A simple C program that implements a custom encryption algorithm for text files based on the Feistel network structure.

This program provides basic file encryption and decryption functionality using a simplified Feistel network cipher. It processes input files byte-by-byte, converting each character using a non-linear function and rounds of encryption.

IMPORTANT: This implementation is a proof-of-concept only and should NOT be used for securing sensitive information. It lacks many essential security features of modern cryptographic systems including:

• Insufficient key length and key management
• Limited block size (only 16 bits)
• Only a single round of encryption
• No initialization vectors or padding
• Vulnerable to numerous cryptographic attacks

This program is intended purely for educational purposes to demonstrate the basic concepts of a Feistel network.

• Encrypt text files to binary format
• Decrypt previously encrypted files
• Command-line interface with simple parameters
• Demonstration of basic Feistel network principles

Compile the program:

gcc -o cipher cipher.c

./cipher encrypt

This reads from sample.txt and creates an encrypted cipher.dat file.

./cipher decrypt

This reads from cipher.dat and creates a decrypted decrypted.txt file.

This program implements a simplified version of a Feistel network, a structure used in many block ciphers including DES. Key characteristics of Feistel networks:

• Data is split into two halves (left and right)
• The same algorithm is used for both encryption and decryption
• Multiple rounds are typically used to increase security (though this implementation uses only one)
• The encryption function only needs to be one-way, not invertible

1. The program uses a simplified Feistel structure with:

   • A 16-bit block size (split into two 5-bit halves)
   • A custom non-linear function (non_linear())
   • A single round of encryption/decryption

2. The encryption process (fn_round()):

   • Splits each 16-bit value into left (5 bits) and right (5 bits) halves
   • Applies the non-linear function to the right half
   • XORs the result with the left half to create the new right half
   • Uses the original right half as the new left half

3. The decryption process (fn_deround()) reverses these operations

• By default, only handles ASCII content correctly
• When processing UTF-8 text, each byte is encrypted separately rather than whole characters
• Uses only 16 bits for encryption, providing minimal security
• Implements only a single round (production Feistel ciphers use many rounds)

• Feistel Network - Wikipedia
• The Feistel Cipher - Cryptography Tutorial
• Understanding Block Cipher Construction - Bruce Schneier