Why not javascript session value getting into code behind into asp.net

Hi,
I am trying getting value of session value from javascript. First time it not getting
but if I try second time it is getting becuase of first time session value is getting null
and second time value is storing which is i am trying of password. I can use hidden value
because of software auditing. I tyring getting value from 'Session["KEY"]' which i have
used in javascript. and i am using master page. so my script in master page

 <asp:ToolkitScriptManager ID="tsmMains" runat="server" EnablePageMethods="true">
 </asp:ToolkitScriptManager>


 <script type="text/javascript">
        function EncryptPassword() {

            var key = '<%= Session["KEY"] %>';

            if (!document.getElementById('<%=tbPassword.ClientID %>').value == '') {

                var password = document.getElementById('<%=tbPassword.ClientID %>').value;
                var encrypted = CryptoJS.TripleDES.encrypt(password, key);

                document.getElementById('<%=tbPassword.ClientID %>').value = encrypted.toString();

                InitializeRequest(password);
            }
        }     
    </script>

cs.code
=========

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {         
            this.bLogin.Attributes.Add("onClick", "EncryptPassword()");           
        }
    }

[WebMethod]

public static string SetDownloadPath(string strpath)
    {
        //Page objp;

        //objp = new Page();

        //objp.Session["KEY"] = strpath;

        nMess = strpath;

        return strpath;
    }