Self-synchronizing stream ciphers and dynamical systems: state of the art and open issues

International audienceDynamical systems play a central role in the design of symmetric cryptosystems. Their use has been widely investigated both in ''chaos-based'' private communications and in stream ciphers over finite fields. In the former case, they get the form of automata named as Moore or Mealy machines. The main charateristic of stream ciphers lies in that they require synchronization of complex sequences generated by the dynamical systems involved at the transmitter and the receiver part. In this paper, we focus on a special class of symmetric ciphers, namely the Self-Synchronizing Stream Ciphers. Indeed, such ciphers have not been seriously explored so far although they get interesting properties of synchronization which could make them very appealing in practice. We review and compare different design approaches which have been proposed in the open literature and fully-specified algorithms are detailed for illustration purpose. Open issues related to the validation and the implementation of Self-Synchronizing Stream Ciphers are developped. We highlight the reason why some concepts borrowed from control theory appear to be useful to this end

A Characterization of Binary Bent Functions

AbstractA recent paper by Carlet introduces a general class of binary bent functions on (GF(2))n(neven) whose elements are expressed by means of characteristic functions (indicators) of (n/2)-dimensional vector-subspaces of (GF(2))n. An extended version of this class is introduced in the same paper; it is conjectured that this version is equal to the whole class of bent functions. In the present paper, we prove that this conjecture is true

Synchronization of Boolean Dynamical Systems: a Spectral Characterization

International audienceIn this paper a spectral characterization of the synchronization property of Boolean dynamical systems is provided. Conditions on the spectrum of the next-state function are derived for two systems coupled in a unidirectional way - also called master-slave configuration - to guarantee self-synchronization. Two kinds of self-synchronization are discussed: the statistical one and the finite one. Next, some conditions are stated for a specific input sequence to allow the system to be self-synchronizing. Some of the results are based on the notion of influence of variables, a notion that is extended to vectorial Boolean functions for the purpose of the paper. A potential application to cryptography is finally given

Security proof of the canonical form of self-synchronizing stream ciphers

International audienceThis paper studies the security level expected by the canon-ical form of the Self-Synchronizing Stream Cipher (SSSC). A SSSC can be viewed as the combination of a shift register together with a filtering function. The maximum security of such a cipher is reached when the filtering function is random. However, in practice, Pseudo Random Functions (PRF) are used as filtering functions. In this case, we show that the security against chosen ciphertext attacks (IND-CCA security) cannot be reached for the canonical form of the SSSC, but it is however secure against chosen plaintext attacks (IND-CPA secure). Then, a weaker property than pseudo-randomness is introduced in order to characterize the security of the canonical SSSC from its filtering function. A connection with the left-or-right indistinguishability (LOR-IND) is made. This property provides a necessary and sufficient condition to characterize the indistinguishablity of SSSC

Towards a spectral approach for the design of self-synchronizing stream ciphers

International audienceThis paper addresses the problem of characterizing the func- tions that can be used in the design of self-synchronizing stream ciphers. We propose a general framework based on a spectral characterization through correlation matrices or equivalently through Walsh matrices. Two modes of self-synchronization are discussed: the finite time one and the statistical one

Auguste Kerckhoffs et la cryptographie militaire

Kerckhoffs, néerlandais d’origine, français d’adoption, polyglotte, adepte du volapük (une de ces « langues universelles », comme l’esperanto, qui fait florès à la fin du xixe siècle, notamment en milieu d’ingénieurs) est une figure originale du tournant du siècle. Il introduit dans cet article « les principes de Kerckhoffs » – le second notamment fonde la cryptographie moderne : « Il faut que le système n’exige pas le secret, et qu’il puisse sans inconvénient tomber entre les mains de l’ennemi ». Paradoxalement en apparence, tout élément secret est un point de faille : le principe lui-même de chiffrement doit être connu de tous, ce sont ses modalités (comme les clefs) qui sont secrètes et qui peuvent être changées facilement en cas d’interception, sans que le principe lui-même soit changé. De nos jours, nos ordinateurs, logiciels et téléphones personnels font du chiffrement/ déchiffrement en permanence (par exemple pour le paiement sur Internet ou pour l’authentification GSM). Ils le font à partir de systèmes cryptographiques connus de tous, comme l’algorithme RSA (Rivest Shamir Adleman) ou PGP (Pretty Good Privacy). Ce sont des systèmes à clef (et parfois même à double clef, publique et privée), fondés sur les grands nombres premiers, qui répondent bien à l’impératif de Kerckhoffs : par exemple, dans l’algorithme RSA, lorsque la clef vient à être « cassée », il ne faut changer ni l’algorithme ni les logiciels embarqués, il suffit de trouver deux nombres premiers plus grands permettant de produire de nouvelles clefs

Influence of hydrogen and oxygen content on the mechanical behavior of zirconium between 275 and 325°C and titanium at 20°C

International audienceThe mechanical behaviors of α-Zr and α-Ti with various oxygen and hydrogen contents were investigated by loading/relaxation/unloading tensile tests and tests with strain rate jumps, at 300°C and 20°C, respectively. Solute oxygen and hydrogen atoms were found to have antagonist effects. While the former increases the yield stress and hardening exponent, reduces the activation volume and enhances static and dynamic strain aging phenomena, the latter significantly reduces these effects, as long as the O content is not too high

Self-synchronizing stream ciphers and dynamical systems: state of the art and open issues

Dynamical systems play a central role in the design of symmetric cryptosystems. Their use has been widely investigated both in "chaos-based" private communications and in stream ciphers over finite fields. In the former case, they get the form of automata named as Moore or Mealy machines. The main charateristic of stream ciphers lies in that they require synchronization of complex sequences generated by the dynamical systems involved at the transmitter and the receiver part. In this paper, we focus on a special class of symmetric ciphers, namely the SelfSynchronizing Stream Ciphers. Indeed, such ciphers have not been seriously explored so far although they get interesting properties of synchronization which could make them very appealing in practice. We review and compare different design approaches which have been proposed in the open literature and fully-specified algorithms are detailed for illustration purpose. Open issues related to the validation and the implementation of Self-Synchronizing Stream Ciphers are developped. We highlight the reason why some concepts borrowed from control theory appear to be useful to this end

Rio + 20 : un sommet délétère ? Les désillusions de la conférence des Nations unies sur le développement durable des 20-22 juin 2012

Les réactions « à chaud » à la publication de la déclaration finale de la Conférence des Nations unies sur le développement durable ont fait étalage de profondes désillusions et déceptions. Après avoir rappelé comment est apparu le concept de développement durable, présenté son interprétation par le sommet de la Terre, réuni à Rio de Janeiro en 1992, et interprété son évolution jusqu’au sommet Rio + 20, cet article analyse la déclaration finale qui confère aux principes juridiques sur lesquels s’appuie le développement durable le caractère de principes du droit international général et comporte quelques avancées. En revanche, la notion d’économie verte destinée à revitaliser le concept de développement durable a été vidée de sa substance et les progrès attendus en matière de gouvernance ne sont pas au rendez-vous car la déclaration revient à des schémas classiques de coopération intergouvernementale. Enfin, il traite de certaines omissions de la déclaration (protection du milieu marin, accès à l’eau, déplacés environnementaux).Deep disillusionment and disappointment were evident in initial reactions to the United Nations Conference on Sustainable Development’s Final Declaration. This article first sets out a summary of how the concept of sustainable development came into being, how it was interpreted by the 1992 Earth Summit in Rio de Janeiro, and an assessment of its evolution up to the Rio+20 Summit. This is followed by an analysis of the Final Declaration, which turns sustainable development’s legal principles into principles of general international law, and in which some steps forward can be found. Nevertheless, the idea of a green economy, which was supposed to revitalise the concept of sustainable development, was completely watered down, and expected improvements in governance were not achieved because the Declaration relies once more on standard forms of intergovernmental cooperation. Lastly, some areas not covered by the Declaration are examined (protection of the marine environment, access to water, people displaced for environmental reasons)