Data encryption in C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;

namespace StringEncryption
{
    class Program
    {
        
        static void Main(string[] args)
        {
            // the message to encrypt
            string messageStr = "Oh what a beautifull morning, oh what a beautifull day!";
            // the secret key used, can be anything make it at least 2 chars long
            string keyStr = "ABCDEFG";

            Console.WriteLine("Encrypted string:");
            StringBuilder OutStr = StringEncrypt(messageStr, keyStr);  
            Console.WriteLine(OutStr.ToString());

            Console.WriteLine("Deciphered string:");
            StringBuilder OutStr2 = StringEncrypt(OutStr.ToString(), keyStr);
            Console.WriteLine(OutStr2.ToString());
            Console.ReadKey();
        }

        private static StringBuilder StringEncrypt(string messageStr, string keyStr)
        {
            const int EOS = -1; //end of string condition

            StringReader Msr = new StringReader(messageStr);
            StringBuilder OutStr = new StringBuilder();
            // alternatively BinaryReader and BinaryWriter could be used here
            int keyLength = keyStr.Length - 1;
            int index = 0;
            char ch;
            int x;

            do
            {
                x = Msr.Read() ^ keyStr[index];     //XOR
                ch = Convert.ToChar(x);
                OutStr.Append(ch);
                index++;
                if (index > keyLength) index = 0;   //start all over with the key               
            } while (Msr.Peek() != EOS);

            return OutStr;
        }
    }
}