## Introduction

All you know what information contains in your NIC number. But do you know what information contains in the Credit Card Number?
Following article provides brief details about what information contain in your credit card and demonstrates to how to validate
credit card number using mod 10 (Luhn) algorithms with C#.

## Background

**Card Length**

Typically, credit card numbers are all numeric and the length of the credit card number is between 12 digits to 19 digits.

- 14, 15, 16 digits – Diners Club
- 15 digits – American Express
- 13, 16 digits – Visa
- 16 digits - MasterCard

For more information please refer http://en.wikipedia.org/wiki/Bank_card_number.

#### Hidden information

* Major Industry Identifier (MII)*
The first digit of the credit card number is the Major Industry Identifier (MII). It designates the category of the entry which issued the card.

- 1 and 2 – Airlines
- 3 – Travel
- 4 and 5 – Banking and Financial
- 6 – Merchandising and Banking/Financial
- 7 – Petroleum
- 8 – Healthcare, Telecommunications
- 9 – National Assignment

**Issuer Identification Number **
The first 6 digits are the Issuer Identification Number. It will identify the institution that issued the card. Following are some of the major IINs.

- Amex – 34xxxx, 37xxxx
- Visa – 4xxxxxx
- MasterCard – 51xxxx – 55xxxx
- Discover – 6011xx, 644xxx, 65xxxx
**Account Number **Taking away the 6 identifier digits and the last digits, remaining digits
are the person’s account number (7th and following excluding last digits)

**Check digits **

Last digit is known as check digits or checksum. It is used to validate the credit card number using Luhn algorithm (Mod 10 algorithm).

For more information refer: http://en.wikipedia.org/wiki/Bank_card_number
and http://en.wikipedia.org/wiki/List_of_Issuer_Identification_Numbers.

**Luhn algorithm (Mod 10) **

The Luhn algorithm or Luhn formula, also known as the “modulus 10″ or “mod 10″ algorithm, is a simple checksum formula used to validate a variety of identification numbers,
such as credit card numbers, IMEI numbers, National Provider Identifier numbers in US and Canadian Social Insurance Numbers. It was created by IBM scientist Hans Peter
Luhn. (http://en.wikipedia.org/wiki/Luhn_algorithm)

When you implementing eCommerce application, It is a best practice validating credit card number before send it to the bank validation.

Here are the Luhn steps which can used to validate the credit card number.

**4 0 1 2 8 8 8 8 8 8 8 8 1 8 8 1**

Step 1 - Starting with the check digit double the value of every other digit (right to left every 2nd digit)

Step 2 - If doubling of a number results in a two digits number, add up the digits to get a single digit number. This will results in eight single digit numbers.

Step 3 - Now add the un-doubled digits to the odd places

Step 4 - Add up all the digits in this number

*If the final sum is divisible by 10, then the credit card number is valid. If it is not divisible by 10, the number is invalid. *

## Using the code

Following code sample validates your credit card number against Mod 10.

public static bool Mod10Check(string creditCardNumber)
{
if (string.IsNullOrEmpty(creditCardNumber))
{
return false;
}
int sumOfDigits = creditCardNumber.Where((e) => e >= '0' && e <= '9')
.Reverse()
.Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))
.Sum((e) => e / 10 + e % 10);
return sumOfDigits % 10 == 0;
}