Masking Fields

When I add fields to my form, how do I make them so that only certain assignees can see certain fields? That is, an assignee can still see that there is a field and a value, but if you're not the correct assignee, the value is blurred to you. If you are the correct assignee, then you see the value unblurred. I'm assuming I have to use the box shield? If so, how do I implement that. If not, what do I use instead. And, is there a limitation on which fields can be masked? Thanks.