The secure column is a holdover (originally had UI attached to it like ignorecol) to indicate a column containing things like raw passwords or credit card numbers that should be treated with care. The idea is that it should be available at the data level in views (so the migrate module can access it) but a handler would be implemented to obfuscate it on display of views (e.g, xxxxxxxxxxxx79). So, we need to implement the views handler, and add UI to set the value.