**Definition 1**

An *l* -bit block encryption is a function , such that for each *m* -bit key and block , has an invertible mapping. The
inverse mapping is block decryption, denoted .

**Definition 2**

A plaintext-ciphertext pair is an ordered 2-tuple

**Definition 3**

A plaintext-ciphertext pair under key, *k* ,
is an ordered 3-tuple

We will use the term *pair* to refer to a plaintext-ciphertext pair.