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[How-To] Check if a Number Is A Valid Credit Card Number

Monday, 9 February 2009, 12:17 | How-To | 14 Comments | Read 421 Times
by Vaibhav

The hack in this post makes use of Luhn’s algorithm which was developed by Hans Peter Luhn who was awarded US Patent 2950048 (“Computer for Verifying Numbers”) for the technique in 1960. In Practice, Using the Luhn’s algorithm, the last digit of the credit card number is arrived at. The same algorithm can be used to check if a number is actually a valid credit card number. 

Credit card numbers are usually of 16 digits, however, they may go upto 19 digits. In order to verify if a number is a valid credit card number, follow the steps below.

In order to arrive at the answer using Luhn’s Algorithm, we will need a 4 row table with number of column’s equal to the number of credit card digits ( most probably 16 or more in special cases)

1.) Write the credit card number in the first row. Lets take the example as 5189 3691 3883 7916 as a credit card number which needs to be verified as a correct number. 

2.) In the second row of the table, alternatively multiply the digit by 2. 

5 1 8 9 3 6 9 1 3 8 8 3 7 9 1 6
5 x 2 = 10 1 8 x 2 = 16 9 3 x 2 = 6 6 9 x 2 = 18 1 3 x 2 = 6 8 8 x 2 = 16 3 7 x 2 = 14 9 1 x 2 = 2 6
10-9=1 1 16-9=7 9 6 6 18-9=9 1 6 8 16 – 9 = 7 3 14 – 9 = 5 9 2 6
1 1 7 9 6 6 9 1 6 8 7 3 5 9 2 6

3.) In the third row, force all digits to be less than 10, by subtracting 9 where necessary. 

4.) Enter the digits obtained after the subtraction in fourth row. 

5.) Add up all the digits in fourth row. If the end result is divisible by 10, the number is a valid credit card number. In our case, we got 86, which is not divisible by 10 and hence 5189 3691 3883 7916 is not a valid credit card number.

If you are willing to learn more about how credit card numbers are formed and what is the significance of each digit, i would suggest that you go through this article.

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  • http://harshad.wordpress.com Harshad Joshi

    Creepy indeed….

    I used to try all permutations, combinations on prepaid cards, but that seemed to fail often… :P

  • http://harshad.wordpress.com Harshad Joshi

    I think there some different system for India, which obviously we dont know…

  • http://www.technofriends.in admin

    Hi Harshad

    this is a global way of figuring out a valid credit card number. You can try out with your credit card information.

  • http://www.intuitivo.fr/index.en intuitivo

    Python implementation below. Enjoy ;)

    def check_luhn( card_number ):
    “”" card_number is a string of usually 16 characters “”"
    sum = 0
    num_digits = len(card_number)
    oddeven = num_digits & 1

    for count in range(0, num_digits):
    digit = int(card_number[count])

    if not (( count & 1 ) ^ oddeven ):
    digit = digit * 2
    if digit > 9:
    digit = digit – 9

    sum = sum + digit

    return ( (sum % 10) == 0 )

  • Michael Campbell

    It works the same way, but I learned it instead of subtracting 9 from the product if it was greater than 10, you simply add all the digits. “10″ = 1 + 0; “14″ => 1 + 4, etc.

  • http://typicalprogrammer.com Greg Jorgensen

    Algorithm in Javascript, with explanation:
    http://typicalprogrammer.com/?p=4

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  • http://brownpapertickets.com Ryan Nejman

    Great post, but actually most are 16 digits or less, not more. All American Express numbers are only 15 digits in length. 17 or more digits are exceptionally rare, in fact I’ve never seen one and I’ve administered a database with literally millions of credit card numbers.

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  • /technofriends.in janesimpson

    how would i check if a credit card is still valid…

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