Network Security: Private Communications in a Public World, 3rd Edition

4 Modes of Operation

4.1 Introduction

We’ve covered how to encrypt a 128-bit block with AES, or a 64-bit block with DES, and these primitives have the nice property that if an attacker changes any part of the ciphertext, the result of a decryption will be effectively a random number. Sadly, most useful messages are longer than 128 bits. Modes of operation are techniques for encrypting arbitrary-sized messages using the block encryption algorithms as primitives to be applied iteratively. There are additional desirable properties that such algorithms can have. If we send the same message multiple times, it would be desirable to have the encrypted message be different each time so that an eavesdropper can’t tell that we’re sending the same message ...