Abstract.A fault attack is a powerful cryptanalytic tool which can be applied to many types of cryptosystems which are not vulnerable to direct attacks. The research literature contains many examples of fault attacks on public key cryptosystems and block ciphers, but surprisingly we could not find any systematic study of the applicability of fault attacks to stream ciphers. Our goal in this paper is to develop general techniques which can be used to attack the standard constructions of stream ciphers based on LFSR's, as well as more specialized techniques which can be used against specific stream ciphers such as RC4, LILI-128 and SOBER-t32. While most of the schemes can be successfully attacked, we point out several interesting open problems such as an attack on FSM filtered constructions and the analysis of high Hamming weight faults in LFSR's.
Abstract. We develop a new generic long-message 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 Merkle-Damgård 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 time-memory 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.
Abstract. The security of hash functions has recently become one of the hottest topics in the design and analysis of cryptographic primitives. Since almost all the hash functions used today (including the MD and SHA families) have an iterated design, it is important to study the general security properties of such functions. At Crypto 2004 Joux showed that in any iterated hash function it is relatively easy to find exponential sized multicollisions, and thus the concatenation of several hash functions does not increase their security. However, in his proof it was essential that each message block is used at most once. In 2005 Nandi and Stinson extended the technique to handle iterated hash functions in which each message block is used at most twice. In this paper we consider the general case and prove that even if we allow each iterated hash function to scan the input multiple times in an arbitrary expanded order, their concatenation is not stronger than a single function. Finally, we extend the result to tree-based hash functions with arbitrary tree structures.
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