www.merriampark.com/anatomycc.htm
This essay has a narrower focus -- to explore the anatomy of your credit card number, and to provide Java source code which determines if a given credit card number might be valid. I have not linked to this URL, because individual versions of IETF drafts are notoriously ephemeral. Major Industry Identifier The first digit of your credit card number is the Major Industry Identifier (MII), which represents the category of entity which issued your credit card. Different MII digits represent the following issuer categories: MII Digit Value Issuer Category 0 ISO/TC 68 and other industry assignments 1 Airlines 2 Airlines and other industry assignments 3 Travel and entertainment 4 Banking and financial 5 Banking and financial 6 Merchandizing and banking 7 Petroleum 8 Telecommunications and other industry assignments 9 National assignment For example, American Express, Diner's Club, and Carte Blanche are in the travel and entertainment category, VISA, MasterCard, and Discover are in the banking and financial category, and SUN Oil and Exxon are in the petroleum category. Issuer Identifier The first 6 digits of your credit card number (including the initial MII digit) form the issuer identifier. This means that the total number of possible issuers is a million (10 raised to the sixth power, or 1,000,000). Some of the better known issuer identifiers are listed in the following table: Issuer Identifier Card Number Length Diner's Club/Carte Blanche 300xxx-305xxx, 36xxxx, 38xxxx 14 American Express 34xxxx, 37xxxx 15 VISA 4xxxxx 13, 16 MasterCard 51xxxx-55xxxx 16 Discover 6011xx 16 If the MII digit is 9, then the next three digits of the issuer identifier are the 3-digit country codes defined in ISO 3166, and the remaining final two digits of the issuer identifier can be defined by the national standards body of the specified country in whatever way it wishes. Account Number Digits 7 to (n - 1) of your credit card number are your individual account identifier. The maximum length of a credit card number is 19 digits. Since the initial 6 digits of a credit card number are the issuer identifier, and the final digit is the check digit, this means that the maximum length of the account number field is 19 - 7, or 12 digits. Each issuer therefore has a trillion (10 raised to the 12th power, or 1,000,000,000,000) possible account numbers. If we consider the large number of potential customers and usurious interest rates charged by issuers, there is obviously a lot of money to be made in the credit card industry. In more civilized ages, people believed that usury was a grievous offense contrary to nature or a mortal sin, not an acceptable business practice (Aristotle, Politics 110; Check Digit The final digit of your credit card number is a check digit, akin to a checksum. The algorithm used to arrive at the proper check digit is called the Luhn algorithm, after IBM scientist Hans Peter Luhn (1896-1964), who was awarded US Patent 2950048 ("Computer for Verifying Numbers") for the technique in 1960.
phrack 47-8: "For a card with an even number of digits, double every odd numbered digit and subtract 9 if the product is greater than 9 Add up all the even digits as well as the doubled-odd digits, and the result must be a multiple of 10 or it's not a valid card. If the card has an odd number of digits, perform the same addition doubling the even numbered digits instead." The main point is that you don't want to double the check digit, and this can easily be done by starting with the check digit, going backwards, and doubling every other digit.
Examples These examples are drawn from junk mail I received from credit card issuers in August 2001. Some of this junk mail contained glossy pictures of credit cards, and the sample numbers come directly from two of these pictures. The Major Industry Identifier (MII) is 4 (banking and financial), the issuer identifier is 440804 (a VISA partner), the account number is 123456789, and the check digit is 0 Let's apply the Luhn check to 4408 0412 3456 7890. In the following table, * The top row is the original number. By changing the check digit from 0 to 3, we arrive at the number 4408 0412 3456 7893, which does pass the Luhn check, since the sum of the digits in the bottom row would be 70, which is divisible by 10. The Major Industry Identifier (MII) is 4 (banking and financial), the issuer identifier is 441712 (a VISA partner), the account number is 345678911, and the check digit is 2 Let's apply the Luhn check to 4417 1234 5678 9112, as we did in the previous example. By changing the check digit from 2 to 3, we arrive at the number 4417 1234 5678 9113, which does pass the Luhn check, since the sum of the digits in the bottom row would be 70, which is divisible by 10. Warning These two credit card offers contained pictures with numbers which the Luhn check proved to be invalid. A change to their check digits made them ostensibly valid. But if I were you, I wouldn't try to charge anything with them. Java Source Code The following simple Java class is free for you to use as you wish, without any restrictions or guarantees.
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