Feistel cipher

Author: Leandro Alegsa Creation date: November 13, 2022

In cryptography, a Feistel cipher is a symmetric structure used in the construction of block ciphers, named after the German IBM cryptographer Horst Feistel; it is also commonly known as a Feistel network. A large set of block ciphers use the scheme, including the Data Encryption Standard

The Feistel structure has the advantage that 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.

Feistel construction is iterative in nature which makes implementing the cryptosystem in hardware easier.

Feistel networks and similar constructions are product ciphers, and so combine multiple rounds of repeated operations, such as:

Bit-shuffling (often called permutation boxes or P-boxes)
Simple non-linear functions (often called substitution boxes or S-boxes)
Linear mixing (in the sense of modular algebra) using XOR to produce a function with large amounts of what Claude Shannon described as "confusion and diffusion".

Bit shuffling creates the diffusion effect, while substitution is used for confusion.

Questions and Answers

Q: What is a Feistel cipher?

A: A Feistel cipher is a symmetric structure used in the construction of block ciphers, named after the German IBM cryptographer Horst Feistel. It is also commonly known as a Feistel network.

Q: What are some advantages of using a Feistel structure?

A: The main advantage of using a Feistel structure is that encryption and decryption operations are very similar, even identical in some cases, requiring only a reversal of the key schedule. This reduces the size of code or circuitry required to implement such a cipher by nearly half. Additionally, it's iterative nature makes implementing the cryptosystem in hardware easier.

Q: How does Claude Shannon describe "confusion and diffusion"?

A: Claude Shannon described "confusion and diffusion" as having large amounts of both elements present in order to make it difficult for an attacker to decipher an encrypted message.

Q: What techniques are used to create confusion and diffusion?

A: Confusion and diffusion are created through bit shuffling (often called permutation boxes or P-boxes) and simple non-linear functions (often called substitution boxes or S-boxes), as well as linear mixing (in the sense of modular algebra) using XOR. Bit shuffling creates the diffusion effect, while substitution is used for confusion.

Q: What type of cipher is a Feistel network?

A: A Feistel network is a type of product cipher which combines multiple rounds of repeated operations in order to encrypt data securely.

Q: Who developed this type of cryptography?

A: The Feistel structure was developed by German IBM cryptographer Horst Feistel.

Q: Is Data Encryption Standard based on this type of cryptography?

A: Yes, Data Encryption Standard uses this type of cryptography which utilizes the same principles outlined above for creating confusion and diffusion within an encrypted message.