Using LOOKUP from user input in a single cell

I need to create a PIN-protected digital signature. My idea is to have a cell where the user types in his PIN, and from a table on a linked worksheet containing a column of PINs and their respective names, LOOKUP finds his name and replaces the PIN number with his name. For aesthetic and security reasons, I'd like this to all happen in a single cell so that the PIN disappears when the user hits enter, to be replaced by his name.

Any help appreciated!