in reply to
Encrypting Credit card numbers
Check with your card processor. You can't help but break one or more rules unless you do so, as there are so many.
PCI compliance is a "big deal" at the moment, and getting your software approved could be difficult (if possible at all). It may be that your processor has a solution on the shelf that may help you.
Disclaimer: I don't know much about the front-end part of credit-card processing, but I do work work for a very large processor of credit card transactions.