Practical Internet of Things Security - Second Edition

Counter modes

Encryption does not have to be performed on complete blocks of data, however; some modes make use of a counter such as Counter Mode (CTR) and Galois Counter Mode (GCM). In these, the plaintext data is not actually encrypted with the cipher and key, not directly anyway. Rather, each bit of plaintext is XOR'd with a stream of continuously produced ciphertext, consisting of encrypted counter values that continuously increment. In this mode, the initial counter value is the IV. It is encrypted by the cipher (using a key), providing a block of ciphertext. This block of ciphertext is XOR'd with the block (or partial block) of plaintext to be encrypted. CTR mode is frequently used in wireless communications because bit errors that ...