XOR encrypt to file, then decrypt it, and display it?

See more:

C#

C#4.0

Hello all, i am using a function to get the TEXT from a texbox, XOR it, then save to a file:

private static string encryptDecrypt(string input) {
        char[] key = {'K', 'C', 'Q'}; //Any chars will work, in an array of any size
        char[] output = new char[input.Length];

        for(int i = 0; i < input.Length; i++) {
            output[i] = (char) (input[i] ^ key[i % key.Length]);
        }

        return new string(output);
    }

Writing to file, XOR all working, BUT i have a richtextbox, in wich i want to open the file and see the current content, of course line by line ( because the file is a list ).
The problem is, i get a really wierd Exception: "Error in opening file, illegal characters found" bla bla...

try
               {
                   if ((myStream = openFileDialog1.OpenFile()) != null)
                   {
                       using (myStream)
                       {
                           richTextBox1.Text = System.IO.File.ReadAllText(encryptDecrypt(openFileDialog1.FileName));
                       }
                   }
               }

Can somebody explain me what am i doing wrong? It should decrypt the file, and output it to my richtextbox1.

Note: without xor all works ok, and without xor... wel it`s useless.