CARD VALIDATION
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.
Here is a table outlining the major credit cards that you might
want to validate.
| CARD TYPE
| Prefix
| Length
| Check digit algorithm
|
| MASTERCARD | 51-55 | 16
| mod 10 |
| VISA | 4 | 13, 16
| mod 10 |
| AMEX | 34
37 | 15
| mod 10 |
Diners Club/
Carte Blanche | 300-305
36
38
| 14 | mod 10 |
| Discover | 6011 | 16
| mod 10 |
| enRoute | 2014
2149 |
15 | any |
| JCB | 3 | 16 |
mod 10 |
| JCB | 2131
1800 | 15
| mod 10 |
Here is the mod-10 LUHN forumula, used 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.
Make sure that you:
- have started with the rightmost digit (including the check
digit) (figure odd and even based upon the rightmost digit being
odd, regardless of the length of the Credit Card.) ALWAYS work
right to left.
- the check digit counts as digit #1 (assuming that the rightmost
digit is the check digit) and is not doubled
- double every second digit (starting with digit # 2 from the
right)
- remember that when you double a number over 4, (6 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 6*2=12 so you would
add 1+2 to your total (not 12).
- always include the Visa or M/C/ prefix.