knaveconwy

I have a Diners Club Premier from when applications were opened briefly a couple years ago. I have a question about the format of the card numbers - the first six digits refer to the bank, and the last four are not considered sensitive. Has anyone noticed anything odd about the middle six numbers? I realize this is a bit vague, but I don't want to post more and reveal what my number is.

sdsearch

Welcome to FlyerTalk, knaveconwy.

It's very simple. Your number actually includes much of those last 4 numbers, but also includes earlier digits. It's just that it's considered an industry standard balance between privacy and public account number documentation that the last 4 digits will be printed on receipts but other digits generally won't be.

So your number is spread between those last 4 digits that show up on receipts, etc, as well as more digits before that (which generally don't show up on receipts).

Card number have more digits than might seem needed so that it's easier to detect a wrong number if one of the digits is wrong. There are check digits in there to ensure that most mistakes in giving the number are easily caught.

And see here: It may be less than 6 digits which identifies the bank. It depends very much on the card type:

https://en.wikipedia.org/wiki/Payment_card_number

tmiw

I'm actually fairly confident that BMO Harris assigns PANs sequentially, which is why I take more care than usual with receipts using that card. That said, the first few digits aren't usually printed out, so it's probably unlikely any fraud would happen.

knaveconwy

Well, whatever the format of PANs the middle six (digits 7-12) are the ones PCI requires be masked (PCI DSS Requirement 3.3 - and yes, other requirements might be stricter). And those numbers on my card do not look particularly random, as tmiw mentioned. I was wondering if others had noticed that too.

I don't mind - I actually think it's an interesting number to have, and don't want it to get compromised because I don't want to change it.

And sorry if this is vague - I'm trying not to reveal too much information about those sensitive six digits.

sdsearch

Keep in mind that the Diners Club USA card number formatting originated while it was with Citi. And if I look at the one Citi card currently in my wallet (Hilton HHonors Visa, no AF version), I see a similarity with the number patterns on my DC USA card (which is a Professional card that indeed was first issued by Citi, then taken over by BMO/Harris) and my Citi HH Visa card (while not seeing that pattern on any other bank's card).

Do you have any Citi credit cards to compare against?

mia

I am not sure there is any benefit to being coy about the six digits, but the pattern on the Consumer cards is not the same as on the Professional accounts inherited from Citi.

Kixo

Most credit card numbers have an algorithm that provides a check digit. This helps purchasing systems automatically reject cards that are randomly numbered.

For MasterCards, here is how it works:

LUHN Formula (Mod 10) for Validation of Primary Account Number

The following steps are required to validate the primary account number:
Step 1: Double the value of alternate digits of the primary account number beginning with the second digit from the right (the first right--hand digit is the check digit.)

Step 2: Add the individual digits comprising the products obtained in Step 1 to each of the unaffected digits in the original number.

Step 3: The total obtained in Step 2 must be a number ending in zero (30, 40, 50, etc.) for the account number to be validated.