Tip of the Day: Unfortunately, social security numbers aren't random either -- existing numbers can be somewhat predictable. However, the US Social Security Administration started assigning random numbers in June. Remember that SSNs aren't meant to be used for identification -- you're generally not obligated to provide your SSN if you're asked for it by a business
Credit Card Numbers Are Not Random: How To Read & Understand Them Yourself by Angela Alcorn
You may have heard before that credit card numbers follow a certain pattern and structure so that they can be validated before a transaction is accepted. However, it's one thing to know that the structure is there and another thing entirely to understand how credit card numbers work.
Why would this knowledge be useful? Well, if you run a small business that doesn't process credit card payments immediately, you could save yourself money by ensuring the card details are valid. If you don't, well it could still be fun to show off your skills at parties. Here's how you do it.
Learning Where The Numbers Come From Credit card numbers are not random. There's a special set of numbers to show information about the card issuer and another set to show information about the card holder. One other number is also important, but we'll come to that later.
The very first number is the Major Industry Identifier (MII) and it tells you what sort of institution issued the card.
- 1 and 2 are issued by airlines.
- 3 is issued by travel and entertainment.
- 4 and 5 are issued by banking and financial institutions.
- 6 is issued by merchandising and banking.
- 7 is issued by petroleum companies.
- 8 is issued by telecommunications companies.
- 9 is issued by national assignment.
Here's a few you might recognise:
- Visa: 4*****
- American Express (AMEX): 34**** or 37****
- Diner's Club International: 36****
- Mastercard: 51**** to 55****
We often see 16-digit credit card numbers today, but it's possible for a card issuer to issue a card with up to 19 digits using the current system. In the future, we may see longer numbers becoming more common.
The very last digit of each credit card is the check digit, or checksum. It is used to validate the credit card number using the Luhn algorithm, which we will now explain in detail.
The Luhn Algorithm Validation Check The Luhn Algorithm is used to validate all sorts of numbers, including credit cards, IMEI numbers and some social security numbers. It's not designed to be a cryptographically secure hash function, but merely a way to check errors are not made when recording numbers. It is not foolproof, but is generally considered to be useful.
Take the credit card number and read the digits from the right. Double every other number and write them down - if you do it in the same order as your card is written it will help with clarity. Now, wherever you have calculated a double-digit number, change it so that it reads as "first digit + second digit" (in other words, sum the digits of the products). Finally, take your calculations and add those numbers to the numbers remaining on your card that you didn't double. A legitimate credit card number will give you a result that is divisible by 10.
For instance, let's use a number I've just made up: 4634 8932 1298 2767. I'll enter it into a table to make it easier to understand the steps.
Try it yourself using the card from the picture earlier in this article. What can you learn from it?