Asymptotic Analysis of Plausible Tree Hash Modes for SHA-3

Discussions about the choice of a tree hash mode of operation for a standardization have recently been undertaken. It appears that a single tree mode cannot address adequately all possible uses and specifications of a system. In this paper, we review the tree modes which have been proposed, we discuss their problems and propose remedies. We make the reasonable assumption that communicating systems have different specifications and that software applications are of different types (securing stored content or live-streamed content). Finally, we propose new modes of operation that address the resource usage problem for the three most representative categories of devices and we analyse their asymptotic behavior

Unifom Generators and Combinatorial Design

International audienceThe concept of randomness is fundamental in many domains and in particular in cryptography. Intuitively, a system, which is unpredictable is more difficult to attack and as a consequence, creating sequences that look like random represents a major issue. In this paper, we first study theoretically how a source of symbols with positive entropy can be turned into a true random generator called Bernoulli. We concentrate on a special type of generators, which consists in randomly choosing k elements out of n elements. After studying some existing algorithms, which are of Las Vegas type, we introduce new constructions from a binary generator taken as a primary random source of symbols. Our method is based on combinatorial block designs and we construct algorithms of Monte Carlo type involving random walks. We analyze in detail properties of our general method. Several explicit constructions of k-out-of-n generators are given. We show that the speed of convergence to the uniform distribution is better than any known method using algorithms with bounded running times

The extended binary quadratic residue code of length 42 holds a 3-design

The codewords of weight $10$ of the $[42,21,10]$ extended binary quadratic residue code are shown to hold a design of parameters $3-(42,10,18).$ Its automorphism group is isomorphic to $PSL(2,41)$. Its existence can be explained neither by a transitivity argument, nor by the Assmus-Mattson theorem.Comment: 6 pages. Second versio

On the construction of elliptic Chudnovsky-type algorithms for multiplication in large extensions of finite fields

International audienceWe indicate a strategy in order to construct bilinear multiplication algorithms of type Chudnovsky in large extensions of any finite field. In particular, using the symmetric version of the generalization of Randriambololona specialized on the elliptic curves, we show that it is possible to construct such algorithms with low bilinear complexity. More precisely, if we only consider the Chudnovsky-type algorithms of type symmetric elliptic, we show that the symmetric bilinear complexity of these algorithms is in O(n(2q)^log * q (n)) where n corresponds to the extension degree, and log * q (n) is the iterated logarithm. Moreover, we show that the construction of such algorithms can be done in time polynomial in n. Finally, applying this method we present the effective construction, step by step, of such an algorithm of multiplication in the finite field F 3^57. Index Terms Multiplication algorithm, bilinear complexity, elliptic function field, interpolation on algebraic curve, finite field

New models for efficient authenticated dictionaries

International audienceWe propose models for data authentication which take into account the behavior of the clients who perform queries. Our models reduce the size of the authenticated proof when the frequency of the query corresponding to a given data is higher. Existing models implicitly assume the frequency distribution of queries to be uniform, but in reality, this distribution generally follows Zipf's law. Our models better reflect reality and the communication cost between clients and the server provider is reduced allowing the server to save bandwidth. The obtained gain on the average proof size compared to existing schemes depends on the parameter of Zipf law. The greater the parameter, the greater the gain. When the frequency distribution follows a perfect Zipf's law, we obtain a gain that can reach 26%. Experiments show the existence of applications for which Zipf parameter is greater than 1, leading to even higher gains

AES Side-Channel Countermeasure using Random Tower Field Constructions

International audienceMasking schemes to secure AES implementations against side-channel attacks is a topic of ongoing research. The most sensitive part of the AES is the non-linear SubBytes operation, in particular, the inversion in GF(2^8), the Galois field of 2^8 elements. In hardware implementations, it is well known that the use of the tower of extensions GF(2) ⊂ GF(2^2) ⊂ GF(2^4) ⊂ GF(2^8) leads to a more efficient inversion. We propose to use a random isomorphism instead of a fixed one. Then, we study the effect of this randomization in terms of security and efficiency. Considering the field extension GF(2^8)/GF(2^4), the inverse operation leads to computation of its norm in GF(2^4). Hence, in order to thwart side-channel attack, we manage to spread the values of norms over GF(2^4). Combined with a technique of boolean masking in tower fields, our countermeasure strengthens resistance against first-order differential side-channel attacks

Optimization of the scalar complexity of Chudnovsky2^2 multiplication algorithms in finite fields

We propose several constructions for the original multiplication algorithm of D.V. and G.V. Chudnovsky in order to improve its scalar complexity. We highlight the set of generic strategies who underlay the optimization of the scalar complexity, according to parameterizable criteria. As an example, we apply this analysis to the construction of type elliptic Chudnovsky$^2$ multiplication algorithms for small extensions. As a case study, we significantly improve the Baum-Shokrollahi construction for multiplication in $\mathbb F_{256}/\mathbb F_4$.Comment: 25 pages, 0 figur

Effective arithmetic in finite fields based on Chudnovsky's multiplication algorithm

International audienceThanks to a new construction of the Chudnovsky and Chudnovsky multiplication algorithm, we design efficient algorithms for both the exponentiation and the multiplication in finite fields. They are tailored to hardware implementation and they allow computations to be parallelized, while maintaining a low number of bilinear multiplications.À partir d'une nouvelle construction de l'algorithme de multiplication de Chudnovsky et Chudnovsky, nous concevons des algorithmes efficaces pour la multiplication et l'exponentiation dans les corps finis. Ils sont adaptés à une implémentation matérielle et sont parallélisables, tout en gardant un nombre de multiplications bilinéaires très bas

CONSTRUCTION OF ASYMMETRIC CHUDNOVSKY ALGORITHMS WITHOUT DERIVATED EVALUATION FOR MULTIPLICATION IN FINITE FIELDS

The Chudnovsky and Chudnovsky algorithm for the multiplication in extensions of finite fields provides a bilinear complexity which is uniformly linear with respect to the degree of the extension. Recently, Ran-driambololona has generalized the method, allowing asymmetry in the interpolation procedure and leading to new upper bounds on the bilinear complexity. In this article, we first translate this generalization into the language of algebraic function fields. Then, we propose a strategy to effectively construct asymmetric algorithms using places of higher degrees and without derivated evaluation. Finally, we provide examples of three multiplication algorithms along with their Magma implementation: in F 16 13 using only rational places, in F 4 5 using also places of degree two, and in F 2 5 using also places of degree four