I'll be making a website with a membership type of option for the website, and I'll be most likely using Authorize.Net to make the transactions, however, I need to know what kind of encryption I can use to store credit card numbers in a MySQL database?
Why even store it in a MySQL database when you can be using Authorize.Net's Customer Information Manager API and taking PCI compliance and security issues right out of your hands completely and letting them do all of the heaving lifting for you? CIM let's you create customer payment profiles by storing the customer's credit card information on their end and then charging against that profile at a future date whenever you need to simply by referring to the payment profile ID at the time of the transaction.
The key regulation you must follow is the Payment Card Industry Data Security Standard (PCI DSS) and of specific interest here is section 3.4 -
Protect Stored Cardholder Data
Render PAN unreadable anywhere it is stored (including on portable digital media, backup media, and in logs) by using any of the following approaches:
- One-way hashes based on strong cryptography (hash must be of the entire PAN)
- Truncation (hashing cannot be used to replace the truncated segment of PAN)
- Index tokens and pads (pads must be securely stored)
- Strong cryptography with associated key-management processes and procedures
The rest of section 3 is also worth reading in depth!
It probably (see note below) satisfies the requirements if you use a well-known symmetric cipher (such as AES or blowfish/twofish) on the sensitive data using a key stored someplace not accessible from the database. Since the result is non-printable, you can either hex or base64 encode the result for storage.
Obviously any system that does automatic encryption and decryption is going to be inherently less secure than a manual system, as the keys will have to be stored right there on the server for the thing to work. But the more you can separate them, the better.
Better yet, many gateway providers allow for mechanisms where you do not have to store the customer's card on your server, including Authorize.net's CIM and Paypal's Reference Transactions just to name a couple of examples.
Note: This does not constitute legal or security advice and should not be interpreted as such. You should always hire a local security professional if you have questions or concerns.