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Assche. Sponge functions
, 2007
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How Risky is the RandomOracle Model?
"... Abstract. RSAFDH and many other schemes secure in the RandomOracle Model (ROM) require a hash function with output size larger than standard sizes. We show that the randomoracle instantiations proposed in the literature for such cases are weaker than a random oracle, including the proposals by Be ..."
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Abstract. RSAFDH and many other schemes secure in the RandomOracle Model (ROM) require a hash function with output size larger than standard sizes. We show that the randomoracle instantiations proposed in the literature for such cases are weaker than a random oracle, including the proposals by Bellare and Rogaway from 1993 and 1996, and the ones implicit in IEEE P1363 and PKCS standards: for instance, we obtain a practical preimage attack on BR93 for 1024bit digests (with complexity less than 2 30). Next, we study the security impact of hash function defects for ROM signatures. As an extreme case, we note that any hash collision would suffice to disclose the master key in the IDbased cryptosystem by Boneh et al. from FOCS ’07, and the secret key in the RabinWilliams signature for which Bernstein proved tight security at EUROCRYPT ’08. We also remark that collisions can be found as a precomputation for any instantiation of the ROM, and this violates the security definition of the scheme in the standard model. Hence, this gives an example of a natural scheme that is proven secure in the ROM but that in insecure for any instantiation by a single function. Interestingly, for both of these schemes, a slight modification can prevent these attacks, while preserving the ROM security result. We give evidence that in the case of RSA and Rabin/RabinWilliams, an appropriate PSS padding is more robust than all other paddings known. 1
Constructing an Ideal Hash Function from Weak Ideal Compression Functions
 In Selected Areas in Cryptography, Lecture Notes in Computer Science
, 2006
"... Abstract. We introduce the notion of a weak ideal compression function, which is vulnerable to strong forms of attack, but is otherwise random. We show that such weak ideal compression functions can be used to create secure hash functions, thereby giving a design that can be used to eliminate attack ..."
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Abstract. We introduce the notion of a weak ideal compression function, which is vulnerable to strong forms of attack, but is otherwise random. We show that such weak ideal compression functions can be used to create secure hash functions, thereby giving a design that can be used to eliminate attacks caused by undesirable properties of compression functions. We prove that the construction we give, which we call the “zipper hash, ” is ideal in the sense that the overall hash function is indistinguishable from a random oracle when implemented with these weak ideal building blocks. The zipper hash function is relatively simple, requiring two compression function evaluations per block of input, but it is not streamable. We also show how to create an ideal (strong) compression function from ideal weak compression functions, which can be used in the standard iterated way to make a streamable hash function. Keywords: Hash function, compression function, MerkleDamg˚ard, ideal primitives, nonstreamable hash functions, zipper hash.
Nontrivial blackbox combiners for collisionresistant hashfunctions don’t exist
 In Proc. Eurocrypt ’07
, 2007
"... 1 Introduction A function H: f0; 1g ..."
A CollisionResistant Rate1 DoubleBlockLength Hash Function
"... (on the leave to BauhausUniversity Weimar, Germany) Abstract. This paper proposes a construction for collision resistant 2nbit hash functions, based on nbit block ciphers with 2nbit keys. The construction is analysed in the ideal cipher model; for n = 128 an adversary would need roughly 2 122 un ..."
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Cited by 8 (0 self)
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(on the leave to BauhausUniversity Weimar, Germany) Abstract. This paper proposes a construction for collision resistant 2nbit hash functions, based on nbit block ciphers with 2nbit keys. The construction is analysed in the ideal cipher model; for n = 128 an adversary would need roughly 2 122 units of time to find a collision. The construction employs “combinatorial ” hashing as an underlying building block (like Universal Hashing for cryptographic message authentication by Wegman and Carter). The construction runs at rate 1, thus improving on a similar rate 1/2 approach by Hirose (FSE 2006). 1
Inside the hypercube
 In ACISP’09
, 2009
"... Some force inside the Hypercube occasionally manifests itself with deadly results. ..."
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Cited by 8 (1 self)
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Some force inside the Hypercube occasionally manifests itself with deadly results.
Preimages for StepReduced SHA2
 ASIACRYPT
"... Abstract. In this paper, we present a preimage attack for 42 stepreduced SHA256 with time complexity 2 251.7 and memory requirements of order 2 12. The same attack also applies to 42 stepreduced SHA512 with time complexity 2 502.3 and memory requirements of order 2 22. Our attack is meetinthem ..."
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Abstract. In this paper, we present a preimage attack for 42 stepreduced SHA256 with time complexity 2 251.7 and memory requirements of order 2 12. The same attack also applies to 42 stepreduced SHA512 with time complexity 2 502.3 and memory requirements of order 2 22. Our attack is meetinthemiddle preimage attack. Keywords: preimage attack, SHA256, SHA512, meetinthemiddle, hash function 1
Do broken hash functions affect the security of timestamping schemes
 In Proc. of ACNS’06, LNCS 3989
, 2006
"... Abstract. We study the influence of collisionfinding attacks on the security of timestamping schemes. We distinguish between clientside hash functions used to shorten the documents before sending them to timestamping servers and serverside hash functions used for establishing one way causal rel ..."
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Cited by 8 (3 self)
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Abstract. We study the influence of collisionfinding attacks on the security of timestamping schemes. We distinguish between clientside hash functions used to shorten the documents before sending them to timestamping servers and serverside hash functions used for establishing one way causal relations between time stamps. We derive necessary and sufficient conditions for client side hash functions and show by using explicit separation techniques that neither collisionresistance nor 2nd preimage resistance is necessary for secure timestamping. Moreover, we show that server side hash functions can even be not oneway. Hence, it is impossible by using blackbox techniques to transform collisionfinders into wrappers that break the corresponding timestamping schemes. Each such wrapper should analyze the structure of the hash function. However, these separations do not necessarily hold for more specific classes of hash functions. Considering this, we take a more detailed look at the structure of practical hash functions by studying the MerkleDamg˚ard (MD) hash functions. We show that attacks, which are able to find collisions for MD hash functions with respect to randomly chosen initial states, also violate the necessary security conditions for clientside hash functions. This does not contradict the blackbox separations results because the MD structure is already a deviation from the blackbox setting. As a practical consequence, MD5, SHA0, and RIPEMD are no more recommended to use as clientside hash functions in timestamping. However, there is still no evidence against using MD5 (or even MD4) as serverside hash functions. 1
Domain extension of public random functions: Beyond the birthday barrier
 In Advances in Cryptology – CRYPTO ’07 (2007), Lecture Notes in Computer Science
, 2007
"... Combined with the iterated constructions of Coron et al., our result leads to the first iterated construction of a hash function f0; 1g\Lambda ! f0; 1gn from a component function f0; 1gn! f0; 1gn that withstands all recently proposed generic attacks against iterated hash functions, like Joux's multi ..."
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Combined with the iterated constructions of Coron et al., our result leads to the first iterated construction of a hash function f0; 1g\Lambda ! f0; 1gn from a component function f0; 1gn! f0; 1gn that withstands all recently proposed generic attacks against iterated hash functions, like Joux's multicollision attack, Kelsey and Schneier's secondpreimage attack, and Kelsey and Kohno's herding attacks. 1 Introduction 1.1 Secret vs. Public Random Functions Primitives that provide some form of randomness are of central importance in cryptography, both as a primitive assumed to be given (e.g. a secret key), and as a primitive constructed from a weaker one to "behave like " a certain ideal random primitive (e.g. a random function), according to some security notion.
Second Preimage Attacks on Dithered Hash Functions
"... Abstract. We develop a new generic longmessage second preimage attack, based on combining the techniques in the second preimage attacks of Dean [8] and Kelsey and Schneier [16] with the herding attack of Kelsey and Kohno [15]. We show that these generic attacks apply to hash functions using the Mer ..."
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Abstract. We develop a new generic longmessage second preimage attack, based on combining the techniques in the second preimage attacks of Dean [8] and Kelsey and Schneier [16] with the herding attack of Kelsey and Kohno [15]. We show that these generic attacks apply to hash functions using the MerkleDamgård construction with only slightly more work than the previously known attack, but allow enormously more control of the contents of the second preimage found. Additionally, we show that our new attack applies to several hash function constructions which are not vulnerable to the previously known attack, including the dithered hash proposal of Rivest [25], Shoup’s UOWHF[26] and the ROX hash construction [2]. We analyze the properties of the dithering sequence used in [25], and develop a timememory tradeoff which allows us to apply our second preimage attack to a wide range of dithering sequences, including sequences which are much stronger than those in Rivest’s proposals. Finally, we show that both the existing second preimage attacks [8,16] and our new attack can be applied even more efficiently to multiple target messages; in general, given a set of many target messages with a total of 2 R message blocks, these second preimage attacks can find a second preimage for one of those target messages with no more work than would be necessary to find a second preimage for a single target message of 2 R message blocks.