Credit Card Validation - Check Digits


This document outlines procedures and algorithms for Verifying
the accuracy and validity of credit card numbers. Most credit
card numbers are encoded with a "Check Digit". A check digit is
a digit added to a number (either at the end or the beginning)
that validates the authenticity of the number. A simple algorithm
is applied to the other digits of the number which yields the
check digit. By running the algorithm, and comparing the check
digit you get from the algorithm with the check digit encoded
with the credit card number, you can verify that you have
correctly read all of the digits and that they make a valid
combination.


1. Prefix, Length, and Check Digit Criteria

Here is a table outlining the major credit cards that you might
want to validate.
                               Check digit
CARD TYPE     Prefix   Length   algorithm
----------    ------   ------  -----------
MASTERCARD    51-55    16      mod 10

VISA          4        13, 16  mod 10

AMEX          34, 37   15      mod 10

Diners Club/  300-305, 14      mod 10
Carte Blanche 36, 38 

Discover      6011     16      mod 10

enRoute       2014,    15      any
              2149

JCB           3        16      mod 10

JCB           2131,    15      mod 10
              1800


2. LUHN Formula (Mod 10) for Validation of Primary Account Number

The following steps are required to validate the primary account
number:

Step 1: Double the value of alternate digits of the primary
account number beginning with the second digit from the right
(the first right--hand digit is the check digit.)

Step 2: Add the individual digits comprising the products
obtained in Step 1 to each of the unaffected digits in the original
number.

Step 3: The total obtained in Step 2 must be a number ending
in zero (30, 40, 50, etc.) for the account number to be validated.

For example, to validate the primary account number 49927398716:

Step 1:

        4 9 9 2 7 3 9 8 7 1 6
         x2  x2  x2  x2  x2 
        ---------------------
         18   4   6  16   2


Step 2: 4 +(1+8)+ 9 + (4) + 7 + (6) + 9 +(1+6) + 7 + (2) + 6 

Step 3: Sum = 70 : Card number is validated

Note: Card is valid because the 70/10 yields no remainder.

Note: remember that when you double a number over 4, (9 for example)
you don't add the result to your total, but rather the sum of
the digits of the result (in the above example 9*2=18 so you would
add 1+8 to your total (not 18).


3. C sourcecode by CodeBuster

static int verify_checksum(char *credit_card)
{
   char *cp;
   int dbl;
   int check_sum;

   check_sum = 0;
   dbl = 0;
   cp = credit_card + strlen(credit_card) -1;
   while (cp >= credit_card)
   {
      int c;

      c = *cp-- -'0';

      if (dbl)
      {
         c *= 2;
         if (c >= 10)
         c -= 9;
      }
      check_sum += c;
      dbl = !dbl;
   }
   return ((check_sum % 10) == 0);
}


//eof