RC4 Encryption Implementation in C

This project contains a simple implementation of the RC4 (Rivest Cipher 4) encryption algorithm written in C. The code includes both a header file (arcfour.h) and a source file (arcfour.c) for encrypting and decrypting data using the RC4 stream cipher.

Table of Contents

• RC4 Encryption Implementation in C
  • Table of Contents
  • Introduction
    • Features
  • Installation & Usage
    • Installation
    • Functions
    • Usage
  • Example usage

Introduction

RC4 is a stream cipher that is widely known for its simplicity and speed. It's a symmetric key cipher, which means the same key is used for both encryption and decryption. This implementation uses the basic RC4 algorithm to initialize the key, generate a pseudo-random byte stream, and perform XOR operations to encrypt and decrypt data. You can read more about RC4 from here

Features

• Basic RC4 encryption and decryption.
• A key-scheduling algorithm (KSA) to initialize the state.
• A pseudo-random generation algorithm (PRGA) to produce the keystream.

Installation & Usage

Installation

    git clone https://github.com/LilSuperUser/rc4-c
    cd ./rc4-c

Functions

• Arcfour *rc4init(int8 *key, int16 ksize);
  • Initializes the RC4 state with a provided key (dynamically allocates memory for Arcfour structure).
  • The key should be a string of maximum length 256.
  • Returns a pointer to the allocated memory.
• int8 rc4byte(Arcfour *p);
  • Generates a single byte of pseudo-random data from the RC4 state.
• int8 *rc4encrypt(Arcfour *p, int8 *clear_text, int16 size);
  • Encrypts the clear_text of length size using the RC4 state.
  • Dynamically allocates memory for the resultig ciphertext.
  • Returns a pointer to dynamically allocated resulting ciphertext.
• void printbin(int8*, const int16)
  • Takes a pointer to binary data and number of bytes to read
  • prints the binary data in the hexadecimal format (to print cipher text)
• #define rc4uninit(x) free(x)
  • function macro to unintialize rc4 state.
• #define rc4decrypt(x, y, z) rc4encrypt(x, y, z)
  • function macro to decrypt a cipher text.
  • takes three arguments (Arcfour pointer, pointer to cipher text, size of cipher text).
  • calls the rc4encrypt with these arguments
  • returns a pointer to dynamically allocated char array (clear text) after decrypting.

Usage

1. Include the rc4 header file arcfour.h.
2. Initialize the rc4 state using rc4init and a key.
3. Use rc4encrypt to encrypt a clear text.
4. Print the clear text using printbin function.
5. Re-initialize the rc4 state using rc4init using the same key.
6. Use rc4decrypt to decrypt a clear text.
7. Print the clear text using normal printf or some other function.
8. Once you're done, free the allocated memory for Arcfour structure, cipher and clear text.

Example usage

#include "arcfour.h"
int main(void){
    // defining char pointers to hold key, clearText, encrypted and decrypted text
    char *key, *clearText;
    key = "This is a dummy key";    // init key char pointer with key
    clearText = "This is a dummy clear text";  // init clearText char pointer with clear text

    // defining int8 pionters for encrypted and decrypted texts
    int8 *encrypted, *decrypted;

    // defining & init int16 variables to hold size of key and text
    int16 skey = (int16)strlen(key), size = (int16)strlen(clearText);

    // Initializing rc4 state
    Arcfour *rc4 = rc4init(key, skey);

    // encrypting the clearText
    encrypted = rc4encrypt(rc4, (int8*)clearText, size);

    // printing the encrypted text using printbin (binary data)
    printf("The encrypted text is: ");
    fflush(stdout);
    printbin(encrypted, size);

    // reinitializing the rc4 state to decrypt the encrypted text
    rc4uninit(rc4);
    rc4 = rc4init(key, skey);

    // decrypting the encrypted text, and printing the clear text
    decrypted = rc4decrypt(rc4, encrypted, size);
    printf("The decrypted string is: %s\n", (char*)decrypted);  

    // uninitliazing (freeing) rc4 state, encrypted and decrypted strings
    rc4uninit(rc4);
    free(encrypted);
    free(decrypted);

    return 0;
}