Quantitative Security of Block Ciphers: Designs and Cryptanalysis Tools
Thomas Baignères · 2008
Abstract Block ciphers probably figure in the list of the most important cryptographicprimitives. Although they are used for many different purposes, their essential goal isto ensure confidentiality. This thesis is concerned by their quantitative security , that is,by measurable attributes that reflect their ability to guarantee this confidentiality.The first part of this thesis deals with well know results. Starting with Shan-non’s Theory of Secrecy, we move to practical implications for block ciphers, recall themain schemes on which nowadays block ciphers are based, and introduce the Luby-Rackoff security model. We describe distinguishing attacks and key-recovery attacksagainst block ciphers and show how to turn the firsts into the seconds. As an illustration,we recall linear cryptanalysis which is a classical example of statistical cryptanalysis.In the second part, we consider the (in)security of block ciphers against sta-tistical cryptanalytic attacks and develop some tools to perform optimal attacks andquantify their efficiency. We start with a simple setting in which the adversary hasto distinguish between two sources of randomness and show how an optimal strategycan be derived in certain cases. We proceed with the practical situation where thecardinality of the sample space is too large for the optimal strategy to be implementedand show how this naturally leads to the concept of