public static byte[] KEY = Encoding.ASCII.GetBytes("unknownkey");

static public void Encrypt(byte[] Buffer)
{
    byte Incrementor = Buffer[4];

    int Group = 0;
    int GroupCount = 0;
    for (int i = 5; i < Buffer.Length; i++)
    {
        int keyPoint = ((i - 5) % KEY.Length); // First Stage
        Buffer[i] ^= KEY[keyPoint];

        byte KeyVal = (byte)(Group % 256); // Second Stage
        if (KeyVal != Incrementor)
        {
            Buffer[i] ^= KeyVal;
        }

        Buffer[i] ^= Incrementor; // Third Stage

        GroupCount++;
        if (GroupCount == KEY.Length)
        {
            Group++;
            GroupCount = 0;
        }
    }
}