Chapter 4
Block Cipher Modes

Block ciphers encrypt only fixed-size blocks. If you want to encrypt something that isn't exactly one block long, you have to use a block cipher mode. That's another name for an encryption function built using a block cipher.

Before proceeding with this chapter, we have one word of warning. The encryption modes that we talk about in this chapter prevent an eavesdropper from reading the traffic. They do not provide any authentication, so an attacker can still change the message—sometimes in any way she wants. Many people find this surprising, but this is simple to see. The decryption function of an encryption mode simply decrypts the data. It might produce nonsense, but it still decrypts a (modified) ciphertext to some (modified and possibly nonsensical) plaintext. You should not rely on the fact that nonsensical messages do no harm. That involves relying on other parts of the system, which all too often leads to grief. Furthermore, for some...