ASP.NET MVC - Secure Temporary Storage of Credit Card Data

Question

I have a checkout process for a shopping cart that is currently storing credit card data in the session for retrieval once the user finalizes the purchase. The purchase process is set up such that the user inputs the credit card, views a confirmation page, and then finalizes the order. The confirmation and finalization actions are the only two actions that need access to the credit card data and to be safe all other actions should discard it.

Short of doing reflection in a base controller to check the current action the user is calling, I cannot think of an elegant way to discard the data on the disallowed requests. Additionally, if the user fails to make another request after entering the data it will linger in the session until they come back to the website- whenever that happens. One suggestion I was offered was encrypting the data into a hidden field and relying on the SSL ticket to prevent caching the markup. This seems like a fairly safe approach, but I don't much like the idea of placing the credit card data in a user-accessible location encrypted or not. Storing in the database is out because the client does not want credit card data saved.

What is the ideal approach to temporarily persisting sensitive data like credit card information across more than one page request?

---

Perhaps someone can tell me if this is a sufficient approach. I have set my Shopping Cart which is stored in the session to have a unique Guid generated every time the object is newed and that Guid is used as a key to encrypt and decrypt the credit card data which i am serializing with the Rijndael algorithm. The encrypted card data is then passed to the user in a hidden field and deserialized after finalize is clicked. The end result is a string much like this:

VREZ%2bWRPsfxhNuOMVUBnWpE%2f0AaX4hPgppO4hHpCvvwt%2fMQu0hxqA%2fCJO%2faOEi%2bX3n9%2fP923mVestb7r8%2bjkSVZDVccd2AJzCr6ak7bbZg8%3d

---

public static string EncryptQueryString(object queryString, Guid encryptionKey)
{
    try
    {
        byte[] key = Encoding.UTF8.GetBytes(ShortGuid.Encode(encryptionKey).Truncate(16));//must be 16 chars
        var rijndael = new RijndaelManaged
                           {
                               BlockSize = 128,
                               IV = key,
                               KeySize = 128,
                               Key = key
                           };

        ICryptoTransform transform = rijndael.CreateEncryptor();

        using (var ms = new MemoryStream())
        {
            using (var cs = new CryptoStream(ms, transform, CryptoStreamMode.Write))
            {
                byte[] buffer = Encoding.UTF8.GetBytes(queryString.ToString());

                cs.Write(buffer, 0, buffer.Length);
                cs.FlushFinalBlock();
                cs.Close();
            }
            ms.Close();
            return HttpUtility.UrlEncode(Convert.ToBase64String(ms.ToArray()));
        }
    }
    catch
    {
        return null;
    }
}

Answer 1

The best way to handle this scenario is to use a payment service that supports two things:

1. Authorization -> Completion semantics.
2. Tokenization

Authorization allows you to reserve the designated charge amount at the time the payment information is received, and then Completion allows you to commit the charge to the payment batch once the payment/order is confirmed. If the order is canceled, you don't have to issue a Completion and you can also attempt to delete the authorization as well.

Regarding tokenization, most gateways that support the aforementioned method of handling payments will return a token, typically a numeric id, for the pending transaction. The token may then be handled however you wish, as it has no value to anyone without access to your authentication credentials at the gateway. This transfers the burden of safety to the gateway as well.

Storing the actual credit card information in any way other than relaying a charge to a gateway/processor is a bad idea. Beyond the problems of securing the data, this will also put your application into card information storage scope for PCI/PABP, which entails a lot of rules and regulations that you won't want to deal with in your application. I believe there is also a regulatory fee that will be imposed in the coming year for compliant applications, reputedly $10k USD. All of this is likely not worth the trouble to you or your client.

Last, during intermediate processing (in-page/event/route handlers), you can use a SecureString to hold the contents of the data until you no longer need them.

SecureString class (System.Security) @ MSDN

Answer 2

What about using TempData? You'd need to put the value back into TempData between the confirmation and finalization actions, but at least it will be discarded with each request. Note that TempData uses the Session for storage so it's no more secure while it's being stored, but it does have the automatic removal feature. I, too, would resist storing the number on the page. I suspect that this violates the PCI rules.

Another alternative would be to store the card info in a database. If you keep it at all in your application you're probably already subject to the PCI rules anyway. Storing it in the DB makes it easier as then you only need to put the billing card id in subsequent requests. This could easily be in a hidden field.