need to Confirm while Clicking Check box checked [Resolved]

<script language="JavaScript" type="text/javascript">  

    function ss(ctrl)

    {

    var checkBox1 = document.getElementById(ctrl.id);

            if (confirm('Are you sure want to submit?')) 

            {

                __doPostBack(ctrl.id, '');

            }

            else 

            {

                return false;  

            }

    }    

    </script>

<asp:CheckBox ID="chkAck" runat="server" Visible="false" Text="Hereby i acknowledge the discussion with the Appraiser"

                                    Font-Bold="true" OnCheckedChanged="chkAck_CheckedChanged" onclick="return ss(this)" />