This repository provides a minimal, academic-oriented implementation of a Feistel block cipher in the C programming language.
Its purpose is strictly educational: to illustrate how Feistel networks operate mathematically and programmatically.

The repository contains:

• feistel.c - A compact 4-round Feistel cipher implementation in C
• index.html - A simple academic explanation of Feistel networks, including the mathematical formulation and diagram

A Feistel cipher divides the input block into two halves:

M = (L0, R0)

For each round (i), with subkey (K_i), the transformation is:

L_{i+1} = R_i
R_{i+1} = L_i XOR F(R_i, K_i)

This process:

• Does not require the round function (F) to be invertible
• Ensures that encryption and decryption share the same structure
• Allows decryption simply by reversing key order

A full mathematical walkthrough and diagram are available in index.html.

The round function may be any transformation, such as:

• XOR operations
• Modular addition
• S-box substitutions
• Bit permutations

In this educational implementation, the round function is:

F(r, k) = r ^ k;