Feistel Cipher Structure

Feistel Cipher Structure, invented by Horst Feistel, is the most common basic #cipher structure adopted by various schemes.

First, it starts its cipher process halving input blocks, which then perform substitution# on left data half based on round function with right halve and subkey as parameters. After that, it performs a permutation# by swapping the halves. The process is then repeated for multiple rounds. The process is shown as formula below:

$$ \begin{aligned} P &= (L_0, R_0)\\ L_i &= R_{i-1}\\ R_i &= L_{i-1} \oplus F(R_{i-1}, K_i)\\ C &= (L_n, R_n) \end{aligned} $$

Where:

$P$ is the plaintext
$L$ indicates the left halve
$R$ indicates teh right halve
$i = 1, 2, \ldots, n$
$n$ is the number of rounds
$K$ is the subkey
$F$ is the round function used

Note: Remember that the after the round function is performed on the left, the right will swap with the left. This means that the right becomes the new left, and the rounded left becomes the new right.

To decrypt the encrypted (by Feistel Cipher) message, simply put the subkeys in reverse order using the same algorithm.

When designing a Feistel Cipher, several elements need to be considered such as block size, key size, number of rounds, subkey generation algorithm, round function, fast software encryption and/or decryption, and ease of analysis.

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Triple Data Encryption Standard (3DES)

3DES is one of the implementations of #Data Encryption Standard (DES). It applies DES algorithm three times, which is also 48 rounds, to each block using 2 or 3 keys, each 56 bits, with a total size of 112 or 168 bits, on 64-bit data in order to encrypt# the messages. However, this comes as a cost of performance. It is standardised in ANSI X9.17 and ISO 8732.

Note: However, due to the nature of Feistel Cipher Structure#, there is no significance differences between encryption and decryption other than order of the subkeys. Decryption algorithm is using the reverse order of the subkeys in encryption algorithm.
TNS3131 Chapter 2: Conventional Encryption and Message Confidentiality

Feistel Cipher Structure
TAC3121 Chapter 5: Symmetric Cryptography

Feistel Cipher Structure
RC5

RC5 is a fast and simple #Block Cipher, invented by Ron Rivest, utilises data-dependent rotations that could have a variable number of rounds of #round funnction (up to 255 rounds) and variable-length key (up to 2048 bits). It can be used to encrypt a 64-bit large data block. It is suited for both software and hardware encryption. Moreover, it can adapt to processors of different word lengths with low memory requirement and provide high security guarantee.
Meet-In-The-Middle (MITM)

It utilises the Feistel Cipher Structure# characteristic where ciphertext could be decrypted as the same way which plaintext is encrypted, only with a reverse order of subkeys.
Lucifer

Lucifer is an earlier version of Data Encryption Standard (DES)# developed by Feistel#.
International Data Encryption Algorithm (IDEA)

IDEA is a #Block Cipher that uses 128-bit key on 64 bits of data and performs 8 rounds of round function# for encryption. It is one of the underlying algorithms used by Pretty Good Privacy (PGP)#.
Double Data Encryption Standard (2DES)

2DES is one of the implementation of #Data Encryption Standard (DES). It does two times DES algorithm on each block to encrypt# the message. Although it is an improvement to DES, it is still vulnerable to meet-in-the-middle (MITM) attack#.
Differential Cryptanalysis

Differential Cryptanalysis is an #Analytic Attacks against Feistel Cipher Structure# by comparing the input (plaintext) and output (ciphertext) differences. It is focus on the non-linearity of the cipher especially the S-boxes.
Data Encryption Standard (DES)

DES is an encryption standard that adheres to #Symmetric Cryptography. It utilises Feistel Cipher Structure#, by doing 16 rounds of encrypting, where the underlying encryption method also exhibits Avalanche Effect# (34 bits and 35 bits difference for the plaintext and the key). Both 64-bit plaintext and the 64-bit key will undergo permutation before going through the Feistel cipher structure where only 56 bits of the key selected. The key will be left circular shifted in order to produce subkeys (in 48 bits), which will be done 16 rounds like the main encryption process. After 16 rounds, a 32-bit swap follows by an inverse initial permutation will produce a 64-bit ciphertext.

For decrypting, see Feistel Cipher Structure# in detail. In short, just encrypt the ciphertext with unwind subkeys order.
Blowfish

Blowfish is a #Block Cipher that has high execution speed and runs in less than 5K of memory. It could have a key size up to 448 bits, and used to encrypt a 64-bit data block. Blowfish will perform 16 rounds of round function#. It is comparatively easier to implement compare to other cryptographic #algorithm.
Block Cipher

Most Block Ciphers are based on Feistel Cipher Structure. And instead of encrypting the entire block which could be costly, the encrypted block is assembled from smaller one utilising Product Cipher.
Advanced Encryption Standard (AES)

AES is one of the implementations of #Data Encryption Standard (DES) invented by Joan Daemen and Vincent Rijmen, which was once called Rijndael. It could use either 128-, 192-, or 256-bit keys to encrypt 128 bit of data with an iterative manner (operates on entire block every round) rather than adopting Feistel Cipher Structure#. There were three initial criteria for AES evaluation: security, computational cost, and algorithm and implementation characteristics. Final criteria for AES evaluation were: general security, software and hardware implementation ease, implementation attacks (practical attacks on the actual implementation), and flexibility (in terms of encryption, decryption, keying etc.).

#cryptography