Feistel cipher

Many modern symmetric block ciphers are based on Feistel networks. Feistel networks were first seen commercially in IBM's Lucifer cipher, designed by Horst Feistel and Don Coppersmith in 1973. Feistel networks gained respectability when the U.S. Federal Government adopted the DES (a cipher based on Lucifer, with changes made by the NSA) in 1976. Like other components of the DES, the iterative nature of the Feistel construction makes implementing the cryptosystem in hardware easier (particularly on the hardware available at the time of DES's design).

A Feistel network uses a round function, a function which takes two inputs – a data block and a subkey – and returns one output of the same size as the data block.[1] In each round, the round function is run on half of the data to be encrypted, and its output is XORed with the other half of the data. This is repeated a fixed number of times, and the final output is the encrypted data. An important advantage of Feistel networks compared to other cipher designs such as substitution–permutation networks is that the entire operation is guaranteed to be invertible (that is, encrypted data can be decrypted), even if the round function is not itself invertible. The round function can be made arbitrarily complicated, since it does not need to be designed to be invertible.[2]^: 465 [3]^: 347 Furthermore, the encryption and decryption operations are very similar, even identical in some cases, requiring only a reversal of the key schedule. Therefore, the size of the code or circuitry required to implement such a cipher is nearly halved.

The structure and properties of Feistel ciphers have been extensively analyzed by cryptographers.

Michael Luby and Charles Rackoff analyzed the Feistel cipher construction and proved that if the round function is a cryptographically secure pseudorandom function, with K_i used as the seed, then 3 rounds are sufficient to make the block cipher a pseudorandom permutation, while 4 rounds are sufficient to make it a "strong" pseudorandom permutation (which means that it remains pseudorandom even to an adversary who gets oracle access to its inverse permutation).[4] Because of this very important result of Luby and Rackoff, Feistel ciphers are sometimes called Luby–Rackoff block ciphers.

Further theoretical work has generalized the construction somewhat and given more precise bounds for security.[5][6]

Let ${\displaystyle \mathrm {F} }$ be the round function and let ${\displaystyle K_{0},K_{1},\ldots ,K_{n}}$ be the sub-keys for the rounds ${\displaystyle 0,1,\ldots ,n}$ respectively.

Then the basic operation is as follows:

Split the plaintext block into two equal pieces: (${\displaystyle L_{0}}$, ${\displaystyle R_{0}}$).

For each round ${\displaystyle i=0,1,\dots ,n}$, compute

${\displaystyle L_{i+1}=R_{i},}$

${\displaystyle R_{i+1}=L_{i}\oplus \mathrm {F} (R_{i},K_{i}),}$

where ${\displaystyle \oplus }$ means XOR. Then the ciphertext is ${\displaystyle (R_{n+1},L_{n+1})}$.

Decryption of a ciphertext ${\displaystyle (R_{n+1},L_{n+1})}$ is accomplished by computing for ${\displaystyle i=n,n-1,\ldots ,0}$

${\displaystyle R_{i}=L_{i+1},}$

${\displaystyle L_{i}=R_{i+1}\oplus \operatorname {F} (L_{i+1},K_{i}).}$

Then ${\displaystyle (L_{0},R_{0})}$ is the plaintext again.

The diagram illustrates both encryption and decryption. Note the reversal of the subkey order for decryption; this is the only difference between encryption and decryption.

Feistel or modified Feistel:

Generalised Feistel:

• CAST-256
• CLEFIA
• MacGuffin
• RC2
• RC6
• Skipjack
• SMS4

• Cryptography
• Stream cipher
• Substitution–permutation network
• Lifting scheme for discrete wavelet transform has pretty much the same structure
• Format-preserving encryption
• Lai–Massey scheme