rc4.h

#pragma once
// This code was originally obtained and modified from the following source
// by Bobin Verton:
// https://gist.github.com/rverton/a44fc8ca67ab9ec32089

#define N 256   // 2^8

void swap(unsigned char* a, unsigned char* b) {
    int tmp = *a;
    *a = *b;
    *b = tmp;
}

int KSA(char* key, unsigned char* S) {
    int len = strlen(key);
    int j = 0;

    for (int i = 0; i < N; i++) {
        S[i] = i;
    }

    for (int i = 0; i < N; i++) {
        j = (j + S[i] + key[i % len]) % N;
        swap(&S[i], &S[j]);
    }

    return 0;
}

int PRGA(unsigned char* S, char* plaintext, unsigned char* ciphertext, int plainTextSize) {
    int i = 0;
    int j = 0;

    for (size_t n = 0, len = plainTextSize; n < len; n++) {
        i = (i + 1) % N;
        j = (j + S[i]) % N;
        swap(&S[i], &S[j]);
        int rnd = S[(S[i] + S[j]) % N];
        ciphertext[n] = rnd ^ plaintext[n];
    }

    return 0;
}

int RC4(char* key, char* plaintext, unsigned char* ciphertext, int plainTextSize) {
    unsigned char S[N];
    KSA(key, S);
    PRGA(S, plaintext, ciphertext, plainTextSize);
    return 0;
}