Credit Card . Do u know ??

What Credit Card Numbers Mean

Although phone companies, gas companies and department stores have their own numbering systems, ANSI Standard X4.13-1983 is the system used by most national credit-card systems.

Illustration by Rosaleah Rautert The front of your credit card has a lot of numbers -- here’s an example of what they might mean.

Here are what some of the numbers stand for:

• The first digit in your credit-card number signifies the system:
  
  
  • 3 - travel/entertainment cards (such as American Express and Diners Club)
  • 4 - Visa
  • 5 - MasterCard
  • 6 - Discover Card
  
  
  
• The structure of the card number varies by system. For example, American Express card numbers start with 37; Carte Blanche and Diners Club with 38.
  
  
  • American Express - Digits three and four are type and currency, digits five through 11 are the account number, digits 12 through 14 are the card number within the account and digit 15 is a check digit.
  • Visa - Digits two through six are the bank number, digits seven through 12 or seven through 15 are the account number and digit 13 or 16 is a check digit.
  • MasterCard - Digits two and three, two through four, two through five or two through six are the bank number (depending on whether digit two is a 1, 2, 3 or other). The digits after the bank number up through digit 15 are the account number, and digit 16 is a check digit.
  
  

The Stripe on a Credit Card

­­The stripe on the back of a credit card is a magnetic stripe, often called a magstripe. The magstripe is made up of tiny iron-based magnetic particles in a plastic-like film. Each particle is really a tiny bar magnet about 20-millionths of an inch long.

Illustration by Rosaleah Rautert Your card has a magstripe on the back and a place for your all-important signature.

The magstripe can be "written" because the tiny bar magnets can be magnetized in either a north or south pole direction. The magstripe on the back of the card is very similar to a piece of cassette tape (see How Cassette Tapes Work for details).

A magstripe reader (you may have seen one hooked to someone’s PC at a bazaar or fair) can understand the information on the three-track stripe. If the ATM isn’t accepting your card, your problem is probably either:

• A dirty or scratched magstripe
• An erased magstripe (The most common causes for erased magstripes are exposure to magnets, like the small ones used to hold notes and pictures on the refrigerator, and exposure to a store’s electronic article surveillance (EAS) tag demagnetizer.)

There are three tracks on the magstripe. Each track is about one-tenth of an inch wide. The ISO/IEC standard 7811, which is used by banks, specifies:

• Track one is 210 bits per inch (bpi), and holds 79 6-bit plus parity bit read-only characters.
• Track two is 75 bpi, and holds 40 4-bit plus parity bit characters.
• Track three is 210 bpi, and holds 107 4-bit plus parity bit characters.

Your credit card typically uses only tracks one and two. Track three is a read/write track (which includes an encrypted PIN, country code, currency units and amount authorized), but its usage is not standardized among banks.

The information on track one is contained in two formats: A, which is reserved for proprietary use of the card issuer, and B, which includes the following:

• Start sentinel - one character
• Format code="B" - one character (alpha only)
• Primary account number - up to 19 characters
• Separator - one character
• Country code - three characters
• Name - two to 26 characters
• Separator - one character
• Expiration date or separator - four characters or one character
• Discretionary data - enough characters to fill out maximum record length (79 characters total)
• End sentinel - one character
• Longitudinal redundancy check (LRC) - one character
  
  LRC is a form of computed check character.
  
  

The format for track two, developed by the banking industry, is as follows:

• Start sentinel - one character
• Primary account number - up to 19 characters
• Separator - one character
• Country code - three characters
• Expiration date or separator - four characters or one character
• Discretionary data - enough characters to fill out maximum record length (40 characters total)
• LRC - one character

For mo re information on track format, see ISO Magnetic Stripe Card Standards.

There are three basic methods for determining whether your credit card will pay for what you’re charging:

• Merchants with few transactions each month do voice authentication using a touch-tone phone.
• Electronic data capture (EDC) magstripe-card swipe terminals are becoming more common -- so is swiping your own card at the checkout.
• Virtual terminals on the Internet

When the acquirer company gets the credit-card authentication request, it checks the transaction for validity and the record on the magstripe for:

• Merchant ID
• Valid card number
• Expiration date
• Credit-card limit
• Card usage

Single dial-up transactions are processed at 1,200 to 2,400 bits per second (bps), while direct Internet attachment uses much higher speeds via this protocol. In this system, the cardholder enters a personal identification number (PIN) using a keypad.

Smart Cards

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­The "smart" credit card is an innovative application that involves all aspects of cryptography (secret codes), not just the authentication we described in the last section. A smart card has a microprocessor built into the card itself. Cryptography is essential to the functioning of these cards in several ways:

• The user must corroborate his identity to the card each time a transaction is made, in much the same way that a PIN is used with an ATM.
• The card and the card reader execute a sequence of encrypted sign/countersign-like exchanges to verify that each is dealing with a legitimate counterpart.
• Once this has been established, the transaction itself is carried out in encrypted form to prevent anyone, including the cardholder or the merchant whose card reader is involved, from "eavesdropping" on the exchange and later impersonating either party to defraud the system.

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­This elaborate protocol is conducted in such a way that it is invisible to the user, except for the necessity of entering a PIN to begin the transaction.