C# get string from cryptostream

I am having problems to extract a string from a memory stream. The Memory stream is decorated with a crypto stream. It appears, unless I flush the cryptostream, i cannot read anything from the memory stream. I am trying to generate multiple strings (which will then be parsed to numerical values) within a for-loop. So far I can't read from a memory stream while the crypto stream is still active. As one can see, I am trying to meassure the runtime but the length of my memory stream always appears to be zero. I also need to find an efficient way to get a string from the encrypted byte array which i extract from the memory stream.

        Stopwatch watch = new Stopwatch();

        MemoryStream ms = new MemoryStream();
        ICryptoTransform encryptor = aesInstance.CreateEncryptor(aesInstance.Key, aesInstance.IV);
        CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write);
        UTF8Encoding encoder = new UTF8Encoding();

        int counter = (int)numericUpDown2.Value;
        byte[] text;
        byte[] num;

        watch.Start();
        for (int k = 0; k < rounds; k++) {

            text = encoder.GetBytes(counter.ToString());
            cs.Write(text, 0, text.Length);
            cs.FlushFinalBlock(); 
            num = new byte[ms.Length];
            ms.Read(num, 0, (int)num.Length);

            ms.Flush();
            counter++;
        }
        watch.Stop();