下载时如何加密文件

我在下载时加密文件时遇到问题。我正在使用此代码将数据库下载到 excel 文件

GridView gv = new GridView();
                gv.DataSource = graphids.ToList();
                gv.DataBind();
                Response.ClearContent();
                Response.Buffer = true;
                Response.AddHeader("content-disposition", "attachment; filename=GraphTable.xls");
                Response.ContentType = "application/ms-excel";
                Response.Charset = "";
                StringWriter sw = new StringWriter();
                HtmlTextWriter htw = new HtmlTextWriter(sw);
                Response.Output.Write(sw.ToString());
                Response.Flush();
                Response.End();

现在，

在此代码文件自动下载到我的PC的下载文件夹之后，现在我想在下载之前加密此xls文件？

FileStream fsInput = new FileStream(sInputFilename, 
                    FileMode.Open, 
                    FileAccess.Read);
    FileStream fsEncrypted = new FileStream(sOutputFilename, 
                    FileMode.Create, 
                    FileAccess.Write);

StringWriter sw = new StringWriter();