We have already seen several ciphers that operate on text that has been divided into blocks of characters. Some operate on small blocks of just 2 or 3 characters, such as Playfair, Two Square, Three Square and Four Square. Some operate on longer blocks, but change only 2 or 3 characters at a time, such as bifid, trifid or FR-Actionated Morse. These ciphers act locally, on just one portion of each block. A change in one character of the plaintext typically changes at most 2 or 3 characters of the ciphertext.
This chapter deals with much stronger types of block ciphers. In these ciphers, changing even a single bit of the plaintext, or a single bit of the key, will change roughly half of the bits of the ciphertext, and nearly all of the bytes of the ciphertext. This indicates that the cipher is highly non-linear (see section 12.3). These ciphers are intended only for computer use, often with special-purpose hardware to speed the encryption.