|Don't ask to ask, just ask|
My advice is don't store the CC details. You cant hack CC details off a server if they are simply not there. Make the user re-enter CC details for every transaction
We had exactly this approach, together with a third-party CC processor on a place I was working on earlier, one of Scandinavias bigger electronics ecommerce sites. We had to change this approach and store the CC numbers because we had quite a lot of trouble with frauds.
We needed to be able to access these numbers when a fraud was suspected, and due to the third party company being quite stupid in what data they could receive and return (their system was lacking a lot, but they were the only ones in the market, more or less after some turbulent dot-com times), we needed to have access to them from our own computers. Maybe others have better luck in other countries. :)
Anyhow, we first made it so that the numbers were stored on another machine and not together with the other details, only a key to identify it by, and they were purged after a certain time limit. Later, we also added encryption to this data. Can but hope that all this was enough.
If possible, I totally agree that these numbers should not be stored. As it was now however, the data was very inaccessible, and only limited damage could be done if someone carried away our machines at night and managed to crack the encryption. :)
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It is pitch black. You are likely to be eaten by a grue.