public static class BaseHelper
{
 public static void EncryptFile(string filename, string output)
 {
  if (File.Exists(filename))
  {
    string b64 = Convert.ToBase64String(EncodingDef(File.ReadAllText(filename)));
    File.WriteAllText(output, b64);
  }
}

public static void DecryptFile(string filename, string output)
{
   if (File.Exists(filename))
   {
     byte[] bb = Convert.FromBase64String(filename);
     string save = EncodingUtf8(bb);
     File.WriteAllText(output, save);
   }
}

 private static byte[] EncodingDef(string file) =>
 Encoding.Default.GetBytes(file);
 private static string EncodingUtf8(byte[] file) =>
 Encoding.ASCII.GetString(file);
}

string filename = "1.txt";
BaseHelper.EncryptFile(filename, "1_enc.txt");
BaseHelper.DecryptFile("1_enc.txt", "1.dec.txt");