The Block Cipher Companion - Hardcoverby Lars R. Knudsen (Author), Matthew Robshaw (Author) 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