Are there any actual crypto experts reading who'd comment on the dangers of encrypting multiple credit card numbers with the same key? Keep in mind for a single bank the first 4 digits of a credit card will all be one of two choices (their Visa or Mastercard prefix) and for a single branch I think the first 6 digits will be the same for every customers Visa card, which only leaves 9 digits (and the checksum).
Surely a naive approach like:
$encrypted_cookie_text = any_encryption_function($sixteen-digit-cc-number,$global_key);
I don't see how that follows. Storing credit card details on the server is generally a bad thing to do and takes a lot of work to get it to be PCI compliant.
What would be wrong with a different key/salt per user stored on the server, with the credit card number stored in a short-lived, secure, httponly cookie?
Also you didn't address the most important point, PCI compliance.
The method Santander employs is unquestionably a bad way to do things.
Say obfuscation = encrypted.
> It's vulnerable to cookie jacking over non HTTPS
That's why you use the secure and httponly flag for the cookie.
> It's vulnerable to theft if you have access to a computer where someone has logged on
Pretty much everything is vulnerable to theft if you have access to a computer where someone else has logged on.