Linear Structures in Blockciphers

A blockcipher maps each pair of plaintext and key onto a ciphertext in such a way that for every fixed key, the relationship between plaintexts and ciphertexts is one-to-one. It is assumed that plaintexts and ciphertexts belong to a message space comprising all bit-strings (sequences of zeros and ones) of a given length; keys are taken from a key space made up of aU bitstrings of a possibly Merent given length. A well-known blockcipher is the NBS Data Encryption Standard (DES) [6], whch is the iteration of sixteen essentially equal “rounds”.