The block cipher companion Block ciphers encrypt blocks of plaintext messages into blocks of ciphertext under the action of a secret key and the process of encryption is reversed by decryption which uses the same user supplied key. Block ciphers are fundamental to modern cryptography in fact they are the most widely used cryptographic primitive useful in their own right and in the construction of other cryptographic mechanisms. In this book the authors provide a technically detailed yet readable account of the state of the art of block cipher analysis design and deployment. The authors first describe the most prominent block ciphers and give insights into their design. They then consider the role of the cryptanalyst the adversary and provide an overview of some of the most important cryptanalytic methods. The book will be of value to graduate and senior undergraduate students of cryptography and to professionals engaged in cryptographic design. An important feature of the presentation is the authors exhaustive bibliography of the field each chapter closing with comprehensive supporting notes. The first author is a professor in the Department of Mathematics and head of the Crypto Group at the Technical University of Denmark; his main interests are the analysis and design of block ciphers hash functions and message authentication codes. The second author is a researcher in the Cryptography Research Group of Orange (France Télécom); his main interests are the design and analysis of symmetric cryptographic primitives cryptographic techniques for constrained devices and the deployment of cryptographic solutions. Both authors have taken lead roles in the related professional associations and academic conferences.