumes the existence of divisionintractable hash functions, or (ii) requires an injective mapping into the prime numbers in both the signing and verification algorithms. In this paper, we revisit the GHR signature scheme and completely remove the extra assumption made on the hash functions without relying on injective prime mappings. As a result, we obtain a practical signature scheme (and an online/offline variant thereof) whose security is solely and tightly related to the strong RSA assumption.
full version. CollisionResistant Hashing: Towards Making UOWHFs Practical
, 1997
"... Recent attacks on the cryptographic hash functions MD4 and MD5 make it clear that (strong) collisionresistance is a hardtoachievegoal. Welooktowards a weaker notion, the universal oneway hash functions (UOWHFs) of Naor and Yung, and investigate their practical potential. The goal is to build UOWH ..."
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Recent attacks on the cryptographic hash functions MD4 and MD5 make it clear that (strong) collisionresistance is a hardtoachievegoal. Welooktowards a weaker notion, the universal oneway hash functions (UOWHFs) of Naor and Yung, and investigate their practical potential. The goal is to build UOWHFs not based on number theoretic assumptions, but from the primitives underlying current cryptographic hash functions like MD5 and SHA1. Pursuing this goal leads us to new questions. The main one is how toextend a compression function to a full edged hash function in this new setting. We show thatthe classic MerkleDamgard method used in the standard setting fails for these weaker kinds of hash functions, and we present some new methods that work. Our main construction is the \XOR tree. " We also consider the problem
1Cryptographic Primitives for Information Authentication  State of the Art
"... Abstract. This paper describes the state of the art for cryptographic primitives that are used for protecting the authenticity of information: cryptographic hash functions and digital signature schemes; the ¯rst class can be divided into Manipulation Detection Codes (MDCs, also known as oneway and ..."
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Abstract. This paper describes the state of the art for cryptographic primitives that are used for protecting the authenticity of information: cryptographic hash functions and digital signature schemes; the ¯rst class can be divided into Manipulation Detection Codes (MDCs, also known as oneway and collision resistant hash functions) and Message Authentication Codes (or MACs). The theoretical background is sketched, but most attention is paid to overview the large number of practical constructions for hash functions and to the recent developments in their cryptanalysis. It is also explained to what extent the security of these primitives can be reduced in a provable way to realistic assumptions. 1