Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!usc!henry.jpl.nasa.gov!elroy.jpl.nasa.gov!mahendo!wlbr!pmdvax!staylor
From: staylor@pmdvax.UUCP (Scott Taylor)
Newsgroups: comp.lang.c
Subject: Re: The 12121 check digit algorithm
Message-ID: <117@pmdvax.UUCP>
Date: 7 Jan 90 20:02:37 GMT
References: <1498@skye.ed.ac.uk>
Lines: 36

in article <1498@skye.ed.ac.uk>, ken@aiai.ed.ac.uk (Ken Johnson) says:
>  * check_digit_for computes and returns the check digit for a given number,
>  * the number having first been stripped of any check digit.
>  * How it works: Assume the number has the digits abcde. Then add together
>  * a+c+e+2*b+2*d. To that total add 1 if b>=5 and another 1 if d>=5. The
>  * check digit is that total modulo 10.

With all the copyright crap aside (why do you people bother arguing?)
a much better method, used by the Visa and MasterCard people, and heavily
in the COBOL world, is the following (which ken's seems to be a variant of):

	a + SUM(b) + c + SUM(d) + e

	Where:  SUM(x) :: The sum of the digits of x added to itself,
				e.g.  1 = 1+1 = 2
				      2 = 2+2 = 4
				      3 = 3+3 = 6
				      4 = 4+4 = 8
				      5 = 5+5 = 10 = 1 + 0 = 1
				      6 = 6+6 = 12 = 1 + 2 = 3
						You got the idea...

	Again, the check digit is MOD 10, however it will be the difference
of 10 less the check digit MOD 10 ( 9 -> 10 - 9 = 1).  This method is very
accurate at determining transposed digits too.  Try it on your Visa or MC.
For 16 digit cards, perform SUM on the odd number digits; for 13
digit Visa numbers, the even digits.  The last digit is the Check so
don't include it. 

I have algorithms in C and COBOL if anyone needs them.  I would have posted
them here already, however I had already posted the article.
-- 
Scott G. Taylor                                    Pmd Resources  (818) 991-0068
{wlbr,mahendo}!snidely!staylor                     31230 Cedar Valley Dr.
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