Encryption and Security of Counter Mode
Bart Mennink · 2023
This chapter considers the ways to encipher messages of arbitrary length using a block cipher. These ways are called modes of use, modes of operation or modes in short. These modes are subdivided into two types depending on how the plaintext is processed: block encryption and stream encryption. The chapter argues about security of modes. It explains a degenerated version of Electronic Codebook (ECB) that can only encrypt messages that have as length a multiple of the block length. Cipher block chaining (CBC) encryption has the disadvantage that it is inherently sequential. As ECB, CBC suffers from message expansion: due to padding, the ciphertext is longer than the plaintext. For encryption, we can use block ciphers to build secure stream ciphers. These are stream cipher modes of use. The chapter discusses two of them: output feedback mode and counter mode.