

Making sense of your credit card - varunkumar
http://www.mint.com/blog/trends/credit-card-code-01202011/

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pmorici
The original was posted a few days ago,
<http://www.mint.com/blog/trends/credit-card-code-01202011/> this article
really doesn't add anything.

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corin_
I feel I'm being moronic, and will come back to this in an hour or so once
I've finished some work and do the maths again... but I've checked with four
cards (two credit cards, two debit cards, all VISA) and none of them result in
a number divisible by ten. Results were 107, 98, 107, 106.

Having read <http://en.wikipedia.org/wiki/Luhn_algorithm> I'm either doing it
right, or I'm way more tired than I thought. Each card number is 16 digits
long, so for (from the left) digits 1, 3, etc. I double them, then I add the
sum of those doubles to digits 2, 4, etc.

i.e.

    
    
      1234 1234 1234 1234
    

Would be:

    
    
      ((1 * 2) + 2 + (3 * 2) + 4) * 4
    

I feel really dumb for writing that out given it's already written out in this
article, and on Wikipedia, but either I _am_ being really dumb or my four
cards are all invalid.

EDIT: Here is an old card number (card is long expired), and to be safe I've
swapped various numbers around (but only swapped even position numbers with
each other, and odd position numbers with each other, so it shouldn't effect
the check sum). Could someone kindly tell me if I'm being an idiot?

    
    
      5658 4612 3826 9730

~~~
akheron
I get 80 with this number. When you sum, remember to handle two-digit numbers
specially, for example 5 * 2 = 10 as 1 and 0, not 10.

~~~
corin_
Bingo. Well that solves the "none of my cards are valid" conundrum. Now I just
need to work out why my mind isn't functioning properly this morning.

Cheers!

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ffffruit
I was under the impression that credit companies relied on the insanely big
number of possible combinations in order to protect them selves. You can
surely create a billion "valid" credit card numbers but the probability of a
portion of them being in-use is very low.

~~~
pmorici
I'm pretty sure the name and expiration date if not the security code on the
reverse side come into play as well.

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Sniffnoy
That's a weird way of doing checksums. Summing digits preserves residues mod
9, yet the final check is mod 10. Weird.

(Now how do the 3 additional verification digits on the back work?)

~~~
gjm11
They're generated independently of the card number and, in principle, known
only to the card issuer and verifiable only by asking them. Merchants are
supposed never to record them.

(They don't really need to be independently generated for each card; instead
they can be computed from the card number, expiry date, etc., using a secret
key known only to the issuer and some cryptographic magic.)

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doki_pen
I wrote this for a site I worked on. It uses prototype.js.

<https://gist.github.com/271597>

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mcantelon
>9 is national assignment

...as in state applications, like national ID cards?

~~~
rdl
Assignment by national assignment agencies (ABA in the USA), for domestic-only
payment cards.

