Attaque par clé apparentée

Résumé

Une attaque par clé apparentée est une forme de cryptanalyse où l'adversaire peut observer les opérations d'un algorithme de chiffrement lorsqu'il est utilisé avec différentes clés, aux valeurs inconnues, mais qui sont liées entre elles par des propriétés mathématiques connues de l'attaquant. Par exemple, une telle propriété pourrait être une séquence comme 1100 qui apparaît dans chaque clé au même endroit, alors que le reste des bits composants la clé sont inconnus. Cela peut sembler être une hypothèse purement théorique : il est improbable d'arriver à persuader un opérateur de chiffrer ses messages avec des clés soumises à des contraintes. Toutefois, un tel scénario peut se produire dans des protocoles faisant appel à des primitives cryptographiques, et où l'intervention d'une tierce personne est limitée. La complexité de ces protocoles, et leur conception parfois faible du point de vue cryptographique, peuvent induire des failles susceptibles d'être exploitées par une attaque par c

Source officielle

https://fr.wikipedia.org/wiki/Attaque_par_cl%C3%A9_apparent%C3%A9e

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Chargement

In this paper, we present improved related-key attacks on the original DESX, and DESX+, a variant of the DESX with its pre- and post-whitening XOR operations replaced with addition modulo

2^{64}

. Compared to previous results, our attack on DESX has reduced text complexity, while our best attack on DESX+ eliminates the memory requirements at the same processing complexity.

2008

Chargement

Skipjack is a block cipher designed by the NSA for use in US government phones, and commercial mobile and wireless products by ATT. Among its initial implementations in hardware were the Clipper chip and Fortezza PC cards, which have since influenced the private communications market to be compatible with this technology. For instance, the Fortezza card comes in PCMCIA interface and is a very easy plug-n-play device to add on to mobile and wireless systems to provide encryption for wireless transmissions. Initially classified when it was first proposed, Skipjack was declassified in 1998, and it sparked numerous security analyses from security researchers worldwide because it provides insight into the state-of-the-art security design techniques used by a highly secretive government intelligence agency such as the NSA. In this paper, commemorating a decade since Skipjack's public revelation, we revisit the security of Skipjack, in particular its resistance to advanced differential-style distinguishers. In contrast to previous work that considered conventional and impossible differential distinguishers, we concentrate our attention on the more recent advanced differential-style and related-key distinguishers that were most likely not considered in the original design objectives of the NSA. In particular, we construct first-known related-key impossible differential, rectangle and related-key rectangle distinguishers of Skipjack. Our related-key attacks (i.e., related-key miss-in-the-middle and related-key rectangle attacks) are better than all the previous related-key attacks on Skipjack. Finally, we characterize the strength of Skipjack against all these attacks and motivate reasons why, influenced by the Skipjack structure, some attacks fare better. What is intriguing about Skipjack is its simple key schedule and a structure that is a cross between conventional Feistel design principles and the unconventional use of different round types. This work complements past results on the security analysis of Skipjack and is hoped to provide further insight into the security of an NSA-designed block cipher; the only one publicly known to date.

2009

Chargement

Bounded Tamper Resilience: How to go beyond the Algebraic Barrier

Sebastian Faust

Related key attacks (RKAs) are powerful cryptanalytic attacks where an adversary can change the secret key and observe the effect of such changes at the output. The state of the art in RKA security protects against an a-priori unbounded number of certain algebraic induced key relations, e.g., affine functions or polynomials of bounded degree. In this work, we show that it is possible to go beyond the algebraic barrier and achieve security against arbitrary key relations, by restricting the number of tampering queries the adversary is allowed to ask for. The latter restriction is necessary in case of arbitrary key relations, as otherwise a generic attack of Gennaro et al. (TCC 2004) shows how to recover the key of almost any cryptographic primitive. We describe our contributions in more detail below. 1. We show that standard ID and signature schemes constructed from a large class of -protocols (including the Okamoto scheme, for instance) are secure even if the adversary can arbitrarily tamper with the prover's state a bounded number of times and obtain some bounded amount of leakage. Interestingly, for the Okamoto scheme we can allow also independent tampering with the public parameters. 2. We show a bounded tamper and leakage resilient CCA secure public key cryptosystem based on the DDH assumption. We first define a weaker CPA-like security notion that we can instantiate based on DDH, and then we give a general compiler that yields CCA-security with tamper and leakage resilience. This requires a public tamperproof common reference string. 3. Finally, we explain how to boost bounded tampering and leakage resilience (as in 1. and 2. above) to continuous tampering and leakage resilience, in the so-called floppy model where each user has a personal hardware token (containing leak- and tamper-free information) which can be used to refresh the secret key. We believe that bounded tampering is a meaningful and interesting alternative to avoid known impossibility results and can provide important insights into the security of existing standard cryptographic schemes.

2013

Chargement

In this paper, we present improved related-key attacks on the original DESX, and DESX+, a variant of the DESX with its pre- and post-whitening XOR operations replaced with addition modulo

2^{64}

. Compared to previous results, our attack on DESX has reduced text complexity, while our best attack on DESX+ eliminates the memory requirements at the same processing complexity.

2008

2009

Bounded Tamper Resilience: How to go beyond the Algebraic Barrier

Sebastian Faust

2013