Comments on: Luhn formula

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.

1. If you insist on calculating and comparing to the check digit removed in step 1, then this calculation seems simpler: 10 – (sum % 10).

2. An even easier solution is to add in the check digit removed in step 1 and confirm that the sum % 10 == 0.

3. 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

By: John Banshee

John Banshee — Tue, 25 Apr 2006 20:28:47 +0000

Hi,

Thanks for this – very useful when developing eCommerce and don’t really wanna use my personal CC number he-he.

Regards

By: V_Man

V_Man — Fri, 03 Mar 2006 21:42:53 +0000

Very interesting and educational site, nice to know a little bit of how this stuff works, good work,
please post more information as you find out.

By: me

me — Wed, 18 Jan 2006 13:23:10 +0000

Howard Beale, I think the reversing has to do about credit cards with 13 digits for example…
If you drop the last digit and do not reverse, you will get the numbers witch were suposed to multiply by 2 (the odd positoion ones) not multiplyed and vice versa… Off course, there are many other ways to deal with this, reversing is just one of them ;)

PS: Sorry my english :-S

By: MTN Thief

MTN Thief — Thu, 22 Dec 2005 01:19:40 +0000

A differenct formular applies to Rcarge cards. That is my area of specialization. MTN have been on their knee only to know my whereabout and arrest me, maybe kill me. I will continue to destroy the company until they begin to do the right thing. Wanna join my syndicacy? Dangerous but lucrative…send me an email, eetracy2003@yahoo.com. Let me have your number and we can talk. But it must cost you something…SOMETHING…ready?…

By: Howard Beale

Howard Beale — Sun, 04 Dec 2005 19:13:37 +0000

Actually, according to the information above and my own personal experimentation, you can save yourself some coding and processing time by NOT reversing the number when calculating the Luhn check digit. Mathematically, addition wouldn’t matter if the number were reversed or not. The reversing of the digits is therefore a waste of time.

If the length of the numeric string which you’re going to add the check digit to is odd in length then you would double-calculate the digits in the even positions. Likewise, if the string is even in length then you will double-calculate the digits in the odd positions.

I have some source code in Quick/Visual BASIC form on my website which demonstrates that fact. In fact it automatically calculates the correct digits based on the length of the input string and whether or not you want to generate or verify the Luhn check digit. Very simple MOD 2 stuff. ;->

Anyhow, LOVE the website and the redesign. I do miss the hysterical lameness of the classic posts by the lamers though (I’m sure they’ll be new ones by the terminally clueless).

Perpetually mad as hell at spammers and scammers…

–Howie

By: Ebuka

Ebuka — Fri, 04 Nov 2005 16:04:00 +0000

Does Lunh’s formula apply to all cards number (recharge cards)