How to verify a credit card number in your head (why? because it is interesting you boring farts!)

[tntcoder]

Getting a number divisible by 10 isn't that reliable though ? A couple of digit changes could still work out to be valid.

 

[siumatfung]

yup works but what use would it be ?

 

[Hatter The Mad]

question is, now we have a formulae for a valid credit card number, can someone give us the formulae for the correct security codes to go with them?

Unfortunately not. To generate the security code the bank takes your long card number, expiration date, service code, encrypts the whole lot using secret encryption keys (not sure how frequently they are regenerated, presumably relatively often) and then hash it into the 3 or 4 digits on the back of your card.

 

[tntcoder]

The question is, now we have a formulae for a valid credit card number, can someone give us the formulae for the correct security codes to go with them?

EDIT: as above

Although IIRC the key is actually quite small and brute forcable, at least it used to be.

 

[Hatter The Mad]

So not really possible unless you obtain the key, although IIRC the key itself is actually quite small

They use two DES keys (56bits each), so not too small but relatively weak by today's encryption standards.

 

[Street]

Thats quite cool, thanks for sharing!

I'll probably try it out when I'm bored in work tomorrow!

 

[tntcoder]

They use two DES keys (56bits each), so not too small but relatively weak by today's encryption standards.

Ahh yer thats right, just checked and they use the two keys with 3DES so still plenty safe enough