International Association for Cryptologic Research

International Association
for Cryptologic Research


Xianhui Lu


Differential-Linear Cryptanalysis from an Algebraic Perspective 📺
The differential-linear cryptanalysis is an important cryptanalytic tool in cryptography, and has been extensively researched since its discovery by Langford and Hellman in 1994. There are nevertheless very few methods to study the middle part where the differential and linear trail connect, besides the Differential-Linear Connectivity Table (Bar-On et al., EUROCRYPT 2019) and the experimental approach. In this paper, we study differential-linear cryptanalysis from an algebraic perspective. We first introduce a technique called Differential Algebraic Transitional Form (DATF) for differential-linear cryptanalysis, then develop a new theory of estimation of the differential-linear bias and techniques for key recovery in differential-linear cryptanalysis. The techniques are applied to the CAESAR finalist ASCON, the AES finalist SERPENT, and the eSTREAM finalist Grain v1. The bias of the differential-linear approximation is estimated for ASCON and SERPENT. The theoretical estimates of the bias are more accurate than that obtained by the DLCT, and the techniques can be applied with more rounds. Our general techniques can also be used to estimate the bias of Grain v1 in differential cryptanalysis, and have a markedly better performance than the Differential Engine tool tailor-made for the cipher. The improved key recovery attacks on round-reduced variants of these ciphers are then proposed. To the best of our knowledge, they are thus far the best known cryptanalysis of SERPENT, as well as the best differential-linear cryptanalysis of ASCON and the best initialization analysis of Grain v1. The results have been fully verified by experiments. Notably, security analysis of SERPENT is one of the most important applications of differential-linear cryptanalysis in the last two decades. The results in this paper update the differential-linear cryptanalysis of SERPENT-128 and SERPENT-256 with one more round after the work of Biham, Dunkelman and Keller in 2003.
Understanding and Constructing AKE via Double-Key Key Encapsulation Mechanism
Motivated by abstracting the common idea behind several implicitly authenticated key exchange (AKE) protocols, we introduce a primitive that we call double-key key encapsulation mechanism (2-key KEM). It is a special type of KEM involving two pairs of secret-public keys and satisfying some function and security property. Such 2-key KEM serves as the core building block and provides alternative approaches to simplify the constructions of AKE. To see the usefulness of 2-key KEM, we show how several existing constructions of AKE can be captured as 2-key KEM and understood in a unified framework, including widely used HMQV, NAXOS, Okamoto-AKE, and FSXY12-13 schemes. Then, we show (1) how to construct 2-key KEM from concrete assumptions, (2) how to adapt the classical Fujisaki-Okamoto transformation and KEM combiner to achieve the security requirement of 2-key KEM, (3) an elegant Kyber-AKE over lattice using the improved Fujisaki-Okamoto technique.