Storing credit card information securely and legally is very difficult and should not be attempted. I have no intention of storing credit card data but I'm dying to figure out the following:

My credit card info is being stored on a server some where in he tworld. This data is (hopefully) not being stored on a merchant's server, but at some point it needs to be stored to verify and charge the account identified by merchant submitted data.

My question is this: if you were tasked with storing credit card data what encryption strategy would you use to secure the data on-disk? From what I can tell submitted credit card info is being checked more or less in real time. I doubt that any encryption key used to secure the data is being entered manually, so decryption is being done on the fly, which implies that the keys themselves are being stored on-disk. How would you secure your data and your keys in an automated system like this?