Best way to prevent duplicate use of credit cards
We have a system where we want to prevent the same credit card number being registered for two different accounts. As we don't store the credit card number internally - just the last four digits and expiration date - we cannot simply compare credit card numbers and expiration dates.
Our current idea is to store a hash (SHA-1) in our system of the credit card information when the card is registered, and to compare hashes to determine if a card has been used before.
Usually, a salt is used to avoid dictionary attacks. I assume we are vulnerable in this case, so we should probably store a salt along with the hash.
Do you guys see any flaws in this method? Is this a standard way of solving this problem?
Answer
Let's do a little math: Credit card numbers are 16 digits long. The first seven digits are 'major industry' and issuer numbers, and the last digit is the luhn checksum. That leaves 8 digits 'free', for a total of 100,000,000 account numbers, multiplied by the number of potential issuer numbers (which is not likely to be very high). There are implementations that can do millions of hashes per second on everyday hardware, so no matter what salting you do, this is not going to be a big deal to brute force.
By sheer coincidence, when looking for something giving hash algorithm benchmarks, I found this article about storing credit card hashes, which says:
Storing credit cards using a simple single pass of a hash algorithm, even when salted, is fool-hardy. It is just too easy to brute force the credit card numbers if the hashes are compromised.
...
When hashing credit card number, the hashing must be carefully designed to protect against brute forcing by using strongest available cryptographic hash functions, large salt values, and multiple iterations.
The full article is well worth a thorough read. Unfortunately, the upshot seems to be that any circumstance that makes it 'safe' to store hashed credit card numbers will also make it prohibitively expensive to search for duplicates.