Block Cipher - Definition

Definition

A block cipher consists of two paired algorithms, one for encryption, E, and the other for decryption, D. Both algorithms accept two inputs: an input block of size n bits and a key of size k bits; and both yield an n-bit output block. The decryption algorithm D is defined to be the inverse function of encryption, i.e., D = E−1. More formally, a block cipher is specified by an encryption function

which takes as input a key K of bit length k, called the key size, and a bit string P of length n, called the block size, and returns a string C of n bits. P is called the plaintext, and C is termed the ciphertext. For each K, the function E_K(P) is required to be an invertible mapping on {0,1}n. The inverse for E is defined as a function

taking a key K and a ciphertext C to return a plaintext value P, such that

For example, a block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. The exact transformation is controlled using a second input – the secret key. Decryption is similar: the decryption algorithm takes, in this example, a 128-bit block of ciphertext together with the secret key, and yields the original 128-bit block of plain text.

For each key K, E_K is a permutation (a bijective mapping) over the set of input blocks. Each key selects one permutation from the possible set of .