Mask and Unmask a Label Control?

I have a requirement that we display a Social Security Number masked on a review page like so: XXX-XX-1234

This is currently just being displayed inside of a Label control, but now they added a requirement that we put a checkbox next to it to display the real SSN if checked.  This supposedly would stop people from looking over their shoulder to get their SSN.  I am OK with just doing this through javascript so long as the JS can be loaded from codebehind as there are multiple SSN fields and an uncertain amount of tabs that contain those SSN fields.

Does anyone know how to do this from code behind while loading my ssn field?  Currently I am doing
'FOR LOOP UP HERE' '...' pnlData.ControlAdd(New Label With { .Text = person.SocialSecurityNumber.Formatted .ID = "lblSSN" & i.ToString }) 'Next' So everything is being added as a control to the Panel.  I will take C# or VB cod ...