Modes of Operation: CBC Mode

Cipher block chaining mode

In cipher block chaining (CBC) mode, each ciphertext block as well as being sent to the receiver serves as an input into the encryption process of the subsequent plaintext block.

Encryption using CBC mode

CBC mode encryption is illustrated in the illustration below, where $E$ represents the encryption algorithm of the block cipher. We’ll assume in our discussion that $E$ has a block size of 128 bits (however, it could be any value). The two rows of boxes represent temporary registers of 128 bits, which will hold intermediate values needed to perform the encryption process. The contents of these registers will change throughout the encryption process.