How can I mask digits from a credit card when exporting a GridView?

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C#

ASP.NET

Oracle

My question is that i want to mask some numbers of credit card number which is of 16 digits.The positions of the numbers which i want to mask are from 7th to 12th the first six numbers and the last four numbers are not masked.The credit card numbers are being stored in boundfield and are of string type.Then i am exporting the web page data into an excel sheet.The process of masking which i want to perform is after the file is exported meaning the card number should show original on web page but when i will download it excel sheet the card number should be masked.I am using Oracle database, Microsoft Visual Studio 2008 as IDE and C# ASP.NET framework 3.5. Thanks in advance and sorry for the question's length.

using System;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;
using System.Drawing;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.IO;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.OracleClient;
using System.Text;

public partial class _Default : System.Web.UI.Page
{
    
 protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
BindGridview();
}
}
protected void BindGridview()
{
DataSet ds = new DataSet();
{
con.Open();
OracleCommand cmd = new OracleCommand("select * from CAPTURED_CARD_REPORTING where CARD_NO is not null", con);
OracleDataAdapter da = new OracleDataAdapter(cmd);
da.Fill(ds);
con.Close();
gvDetails.DataSource = ds;
gvDetails.DataBind();
}
}
protected void gvDetails_RowDataBound(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.DataRow && gvDetails.EditIndex == e.Row.RowIndex)
{
{
con.Open();
DropDownList ddlcard = (DropDownList)e.Row.FindControl("ddlreturn_to_customer");
con.Close();
ddlcard.Items.Insert(0, new ListItem("--Select--","-1"));
ddlcard.Items.Insert(1, new ListItem("Yes","Y"));
ddlcard.Items.Insert(2, new ListItem("No","N"));
}
}
}
protected void gvDetails_RowEditing(object sender, GridViewEditEventArgs e)
{
gvDetails.EditIndex = e.NewEditIndex;
BindGridview();
}
protected void gvDetails_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
{
gvDetails.EditIndex = -1;
BindGridview();
}
protected void gvDetails_PageIndexChanging(object sender, GridViewPageEventArgs e)
{
gvDetails.PageIndex = e.NewPageIndex;
BindGridview();
}
protected void gvDetails_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
    
{
string card_no = Convert.ToString(gvDetails.DataKeys[e.RowIndex].Value.ToString());
string branch_code = Convert.ToString(gvDetails.Rows[e.RowIndex].Cells[1].Text);
DropDownList ddlreturn_to_customer = (DropDownList)gvDetails.Rows[e.RowIndex].FindControl("ddlreturn_to_customer");
con.Open();
OracleCommand cmd = new OracleCommand("update CAPTURED_CARD_REPORTING set RETURN_TO_CUSTOMER='" + ddlreturn_to_customer.SelectedValue + "' where CARD_NO='" + card_no + "'  and BRANCH_CODE='" + branch_code + "' ", con);
cmd.ExecuteNonQuery();
lblresult.ForeColor = Color.Green;
lblresult.Text = " Details Updated successfully";
gvDetails.EditIndex = -1;
BindGridview();
con.Close();
}
}
protected DataSet BindDatatable()
{
    OracleDataAdapter adapter = new OracleDataAdapter();
    DataSet ds = new DataSet();
    string oracle = null;
    oracle = "select CARD_NO , BRANCH_CODE , BANK_NAME , RETURN_TO_CUSTOMER from CAPTURED_CARD_REPORTING where CARD_NO is not null";
    OracleConnection connection = new OracleConnection(connetionString);
    connection.Open();
    OracleCommand command = new OracleCommand(oracle, connection);
    adapter.SelectCommand = command;
    adapter.Fill(ds);
    connection.Close();
    gvDetails.DataSource = ds.Tables[0];
    gvDetails.DataBind();
    return ds;
}
public override void VerifyRenderingInServerForm(Control control)
{
    /* Verifies that the control is rendered */
}
protected void ExportFile(string fileName, string contentType)
{
    //disable paging to export all data and make sure to bind griddata before begin
    Response.ClearContent();
    Response.Buffer = true;
    Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", fileName));
    Response.ContentType = contentType;
    StringWriter objSW = new StringWriter();
    HtmlTextWriter objTW = new HtmlTextWriter(objSW);
    gvDetails.AllowPaging = false;
    BindDatatable();
    gvDetails.Columns[4].Visible = false;
    gvDetails.RenderControl(objTW);
    Response.Write(objSW);
    Response.End();
}
protected void btnExport_Click(object sender, EventArgs e)
{
    string fileName = "ExportToExcel_" + DateTime.Now.ToShortDateString() + ".xls",
    contentType = "application/vnd.ms-excel";

    //call 1st export method with fileName and contentType
    ExportFile(fileName, contentType);

}

protected void gvDetails_SelectedIndexChanged(object sender, EventArgs e)
{

}
}