⚙️ DES Implementation in C

A from-scratch implementation of the Data Encryption Standard (DES) algorithm in plain C. This project was built for educational purposes to understand the internal workings of a classic block cipher, including its Feistel network, S-Boxes, and key schedule.

Warning: This implementation is for educational use only. DES is insecure due to its small 56-bit key size and should never be used for modern, secure applications. Use algorithms like AES instead.

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🚀 Features

This project implements the complete DES algorithm as specified in FIPS 46-3:

• Full Encryption & Decryption: Implements the core 64-bit block cipher.
• Feistel Network: A 16-round Feistel structure.
• Feistel Function (F-function):
  • E-Box (Expansion)
  • S-Boxes (Substitution - the non-linear core)
  • P-Box (Permutation)
• Key Schedule: Generates all 16 48-bit subkeys from a 64-bit master key using PC-1, PC-2, and the specified rotational shifts.
• Permutations: Includes the Initial Permutation (IP) and Final Permutation (IP⁻¹).
• Validated: Passes a full suite of standard DES test vectors (including weak and semi-weak keys) to ensure correctness.

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📂 File Structure

The project is organized into modular files for clarity:

• des.h: The main header file with function prototypes and type definitions.
• des_tables.h: Header file declaring all the standard DES permutation and substitution tables.
• des_tables.c: Implementation file containing the static data for all tables (IP, S-Boxes, PC-1, etc.).
• des_core.c: The core logic for encryption, decryption, the F-function, and the key schedule.
• main.c: A test driver that runs the implementation against a set of known test vectors to verify correctness.

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🛠️ How to Build and Run

This project is written in standard C and can be compiled with gcc or clang.

1. Compilation

Navigate to the project directory and run the following command to compile all source files into a single executable named des_test:

gcc main.c des_core.c des_tables.c -o des_test -W -Wall

2. Running Tests

The compiled main.c acts as a test bench. It will run several standard test vectors against the cipher and report on success or failure.

To run the tests, simply execute the compiled file:

./des_test

3. Expected Output

If the implementation is correct, you will see a success message for all test vectors:

===== Test Implementazione DES =====
Chiave     : 0x133457799BBCDFF1
Plaintext  : 0x0123456789ABCDEF
Cipher Atteso: 0x85E813540F0AB405
--------------------------------------
1. Sotto-chiavi generate.
2. Blocco cifrato.
Cipher Ottenuto: 0x85E813540F0AB405
>>> SUCCESSO: La cifratura è corretta!
--------------------------------------
3. Blocco decifrato.
Plaintext Ottenuto: 0x0123456789ABCDEF
>>> SUCCESSO: La decifratura è corretta! (Roundtrip riuscito)
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