在字节流中如何编辑字节？

Question

My goal is to have a file stream open up a user-chosen file, then, it should stream the files bytes through in chunks (buffers) of about 4mb (this can be changed it's just for fun). As the bytes travel (in chunks) through the stream, I'd like to have a looping if-statement see if the bytes value is contained in an array I have declared elsewhere. (The code below will build a random array for replacing bytes), and the replacement loop could just say something like the bottom for-loop. As you can see I'm fairly fluent in this language but for some reason the editing and rewriting of chunks as they are read from a file to a new one is eluding me. Thanks in advance!

    private void button2_Click(object sender, EventArgs e)
    {
        GenNewKey();

        const int chunkSize = 4096; // read the file by chunks of 4KB
        using (var file = File.OpenRead(textBox1.Text))
        {
            int bytesRead;
            var buffer = new byte[chunkSize];
            while ((bytesRead = file.Read(buffer, 0, buffer.Length)) > 0)
            {
                byte[] newbytes = buffer;
                int index = 0;
                foreach (byte b in buffer)
                {
                    for (int x = 0; x < 256; x++)
                    {
                        if (buffer[index] == Convert.ToByte(lst[x]))
                        {
                            try
                            {
                                newbytes[index] = Convert.ToByte(lst[256 - x]);
                            }
                            catch (System.Exception ex)
                            {
                                //just to show why the error was thrown, but not really helpful..
                                MessageBox.Show(index + ", " + newbytes.Count().ToString());
                            }
                        }
                    }
                    index++;
                }
                AppendAllBytes(textBox1.Text + ".ENC", newbytes);
            }
        }
    }

    private void GenNewKey()
    {
        Random rnd = new Random();

        while (lst.Count < 256)
        {
            int x = rnd.Next(0, 255);
            if (!lst.Contains(x))
            {
                lst.Add(x);
            }
        }

        foreach (int x in lst)
        {
            textBox2.Text += ", " + x.ToString();
            //just for me to see what was generated
        }
    }

    public static void AppendAllBytes(string path, byte[] bytes)
    {
        if (!File.Exists(path + ".ENC"))
        {
            File.Create(path + ".ENC");
        }
        using (var stream = new FileStream(path, FileMode.Append))
        {
            stream.Write(bytes, 0, bytes.Length);
        }
    }

Where textbox1 holds the path and name of file to encrypt, textBox2 holds the generated cipher for personal debugging purposes, button two is the encrypt button, and of course I am using System.IO.

Answer 1

Indeed you have a off by one error in newbytes[index] = Convert.ToByte(lst[256 - x])

if x is 0 then you will have lst[256] , however lst only goes between 0-255. Change that to 255 should fix it.

The reason it freezes up is your program is EXTREMELY inefficient and working on the UI thread (and has a few more errors like you should only go up to bytesRead in size when processing buffer , but that will just give you extra data in your output that should not be there. Also you are reusing the same array for buffer and newbytes so your inner for loop could modify the same index more than once because every time you do newbytes[index] = Convert.ToByte(lst[256 - x]) you are modifying buffer[index] which will get checked again the next itteration of the for loop).

There is a lot of ways you can improve your code, here is a snippet that does similar to what you are doing (I don't do the whole "find the index and use the opposite location", I just use the byte that is passed in as the index in the array ).

while ((bytesRead = file.Read(buffer, 0, buffer.Length)) > 0)
{
    byte[] newbytes = new byte[bytesRead];
    for(int i = 0; i < newbytes.Length; i++)
    {
        newbytes[i] = (byte)lst[buffer[i]]))
    }
    AppendAllBytes(textBox1.Text + ".ENC", newbytes);
}

This may also lead to freezing but not as much, to solve the freeing you should put all of this code in to a BackgroundWorker or similar to run on another thread.