Luhn Algorithm for validating credit cards

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

by mlissner • 11 years ago

53 31 11 years ago

en.wikipedia.org

This was an interview question at both Square and Braintree. Its weird that interview questions trend from year to year.

mdaniel• 11 years ago

To implement it or describe what it's for?

eli• 11 years ago

I was asked to implement it on a whiteboard for a developer gig at an ad agency in like 2009. I thought it seemed disproportionately hard compared to what the job was likely to require.

justinzollars2• 11 years ago

implement in both cases, after a clear definition.

easytiger3• 11 years ago

It was also on the UK GCSE IT syllabus when I were a lad. The IT syllabus was actually incredibly good. I get very sad when i see the state of the modern one.

alextgordon• 11 years ago

Huh? GCSE ICT was about learning to use Microsoft Office.

Osmium2• 11 years ago

Speak for yourself. I found the GCSE IT syllabus absolutely atrocious (this was 10ish years ago). I do remember this though, so that's something, but broadly speaking it was awful. And I mean truly, truly awful. I find it difficult to put into words. I'm so glad they're actually starting to teach programming now.

mikekchar• 11 years ago

It's one of our interview questions too... I swear we did not get it from Square or Braintree. Very wierd.

It's a nice one because it is easy to understand and you can TDD it fairly easily.

crazybob• 11 years ago

Square's "Luhny Bin" challenge: https://corner.squareup.com/2011/11/luhny-bin.html

contingencies• 11 years ago

Many of us from the pre-stripe era have implemented Luhn once or twice I'd say, very often in Javascript and web scripting languages! However, you really should avoid re-implementing this sort of thing, and particularly additional credit card checks as it's easy to make spurious assumptions (eg. credit card prefix should equal country of address of credit card, or of mailing address) which are in fact invalid. This stuff prevents actual sales and frustrates your users.

For a slightly out there and arguably more interesting financial checksum algorithm, and one that is actually amazingly effective, do some statistics on mod97's performance. Realizing how good it was, a few years back for a cool hack in my IBAN library I wrote a mistranscription error suggestion system, which plays off the strengths of mod97. See https://github.com/globalcitizen/php-iban/blob/master/trunk/... and https://github.com/globalcitizen/php-iban/blob/master/trunk/... - for a quick hack, it's amazingly effective.

PS. I am available for remote gigs doing creative or challenging stuff in finance-land at the moment, if anyone appreciates this sort of thing.

Jugurtha• 11 years ago

This threw me 14 years back. First year high-schooler hanging out on Moroccan websites of pure html with a black background and rotating skeletons (funny fact: it was called c00l-hack), and this stuff was there. Right with phreaking, lock-picking and that kind of things. They had a good community over there and we didn't.

I still have some of those files I suppose (downloaded on a dozen floppy disks I carried with me everywhere from cyber café to the other).

DanBC• 11 years ago

I remember "credit card generators" - they'd create a credit card number to pass luhn checking.

For a few years people could get Compuserve access by using a fake number and fake details.

B-Con• 11 years ago

I wrote a bit of code that generates a random number that passes the Luhn test.[0] It was one of those random things I did in high school when I was learning to program.

[0] http://bradconte.com/cc_generator

Nullset14• 11 years ago

I came to know about this algorithm while solving Ruby Quiz[0]. Simple yet powerful [0]: http://rubyquiz.com/quiz122.html

ratpik• 11 years ago

I had come across this while generating test credit card credentials while integrating a payment gateway. Had written a small Python utility for that. Here - https://github.com/ratpik/luhn

jimktrains2• 11 years ago

I wish more common numbers had a built in check-code like that.

Sharlin• 11 years ago

I find the most interesting thing about the algorithm is that, as mentioned in the article, it was originally designed to be computable by a simple mechanical device.

k8tte2• 11 years ago

Luhn is also used for the last digit in Swedish social security numbers, https://en.wikipedia.org/wiki/Personal_identity_number_%28Sw... which makes me remember doing this in school around age of 10. i wish the teacher then would call it an "algorithm", so one wouldnt grow up thinking "algorithms" is something complex and hard to understand

EvanKnowles• 11 years ago

It's also used in South African ID numbers, although it's applied backwards.

dasp• 11 years ago

The first time I've seen the Luhn algorithm was on a compromised web server. The cracker has put several fake ebay login/registration pages, as well as a couple of Javascript files.

The pages were obviously intended to be used for phishing. However, what really made me go "wow" was the extra mile they went: one of the Javascript files contained methods that took the credit card number from the fake registration page and checked its validity w/ the Luhn algorithm before emailing it to a random email address. If the number was invalid, the user got an error and they forced him to re-enter.

I didn't know about the algorithm at the time (this was around 2005), so I was a bit baffled what the heck they are trying to do.

lnanek2• 11 years ago

This was great for getting a free month of AOL every month for years back when that was how many people connected to the internet. You just came up with a fake number that worked with this algorithm and you were good. There were even lots of automated applications to do that for you near the end such as AOHell: https://en.wikipedia.org/wiki/AOHell

woadwarrior01• 11 years ago

There's also the Damm algorithm[1], which is similar but better. [1]: https://en.wikipedia.org/wiki/Damm_algorithm

Luhn Algorithm for validating credit cards

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