**Definition 4**

CBC Mode.

1. Encryption.

Input: *m* -bit key *k* ; *l* -bit IV; *l* -bit plaintext blocks

Output: such that and for

2. Decryption.

Input: *m* -bit key *k* ; *l* -bit IV; *l* -bit ciphertext blocks

Output: such that and for

We will commonly use the notation to denote an encrypted block in a cipher block chain,
where *x* is the previous block of cipher text, *y* is the current
block of plaintext, and *k* is the key.
Likewise, we will use to denote the result
of decrypting a block in a cipher block chain, where *x* is the
previous block of cipher text and *y* is the current block of
cipher text.