Credit Card Validation

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.
Possible uses for this information:
  • When a user has keyed in a credit card number (or scanned it) and you want to validate it before sending it out for debit authorization.
  • When issuing cards, say an affinity card, you might want to add a check digit using the MOD 10 method.
 
1. Prefix, Length, and Check Digit Criteria
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
Discover
6011
16
mod 10
 
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.