Block Cipher Principles on Cryptography
Mayuri Khatri, Nitin Shukla · 2012
Abstract— The history of cryptography is long and goes back at least 4,000 years to the Egyptians, who used hieroglyphic codes for inscription on tombs. Since then many cryptosystems, also called ciphers, have been developed and used. Many of these old ciphers are much too weak to be used in applications today, because of the tremendous progress in computer technology. There are essentially two types of encryption schemes, one-key and two-key ciphers. In one-key ciphers the encryption of a plaintext and the decryption of the corresponding ciphertext are performed using the same key. Until 1976 when Diffie and Hellman introduced public-key or two-key cryptography all ciphers were one-key systems. Therefore one-key ciphers are also called conventional cryptosystems. Conventional cryptosystems are widely used throughout the world today, and new systems are published from time to time. There are two kinds of one-key ciphers, stream ciphers and block ciphers. In stream ciphers a long sequence of bits is generated from a short string of key bits, and is then added bitwise modulo 2 to the plaintext to produce the ciphertext. In block ciphers the plaintext is divided into blocks of a fixed length, which are then encrypted into blocks of ciphertexts using the same key. Block ciphers can be divided into three groups: Substitution ciphers, transposition ciphers and product ciphers. In the following a few examples of the different types of block ciphers are given. Notation: Let AM and AC be the alphabets for plaintexts and ciphertexts, respectively. Let M = m0,m1, . . . ,mn−1 be an n-character plaintext, s.t. for every i, mi ∈ AM and let C = c0, c1, . . . , cn−1 be a ciphertext, s.t. for every i, ci ∈ AC. We assume that an alphabet AX is isomorphic with INAX... Fig 1: State (with Nb = 6) and encryption key (with Nk =4)