WhatsApp Desktop Client Code

using System;

internal class RC4
{
    private int i = 0;
    private int j = 0;
    private int[] s;

    public RC4(byte[] key, int drop)
    {
        s = new int[256];
        while (this.i < this.s.Length)
        {
            this.s[this.i] = this.i;
            this.i++;
        }
        this.j = 0;
        this.i = 0;
        while (this.i < 0x100)
        {
            this.j = ((this.j + key[this.i % key.Length]) + this.s[this.i]) & 0xff;
            Swap<int>(this.s, this.i, this.j);
            this.i++;
        }
        this.i = this.j = 0;
        this.Cipher(new byte[drop]);
    }

    public void Cipher(byte[] data)
    {
        this.Cipher(data, 0, data.Length);
    }

    public void Cipher(byte[] data, int offset, int length)
    {
        for (int i = length; i > 0; i--)
        {
            this.i = (this.i + 1) & 0xff;
            this.j = (this.j + this.s[this.i]) & 0xff;
            Swap<int>(this.s, this.i, this.j);
            int index = offset++;
            data[index] = (byte)(data[index] ^ this.s[(this.s[this.i] + this.s[this.j]) & 0xff]);
        }
    }

    private static void Swap<T>(T[] s, int i, int j)
    {
        T num = s[i];
        s[i] = s[j];
        s[j] = num;
    }
}