Generic Framework for Key-Guessing Improvements

International audienceWe propose a general technique to improve the key-guessing step of several attacks on block ciphers. This is achieved by defining and studying some new properties of the associated S-boxes and by representing them as a special type of decision trees that are crucial for finding fine-grained guessing strategies for various attack vectors. We have proposed and implemented the algorithm that efficiently finds such trees, and use it for providing several applications of this approach, which include the best known attacks on Noekeon, GIFT, and RECTANGLE

New Attacks from Old Distinguishers Improved Attacks on Serpent

International audienceSerpent was originally proposed in 1998 and is one of the most studied block ciphers. In this paper we improve knowledge of its security by providing the current best attack on this cipher, which is a 12-round differential-linear attack with lower data, time and memory complexities than the best previous attacks. Our improvements are based on an improved conditional key guessing technique that exploits the properties of the Sboxes

Further Improving Differential-Linear Attacks: Applications to Chaskey and Serpent

Differential-linear attacks are a cryptanalysis family that has recently benefited from various technical improvements, mainly in the context of ARX constructions. In this paper we push further this refinement, proposing several new improvements. In particular, we develop a better understanding of the related correlations, improve upon the statistics by using the LLR, and finally use ideas from conditional differentials for finding many right pairs. We illustrate the usefulness of these ideas by presenting the first 7.5-round attack on Chaskey. Finally, we present a new competitive attack on 12 rounds of Serpent, and as such the first cryptanalytic progress on Serpent in 10 years

Improved Differential-Linear Attacks with Applications to ARX Ciphers

International audienceWe present several improvements to the framework of differential-linear attacks with a special focus on ARX ciphers. As a demonstration of their impact, we apply them to Chaskey and ChaCha and we are able to significantly improve upon the best attacks published so far

Key guessing strategies for Sbox-based ciphers

In dieser Dissertation untersuchen wir Key-Recovery-Methoden (d.h. Methoden zum Finden eines geheimen Schlüssels) für Schlüssel-alternierende Blockchiffren. Der grobe Aufbau von Angriffen auf Blockchiffren stellt sich, wie folgt, dar: Ein Angreifer erhält oder generiert Daten, rät Teile des Schlüsselmaterials, benutzt diese, um aus den Daten Zustände innerhalb der Chiffre zu berechnen, und wendet einen Unterscheider an. Man nimmt dann an, dass sich die Zustände zufällig verhalten, wenn der Schlüssel falsch geraten wurde. So kann man anhand dieser Zustände den richtigen vom falschen Schlüssel unterscheiden. In dieser Arbeit behandeln wir folgende Themen: 1. Die Darstellung von Key-Recovery-Methoden mithilfe von Entscheidungsbäumen sowie daraus resultierende Verbesserungen bestehender Angriffe auf Blockchiffren. 2. Die Grenzen dieser Betrachtungsweise im Kontext differenziell-linearer Angriffe. 3. Die mathematischen Eigenschaften von Entscheidungsbäumen in diesem Kontext