Credit Card Number Validation

Informal explanation

The formula verifies a number against its included check digit, which is usually appended to a partial account number to generate the full account number. This account number must pass the following test:

1. Counting from the check digit, which is the rightmost, and moving left, double the value of every second digit.
2. Sum the digits of the products (eg, 10 = 1 + 0 = 1, 14 = 1 + 4 = 5) together with the undoubled digits from the original number.
3. If the total modulo 10 is equal to 0 (if the total ends in zero) then the number is valid according to the Luhn formula; else it is not valid.

Assume an example of an account number "4992739871" that will have a check digit added, making it of the form 4992739871x:

Account number	4	9	9	2	7	3	9	8	7	1	x
Double every other	4	18	9	4	7	6	9	16	7	2	x
Sum together all numbers	64	+ x

To make the sum divisible by 10, we set the check digit (x) to 6, making the full account number 49927398716.
The account number 49927398716 can be validated as follows:

1. Double every second digit, from the rightmost: (1×2) = 2, (8×2) = 16, (3×2) = 6, (2×2) = 4, (9×2) = 18
2. Sum all the individual digits (digits in parentheses are the products from Step 1): 6 + (2) + 7 + (1+6) + 9 + (6) + 7 + (4) + 9 + (1+8) + 4 = 70
3. Take the sum modulo 10: 70 mod 10 = 0; the account number is probably valid.

 Sample Code to Check Card No.

public boolean luhnCardValidationMethod(String cardNumber) {

        char[] charArray = cardNumber.toCharArray();
        int[] number = new int[charArray.length];
        int total = 0;
        for (int i = 0; i < charArray.length; i++)
        {
            number[i] = Character.getNumericValue(charArray[i]);
        }

        for (int i = number.length - 2; i > -1; i -= 2)
        {
            number[i] *= 2;

            if (number[i] > 9)
                number[i] -= 9;
        }

        for (int i = 0; i < number.length; i++)
        {
            total += number[i];
        }

        if (total % 10 != 0)
        {
            //TODO if total%10 is not Equals to Zero the Card is Invalid
            return false;
        }

        //TODO Else Card is Valid
        return true;
    }

Source : http://en.wikipedia.org/wiki/Luhn_algorithm

Some More Tips to Check Card Number.

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
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

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.

Source : http://www.beachnet.com/~hstiles/cardtype.html


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