[AccessD] Transactions

This would appear to be an appropriate thread in which to present my
question, rather than beginning a new thread. The app I have just developed
is for a very small market niche (in fact, so far as I have determined, said
niche contains only one competitor, and my client is so unhappy with said
competitor's product that she commissioned me to write a replacement). All
that aside, there is the issue of credit cards.

It's simple enough to mask the content of the credit card column so that
only the last four numbers show, but the data is still there, naked as it
were, and subject to the aforementioned "link tables" attack. This app uses
an MDB as its back end (so far). I know what to do if the BE is SQL Server
(encrypt the column), but I am not sure what to do in the case of an MDB BE.
The firm's accountant is concerned with this data being potentially
available to leakage (either a disgruntled employee or an internet
penetration). Perhaps a code algorithm to scramble and unscramble the
numbers will suffice. I'm not sure. Any thoughts from the esteemed
readership here?

A.