Preimage Attacks against Variants of Very Smooth Hash

Computing discrete logarithms in finite fields is a main concern in cryptography. The best algorithms in large and medium characteristic fields (e.g., G F ( p 2 ) \rm {GF}(p^2) , G F ( p 12 ) \rm {GF}(p^{12}) ) are the Number Field Sieve and its variants (special, high-degree, tower). The best algorithms in small characteristic finite fields (e.g., G F ( 3 6 ⋅ 509 ) \rm {GF}(3^{6 \cdot 509}) ) are the Function Field Sieve, Joux’s algorithm, and the quasipolynomial-time algorithm. The last step of this family of algorithms is the individual logarithm computation. It computes a smooth decomposition of a given target in two phases: an initial splitting, then a descent tree. While new improvements have been made to reduce the complexity of the dominating relation collection and linear algebra steps, resulting in a smaller factor basis (database of known logarithms of small elements), the last step remains at the same level of difficulty. Indeed, we have to find a smooth decomposition of a typically large element in the finite field. This work improves the initial splitting phase and applies to any nonprime finite field. It is very efficient when the extension degree is composite. It exploits the proper subfields, resulting in a much more smooth decomposition of the target. This leads to a new trade-off between the initial splitting step and the descent step in small characteristic. Moreover it reduces the width and the height of the subsequent descent tree.

We essentially improve lower bounds on the linear complexity of a sequence representing the residues of the discrete logarithm in a finite field modulo a divisor of the order of the multiplicative group. More generally, we present the result as a bound on the linear complexity profile. The proof is based on character sum bounds.

A short proof for explicit formulas for discrete logarithms in finite fields

In the past two years there have been several advances in Number Field Sieve (NFS) algorithms for computing discrete logarithms in finite fields $${\mathbb F}_{p^n}$$ where p is prime and $$n > 1$$ is a small integer. This article presents a concise overview of these algorithms and discusses some of the challenges with assessing their impact on keylengths for pairing-based cryptosystems.

The security of pairing-based crypto-systems relies on the difficulty to compute discrete logarithms in finite fields $${\mathbb F}_{p^n}$$ where n is a small integer larger than 1. The state-of-art algorithm is the number field sieve (NFS) together with its many variants. When p has a special form (SNFS), as in many pairings constructions, NFS has a faster variant due to Joux and Pierrot. We present a new NFS variant for SNFS computations, which is better for some cryptographically relevant cases, according to a precise comparison of norm sizes. The new algorithm is an adaptation of Schirokauer’s variant of NFS based on tower extensions, for which we give a middlebrow presentation.

Classifying and generating exact coset representatives of [formula omitted] in [formula omitted

In this paper, we present a new technique to construct a collision attack from a particular preimage attack which is called a partial target preimage attack. Since most of the recent meet-in-the-middle preimage attacks can be regarded as the partial target preimage attack, a collision attack is derived from the meet-in-the-middle preimage attack. By using our technique, pseudo collisions of the 43-step reduced SHA-256 and the 46-step reduced SHA-512 can be obtained with complexities of 2126 and 2254.5, respectively. As far as we know, our results are the best pseudo collision attacks on both SHA-256 and SHA-512 in literature. Moreover, we show that our pseudo collision attacks can be extended to 52 and 57 steps of SHA-256 and SHA-512, respectively, by combined with the recent preimage attacks on SHA-2 by bicliques. Furthermore, since the proposed technique is quite simple, it can be directly applied to other hash functions. We apply our algorithm to several hash functions including Skein and BLAKE, which are the SHA-3 finalists. We present not only the best pseudo collision attacks on SHA-2 family, but also a new insight of relation between a meet-in-the-middle preimage attack and a pseudo collision attack.

Recently, several striking advances have taken place regarding the discrete logarithm problem (DLP) in finite fields of small characteristic, despite progress having remained essentially static for nearly thirty years, with the best known algorithms being of subexponential complexity. In this expository article we describe the key insights and constructions which culminated in two independent quasi-polynomial algorithms. To put these developments into both a historical and a mathematical context, as well as to provide a comparison with the cases of so-called large and medium characteristic fields, we give an overview of the state-of-the-art algorithms for computing discrete logarithms in all finite fields. Our presentation aims to guide the reader through the algorithms and their complexity analyses ab initio.

We study the security of AES in the open-key setting by showing an analysis on hash function modes instantiating AES including Davies-Meyer, Matyas-Meyer-Oseas, and Miyaguchi-Preneel modes. In particular, we propose preimage attacks on these constructions, while most of previous work focused their attention on collision attacks or distinguishers using non-ideal differential properties. This research is based on the motivation that we should evaluate classical and important security notions for hash functions and avoid complicated attack models that seem to have little relevance in practice. We apply a recently developed meet-in-the-middle preimage approach. As a result, we obtain a preimage attack on 7 rounds of Davies-Meyer AES and a second preimage attack on 7 rounds of Matyas-Meyer-Oseas and Miyaguchi-Preneel AES. Considering that the previous best collision attack only can work up to 6 rounds, the number of attacked rounds reaches the best in terms of the classical security notions. In our attacks, the key is regarded as a known constant, and the attacks thus can work for any key length in common.KeywordsAEShash functionDavies-MeyerMatyas-Meyer-OseasMiyaguchi-PreneelPGVpreimagemeet-in-the-middleWhirlpool

Maelstrom-0 is the second member of a family of AES-based hash functions whose designs are pioneered by Paulo Baretto and Vincent Rijmen. According to its designers, the function is designed to be an evolutionary lightweight alternative to the ISO standard Whirlpool. In this paper, we study the preimage resistance of the Maelstrom-0 hash function using its proposed 3CM chaining construction. More precisely, we apply a meet-in-the-middle preimage attack on the compression function and combine it with a guess and determine approach which allows us to obtain a 6-round pseudo preimage for a given compression function output with time complexity of 2496 and memory complexity of 2112. Then, we propose a four stage attack in which we adopt another meet-in-the-middle attack and a 2-block multicollision approach to defeat the two additional checksum chains and turn the pseudo preimage attack on the compression function into a preimage attack on the hash function. Using our approach, preimages of the 6-round reduced Maelstrom-0 hash function are generated with time complexity of 2505 and memory complexity of 2112.

We present new preimage attacks on standard Keccak-224 and Keccak-256 that are reduced to 3 and 4 rounds. An allocating approach is used in the attacks, and the whole complexity is allocated to two stages, such that fewer constraints are considered and the complexity is lowered in each stage. Specifically, we are trying to find a 2-block preimage, instead of a 1-block one, for a given hash value, and the first and second message blocks are found in two stages, respectively. Both the message blocks are constrained by a set of newly proposed conditions on the middle state, which are weaker than those brought by the initial values and the hash values. Thus, the complexities in the two stages are both lower than that of finding a 1-block preimage directly. Together with the basic allocating approach, an improved method is given to balance the complexities of two stages, and hence, obtains the optimal attacks. As a result, we present the best theoretical preimage attacks on Keccak-224 and Keccak-256 that are reduced to 3 and 4 rounds. Moreover, we practically found a (second) preimage for 3-round Keccak-224 with a complexity of \(2^{39.39}\).

Consider the cryptographic hash function Skein-512-256, which became a finalist of the NIST hash function competition in 2010. Skein-512-256 compression function processes a 512-bit block message block in 72 rounds, 12 operations each. A practical algebraic preimage attack on round-reduced versions of the Skein-512-256 compression function is proposed. The corresponding computational problems are reduced to instances of the Boolean satisfiability problem (SAT). Using sequential SAT solvers, preimages of the first round and the first 7 operations of the second round of the compression function have been found. By applying a parallel SAT solver, the number of the second round’s operations has been increased to 8. Earlier, a practical preimage attack on at most 1-round Skein-512-256 compression function was published.

In modern cryptography, hash functions are considered as one of the key components for secure communication. They play a vital role in a wide range of applications such as ensuring the authentication and integrity of the data, in forensic investigation, password storage, random number generations for unique session keys, and for creating a unified view in blockchain. The Avalanche effect (also known as diffusion) is an important characteristic of a hash function where a minor change in the hash function’s input will result in a significantly different output. The absence of this property implies that the hash function is vulnerable to various attacks such as collision attack, length extension attack, and preimage attack. Through this research, we have investigated the Avalanche effect of sixteen hash functions and two hash-based applications, namely Hash-based Message Authentication Code (HMAC) and Public Key Cryptography Standards (PKCS). To measure the performance of these hash functions and hash-based applications, we have implemented a generic circuit using CrypTool for automating the simulation process for multiple trials. Simulation results indicate that around half of the inputs of each hash function failed to exhibit the Strict Avalanche Criterion (SAC) and, Bit Independence Criterion (BIC). Moreover, we ranked the hash functions based on Multi Criteria Decision Metrics (MCDM) using intermediate states of simulation results. Furthermore, a total of fifteen statistical tests were carried out to evaluate the randomization property of the hash functions using NIST (National Institute of Standards and Technology) toolkit. This study is aimed to open up a future scope of research to the need for improvement of various hash functions by analyzing the randomization and non-correlation properties of existing functions in terms of the Avalanche effect.

Preimage and pseudo-collision attacks on step-reduced SM3 hash function

A simple algorithm to find logarithms in a finite field of characteristic two is described. It uses the Euclidean algorithm for polynomials in attempting to reduce an element to a product of factors all of whose logarithms are stored in a database. The algorithm, which is similar to one of Adleman, has a random runtime and constant storage requirements. It is analyzed and problems associated with the construction of the database are considered. The aim of the work is to show that the algorithm is feasible for the field with $2^{127} $ elements on which several proposed public key distribution systems have been based. For such application it is felt that the discrete logarithm is still a viable technique for sufficiently large fields.