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94
MerkleDamg˚ard Revisited: How to Construct a Hash Function
 Advances in Cryptology, Crypto 2005
"... The most common way of constructing a hash function (e.g., SHA1) is to iterate a compression function on the input message. The compression function is usually designed from scratch or made out of a blockcipher. In this paper, we introduce a new security notion for hashfunctions, stronger than col ..."
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Cited by 94 (8 self)
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The most common way of constructing a hash function (e.g., SHA1) is to iterate a compression function on the input message. The compression function is usually designed from scratch or made out of a blockcipher. In this paper, we introduce a new security notion for hashfunctions, stronger than collisionresistance. Under this notion, the arbitrary length hash function H must behave as a random oracle when the fixedlength building block is viewed as a random oracle or an ideal blockcipher. The key property is that if a particular construction meets this definition, then any cryptosystem proven secure assuming H is a random oracle remains secure if one plugs in this construction (still assuming that the underlying fixedlength primitive is ideal). In this paper, we show that the current design principle behind hash functions such as SHA1 and MD5 — the (strengthened) MerkleDamg˚ard transformation — does not satisfy this security notion. We provide several constructions that provably satisfy this notion; those new constructions introduce minimal changes to the plain MerkleDamg˚ard construction and are easily implementable in practice.
MultiPropertyPreserving Hash Domain Extension and the EMD Transform
 Advances in Cryptology – ASIACRYPT 2006
, 2006
"... Abstract We point out that the seemingly strong pseudorandom oracle preserving (PROPr) propertyof hash function domainextension transforms defined and implemented by Coron et. al. [12] can actually weaken our guarantees on the hash function, in particular producing a hash functionthat fails to be ..."
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Abstract We point out that the seemingly strong pseudorandom oracle preserving (PROPr) propertyof hash function domainextension transforms defined and implemented by Coron et. al. [12] can actually weaken our guarantees on the hash function, in particular producing a hash functionthat fails to be even collisionresistant (CR) even though the compression function to which the transform is applied is CR. Not only is this true in general, but we show that all the transformspresented in [12] have this weakness. We suggest that the appropriate goal of a domain extension transform for the next generation of hash functions is to be multiproperty preserving, namelythat one should have a single transform that is simultaneously at least collisionresistance preserving, pseudorandom function preserving and PROPr. We present an efficient new transformthat is proven to be multiproperty preserving in this sense.
D.: Nonmalleable codes
 In: ICS (2010
"... We introduce the notion of “nonmalleable codes ” which relaxes the notion of errorcorrection and errordetection. Informally, a code is nonmalleable if the message contained in a modified codeword is either the original message, or a completely unrelated value. In contrast to errorcorrection and ..."
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Cited by 45 (6 self)
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We introduce the notion of “nonmalleable codes ” which relaxes the notion of errorcorrection and errordetection. Informally, a code is nonmalleable if the message contained in a modified codeword is either the original message, or a completely unrelated value. In contrast to errorcorrection and errordetection, nonmalleability can be achieved for very rich classes of modifications. We construct an efficient code that is nonmalleable with respect to modifications that effect each bit of the codeword arbitrarily (i.e. leave it untouched, flip it or set it to either 0 or 1), but independently of the value of the other bits of the codeword. Using the probabilistic method, we also show a very strong and general statement: there exists a nonmalleable code for every “small enough ” family F of functions via which codewords can be modified. Although this probabilistic method argument does not directly yield efficient constructions, it gives us efficient nonmalleable codes in the randomoracle model for very general classes of tampering functions — e.g. functions where every bit in the tampered codeword can depend arbitrarily on any 99 % of the bits in the original codeword. As an application of nonmalleable codes, we show that they provide an elegant algorithmic solution to the task of protecting functionalities implemented in hardware (e.g. signature cards) against “tampering attacks”. In such attacks, the secret state of a physical system is tampered, in the hopes that future interaction with the modified system will reveal some secret information. This problem, was previously studied in the work of Gennaro et al. in 2004 under the name “algorithmic tamper proof security ” (ATP). We show that nonmalleable codes can be used to achieve important improvements over the prior work. In particular, we show that any functionality can be made secure against a large class of tampering attacks, simply by encoding the secretstate with a nonmalleable code while it is stored in memory. 1
On the compressibility of NP instances and cryptographic applications
 In Electronic Colloquium on Computational Complexity (ECCC
, 2006
"... We initiate the study of compression that preserves the solution to an instance of a problem rather than preserving the instance itself. Our focus is on the compressibility of NP decision problems. We consider NP problems that have long instances but relatively short witnesses. The question is, can ..."
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We initiate the study of compression that preserves the solution to an instance of a problem rather than preserving the instance itself. Our focus is on the compressibility of NP decision problems. We consider NP problems that have long instances but relatively short witnesses. The question is, can one efficiently compress an instance and store a shorter representation that maintains the information of whether the original input is in the language or not. We want the length of the compressed instance to be polynomial in the length of the witness rather than the length of original input. Such compression enables to succinctly store instances until a future setting will allow solving them, either via a technological or algorithmic breakthrough or simply until enough time has elapsed. We give a new classification of NP with respect to compression. This classification forms a stratification of NP that we call the VC hierarchy. The hierarchy is based on a new type of reduction called Wreduction and there are compressioncomplete problems for each class. Our motivation for studying this issue stems from the vast cryptographic implications compressibility has. For example, we say that SAT is compressible if there exists a polynomial p(·, ·) so that given a formula consisting of m clauses over n variables it is possible to come up with an equivalent (w.r.t satisfiability) formula of size at most p(n, logm). Then given a compression algorithm for SAT we provide a construction of collision resistant hash functions from any oneway function. This task was shown to be impossible via blackbox reductions [57], and indeed the construction presented is inherently nonblackbox. Another application of SAT compressibility is a cryptanalytic result concerning the limitation of everlasting security in the bounded storage model when mixed with (time) complexity based cryptography. In addition, we study an approach to constructing an Oblivious Transfer Protocol from any oneway function. This approach is based on compression for SAT that also has a property that we call witness retrievability. However, we mange to prove severe limitations on the ability to achieve witness retrievable compression of SAT. 1
From identification to signatures via the FiatShamir transform: Minimizing assumptions for security and forwardsecurity
 Proceedings of Eurocrypt 2002, volume 2332 of LNCS
, 2002
"... The FiatShamir paradigm for transforming identification schemes into signature schemes has been popular since its introduction because it yields efficient signature schemes, and has been receiving renewed interest of late as the main tool in deriving forwardsecure signature schemes. In this paper, ..."
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Cited by 38 (8 self)
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The FiatShamir paradigm for transforming identification schemes into signature schemes has been popular since its introduction because it yields efficient signature schemes, and has been receiving renewed interest of late as the main tool in deriving forwardsecure signature schemes. In this paper, minimal (meaning necessary and sufficient) conditions on the identification scheme to ensure security of the signature scheme in the random oracle model are determined, both in the usual and in the forwardsecure cases. Specifically, it is shown that the signature scheme is secure (resp. forwardsecure) against chosenmessage attacks in the random oracle model if and only if the underlying identification scheme is secure (resp. forwardsecure) against impersonation under passive (i.e., eavesdropping only) attacks, and has its commitments drawn at random from a large space. An extension is proven incorporating a random seed into the FiatShamir transform so that the commitment space assumption may be removed. Keywords: Signature schemes, identification schemes, FiatShamir transform, forward security,
Efficient Garbling from a FixedKey Blockcipher
, 2013
"... Abstract. We advocate schemes based on fixedkey AES as the best route to highly efficient circuitgarbling. We provide such schemes making only one AES call per garbledgate evaluation. On the theoretical side, we justify the security of these methods in the randompermutation model, where parties h ..."
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Abstract. We advocate schemes based on fixedkey AES as the best route to highly efficient circuitgarbling. We provide such schemes making only one AES call per garbledgate evaluation. On the theoretical side, we justify the security of these methods in the randompermutation model, where parties have access to a public random permutation. On the practical side, we provide the JustGarble system, which implements our schemes. JustGarble evaluates moderatesized garbledcircuits at an
Salvaging MerkleDamg˚ard for Practical Applications
, 2009
"... Many cryptographic applications of hash functions are analyzed in the random oracle model. Unfortunately, most concrete hash functions, including the SHA family, use the iterative (strengthened) MerkleDamg˚ard transform applied to a corresponding compression function. Moreover, it is well known tha ..."
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Cited by 28 (2 self)
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Many cryptographic applications of hash functions are analyzed in the random oracle model. Unfortunately, most concrete hash functions, including the SHA family, use the iterative (strengthened) MerkleDamg˚ard transform applied to a corresponding compression function. Moreover, it is well known that the resulting “structured ” hash function cannot be generically used as a random oracle, even if the compression function is assumed to be ideal. This leaves a large disconnect between theory and practice: although no attack is known for many concrete applications utilizing existing (MerkleDamg˚ard based) hash functions, there is no security guarantee either, even by idealizing the compression function. Motivated by this question, we initiate a rigorous and modular study of developing new notions of (still idealized) hash functions which would be (a) natural and elegant; (b) sufficient for arguing security of important applications; and (c) provably met by the (strengthened) MerkleDamg˚ard transform, applied to a “strong enough ” compression function. In particular, we show that a fixedlength compressing random oracle, as well as the currently used DaviesMeyer compression function (the latter analyzed in the ideal cipher model) are “strong enough ” for the two specific weakenings of the random oracle that we develop. These weaker notions, described below, are quite natural and should be interesting in their own right: • Preimage Aware Functions. Roughly, if an attacker found a “later useful ” output y of the function, then it must
Duplexing the sponge: Singlepass authenticated encryption and other applications
 In SAC 2011 (2011
"... Abstract. This paper proposes a novel construction, called duplex, closely related to the sponge construction, that accepts message blocks to be hashed and—at no extra cost—provides digests on the input blocks received so far. It can be proven equivalent to a cascade of sponge functions and hence in ..."
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Abstract. This paper proposes a novel construction, called duplex, closely related to the sponge construction, that accepts message blocks to be hashed and—at no extra cost—provides digests on the input blocks received so far. It can be proven equivalent to a cascade of sponge functions and hence inherits its security against singlestage generic a�acks. The main application proposed here is an authenticated encryption mode based on the duplex construction. This mode is efficient, namely, enciphering and authenticating together require only a single call to the underlying permutation per block, and is readily usable in, e.g., key wrapping. Furthermore, it is the first mode of this kind to be directly based on a permutation instead of a block cipher and to natively support intermediate tags. The duplex construction can be used to efficiently realize other modes, such as a reseedable pseudorandom bit sequence generators and a sponge variant that overwrites part of the state with the input block rather than to XOR it in.
Obfuscation for cryptographic purposes
 In In TCC 2007, LNCS 4392
, 2007
"... All intext references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately. ..."
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Cited by 24 (0 self)
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All intext references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately.