RE: Modulus 10 formula (REPOST)

["Bob Cozzi" <cozzi@xxxxxxxxx>]

Crap! That's what I thought.  :(
-Bob


-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx [mailto:rpg400-l-bounces@xxxxxxxxxxxx]
On Behalf Of Barr, Regan
Sent: Wednesday, February 25, 2004 9:26 AM
To: RPG400-L
Subject: RE: Modulus 10 formula (REPOST)

Bob,

I'm just finishing up a credit card project, and I found that there seem to
be as many versions of the
Mod10 as there are merchants/card issuers.  I was working primarily with
validating merchant numbers.
One bank used only the last 6 digits of the number, beginning with the left
and working to the right.
Another used the entire number, but if the first 3 digits fell within a
certain range, you changed the
first digit of the number to 0, then worked your Mod10 from right to left.
There was no way I could
use the canned IBM Mod10 function for display files.  I had to write my own
Mod10 validation routine
for every issuing bank.  I'm not sure this answers whether there are 2
"official" versions, but I
found that to be irrelevant, since no one used it without significant
modifications.  I guess everyone
knows how to break Mod 10 these days, so banks are getting creative in how
they apply it.

Regan L. Barr
Cintas - Corp. Systems
Accounting & Finance
Systems Analyst

------------------------------

date: Wed, 25 Feb 2004 08:43:10 -0600
from: "Bob Cozzi" <cozzi@xxxxxxxxx>
subject: RE: Modulus 10 formula (REPOST)

Gosh, is it too early for me to type or what??? :)

-Bob

I've been going back and forth on a Modulus10 routine because, as it turns
out there seems to be two
different formulas: IBM and LUHN. The biggest issue is a subtle one which
follows. With IBM, you
always start with the left most digit (highest order digit) and select the
even position digits by
moving to the right and multiplying hem by two. This seems to be done
regardless of the length of the
original number; therefore, 34567 would use 3, 5 and 7, and 4567 would use 4
and 6. But the LUHN
Modulus 10 formula begins with the rightmost digit (lowest order digit) and
selects every other digit
moving to the left, and multiples them by two.

So with IBM we would get this (starting on the left):

Start with: 3456
Step 1: Multiple 3 and 5 by 2 = 6 10 
Step 2: Add them: 6 + 4 + 1 + 0 + 6 = 17
Step 3: 20 - 17 = 3
Check Digit = 3
Result: 3456-3

But with the LUHN formula we would get this:

Start with 3456
Step 1: Multiple 4 and 6 by 2 = 12 and 8
Step 2: Add them: 3 + 8 + 5 + 1 + 2 = 19
Step 3: 20 - 19 = 1
Check Digit = 1
Result: 3456-1

So is the IBM one screwy or are there two of them?
-Bob Cozzi


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