Помогите ускорить код, выполняет шифрование в режиме простая замена ГОСТ 28147-89

хотел реализовать ГОСТ 28147-89 режим простой замены получилось но с проблема в том что выполняется слишком долго 1мб шифрует за 20 секунд (приблизительно) а надо 1мб до 10 сек посоветуйте что можно сделать

private void button1_Click(object sender, EventArgs e)
{
    Stopwatch sw = new Stopwatch();
    byte[] key_Byte = {0xff, 0xee, 0xdd, 0xcc, 0xbb, 0xaa, 0x99, 0x88,
                       0x77, 0x66, 0x55, 0x44, 0x33, 0x22, 0x11, 0x00,
                       0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7,
                       0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff};
    string key256 = Convert.ToString(TO_BIT(key_Byte));
    openFileDialog1.Filter = "Все файлы (*.*).|*.*";
    if (openFileDialog1.ShowDialog() == DialogResult.OK)
    {
        sw.Start();
        string filePath = openFileDialog1.FileName;
        byte[] fileBytes = File.ReadAllBytes(filePath);
        string fromfile_Bin = TO_BIT(fileBytes);
        string output_Bin = PZENCRYPT(fromfile_Bin, key256);
        int length = output_Bin.Length / 8;
        byte[] output_Byte = TO_BYTE(output_Bin, length);
        File.WriteAllBytes("cipher.txt", output_Byte);
        richTextBox1.Text = Convert.ToString(sw.Elapsed.TotalSeconds);
    }
}
private static string PZENCRYPT(string ot, string full_key)
{
    char[] xor_chars;
    string key = "";
    string ot_64, nblok1, nblok2, nblok = "", sblok2, ztblok = "", zt_64;
    StringBuilder zt = new StringBuilder();

    string[] keys = new string[8];
    for (int i = 0; i < 8; i++)
    {
        keys[i] = full_key.Substring(i * 32, 32);
    }


    //----------------------------------------------------- 64 bit OT filling
    int otLen = ot.Length;
    if (otLen % 64 != 0)
    {
        ot = ot.PadRight(otLen + 64 - otLen % 64, '0');
    }

    int blockSize = 64;
    int size = otLen / 64;
    for (int blk = 0; blk < size; blk++)
    {
        ot_64 = ot.Substring(blk * blockSize, blockSize);
        nblok2 = ot_64.Substring(0, 32);
        nblok1 = ot_64.Substring(32, 32);


        //----------------------------------------------------- start of 32 rounds
        for (int r = 0; r < 32; r++)
        {
            if (r > 0 && r < 32) { nblok2 = nblok; nblok1 = ztblok; }
            nblok = nblok1;
            key = r > 23 ? keys[r % 8] : keys[7 - (r % 8)];


            //----------------------------------------------------- addition modulo 2 to the power of 32
            long key_long = Convert.ToInt64(key, 2);
            long nblok1_long = Convert.ToInt64(nblok1, 2);
            long nblok_long = (key_long + nblok1_long) % 4294967296;
            nblok1 = Convert.ToString(nblok_long, 2).PadLeft(32, '0');


            //----------------------------------------------------- replacement by S blocks
            int[,] sblok_array = { { 4, 10, 9, 2, 13, 8, 0, 14, 6, 11, 1, 12, 7, 15, 5, 3 },
                                   { 14, 11, 4, 12, 6, 13, 15, 10, 2, 3, 8, 1, 0, 7, 5, 9 },
                                   { 5, 8, 1, 13, 10, 3, 4, 2, 14, 15, 12, 7, 6, 0, 9, 11 },
                                   { 7, 13, 10, 1, 0, 8, 9, 15, 14, 4, 6, 12, 11, 2, 5, 3 },
                                   { 6, 12, 7, 1, 5, 15, 13, 8, 4, 10, 9, 14, 0, 3, 11, 2 },
                                   { 4, 11, 10, 0, 7, 2, 1, 13, 3, 6, 8, 5, 9, 12, 15, 14 },
                                   { 13, 11, 4, 1, 3, 15, 5, 9, 0, 10, 14, 7, 6, 8, 2, 12 },
                                   { 1, 15, 13, 0, 5, 7, 10, 4, 9, 2, 3, 14, 6, 11, 8, 12 }};
            sblok2 = "";
            StringBuilder sblok2Builder = new StringBuilder();
            string sblok = "";
            for (int i = 0; i < 8; i++)
            {
                int sblok_int = Convert.ToInt32((nblok1.Substring(0, 4)), 2);
                nblok1 = nblok1.Remove(0, 4);
                sblok = Convert.ToString(sblok_array[i, sblok_int], 2);
                sblok = sblok.PadLeft(4, '0');
                sblok2Builder.Append(sblok);
            }
            sblok2 = sblok2Builder.ToString();


            //----------------------------------------------------- cyclic displacement
            string for_sikl = sblok2;
            int shift = 11 % sblok2.Length;
            sblok2 = sblok2.Substring(shift) + sblok2.Substring(0, shift);

            //----------------------------------------------------- XOR with N block
            xor_chars = sblok2.ToCharArray();
            for (int i = 0; i < 32; i++)
            {
                xor_chars[i] = (sblok2[i] == nblok2[i]) ? '0' : '1';
            }
            ztblok = new string(xor_chars);
        }
        zt_64 = ztblok + nblok;
        zt.Append(zt_64);
    }
    return zt.ToString();
}

private static string TO_BIT(byte[] bytes)
{
    StringBuilder bynarystring = new StringBuilder();
    foreach (byte b in bytes)
    {
        bynarystring.Append(Convert.ToString(b, 2).PadLeft(8, '0'));
    }
    return Convert.ToString(bynarystring);
}


private static byte[] TO_BYTE(string binarystr, int ff_len)
{
    byte[] bytes = new byte[ff_len];
    int k = 0;
    for (int i = 0; i < binarystr.Length; i += 8)
    {
        string bitsegment = binarystr.Substring(i, Math.Min(8, binarystr.Length - 1));
        byte bytevalue = Convert.ToByte(bitsegment, 2);
        bytes[k] = bytevalue;
        k++;
    }
    return bytes;
}