Comments on: Luhn formula

By: Sanjeev Thakur

Sanjeev Thakur — Tue, 04 Oct 2011 04:17:49 +0000

Great content Man.. hats off :D

By: Cosmos

Cosmos — Fri, 08 Jul 2011 11:51:25 +0000

infact u are the best site ever i have seen because u have done a good calculation.thank u for your formula it is working now .

By: john

john — Sat, 24 Apr 2010 21:34:19 +0000

when you reverse and take off the check digit. for a 16 digit for example, do you simply just disregard the fact that it was an odd number and then start by multiplying 14th digit and the 12 digit, and skipping the last even digit? and then comming back to it later for the even multiplications?

By: will

will — Sat, 27 Mar 2010 02:11:47 +0000

nice, i used the formula for websites where they want you to put your credit card # in.(websites that dont charge but want your number) i personally thought what idiot would do that so i checked out your website and found this! Good Work.Is this as a hobby or does your job like need you to do this? Well anyways kool and i’ll probaly have to re-visit the site so i can tell my friends how to make there credit card numbers into fake ones.

By: Free Credit Cards (kind of) | Villager With Wheel

Free Credit Cards (kind of) | Villager With Wheel — Sat, 19 Apr 2008 04:54:34 +0000

[...] number” means reverse the whole credit card number. The Darkcoding site explanations right here and right here as well as the Wikipedia explanation here help clarify what is going on. The [...]

By: Rand Al'Thor

Rand Al'Thor — Sun, 20 Jan 2008 20:09:26 +0000

though this is a bit freighting, its fun to see things like this everyday. its not everyday you get a funny bunch like this :) thanks John

By: James Brown

James Brown — Mon, 03 Sep 2007 21:45:50 +0000

It would seem to me that step 7 of your algorithm is overly complicated. Let me suggest three alternatives to make the algorithm simpler.

If you insist on calculating and comparing to the check digit removed in step 1, then this calculation seems simpler: 10 - (sum % 10).
An even easier solution is to add in the check digit removed in step 1 and confirm that the sum % 10 == 0.
One more simplification presents itself if you opt for solution 2 above. Simply do not remove the check digit at all (skip step 1), change step 3 to double all the digits in EVEN positions, and change step 4 to add in the digits in ODD positions. With these changes the new instructions become ...

Reverse the number

Multiply all the digits in even positions (The second digit, the fourth digit, etc) by 2.

If any one is greater than 9 subtract 9 from it.

Sum those numbers up

Add the digits in odd positions (the first, third, etc) to the number you got in the previous step

Confirm that the sum is an even multiple of 10. You can do this in code with something like (sum % 10) == 0

James

By: Tamlyn Rhodes

Tamlyn Rhodes — Sat, 31 Mar 2007 11:52:03 +0000

Thanks for this! I thought I’d found a clever way to human-readably encrypt my credit card number in an email by multiplying it with a number that only me and the recipient knew. However it turns out that due to the luhn-induced redundancy it is trivial to retrieve the factors from the product. Drat!

By: anon

anon — Thu, 09 Nov 2006 20:29:39 +0000

Thanks for the info, this helps me alot with the e-commerce site that I’m making for my work right now. And it also gives me a laugh reading through the comments ;)

By: blessyn

blessyn — Fri, 11 Aug 2006 14:44:17 +0000

am not good at maths pls