Mask Credit Card Number: Show only the last four digit of a credit card number C#

HTML

<asp:TextBox ID="txtCreditCardNum" runat="server" Text="1111222233334444" />
<br />
<hr />
<asp:Label ID="lblCreditCardNum" runat="server" />
<br />
<asp:Button runat="server" Text="Hide Characters" OnClick="HideCharacters" />

C#:

protected void HideCharacters(object sender, EventArgs e)
{
    this.lblCreditCardNum.Text = string.Format("************{0}", this.txtCreditCardNum.Text.Trim().Substring(12, 4));
}

Screenshot