Ph.D. Defence Vesselin Velichkov - Recent Methods for Cryptanalysis of Symmetric-key Cryptographic Algorithms
Start date: 15/02/2012
Location: Auditorium Kasteel Arenberg
In order for cryptography to serve its purposes well, secure and reliable cryptographic algorithms are necessary. The design of such algorithms is intimately linked to the ability to analyze and understand their properties. The latter is the subject of study of cryptanalysis. The goal of this thesis is to study new techniques for cryptanalysis of symmetric-key cryptographic algorithms. These are algorithms that use the same key for both encryption and decryption. Two recent techniques are researched in particular: the differential analysis of ARX-based cryptographic algorithms and algebraic cryptanalysis.
ARX algorithms are based on the operations modular addition, bit rotation and eXclusive-OR (ARX). Although many contemporary algorithms fall into this class, their cryptographic properties are not well understood. In the first part of the thesis we address this problem by investigating the properties of ARX with respect to one of the most powerful cryptanalytic techniques - differential cryptanalysis. We propose a general framework for the differential analysis of ARX and apply it to the analysis of the stream cipher Salsa20.
The second part of the thesis is dedicated to algebraic cryptanalysis. In algebraic cryptanalysis a cipher is represented as a system of algebraic equations with unknowns - the bits of the secret key. Solving this system is equivalent to recovering the secret key of the cipher. We focus our analysis on equation systems arising from algorithms based on the most widely used cipher today - the Advanced Encryption standard (AES). We describe a tool that automatically generates equation systems for AES and we apply it to the algebraic cryptanalysis of the AES-based stream cipher LEX.