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I have the following visa card from comdirect.de:

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I guess:

  • Number A is my credit card number. The 4 groups don't have a meaning.
  • Number B: I have no idea. Might be my security code?
  • Number C is my security code.
  • Number D is eventually a card code (GD is Giesecke & Devrient)

So my question is mainly about number B and C.

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2 Answers 2

up vote 28 down vote accepted

Number A is my credit card number. The 4 groups don't have a meaning.

They do, actually. First digit is 4 for VISA, 5 for Mastercard, 6 for Discover/Diners Club, 3 for American Express/Diners Club (those are shorter than 16).

Also, first 6 digits for Visa and Mastercard are code numbers for the issuing institution. By these 6 digits anyone can know which institution issued the card, and what type of card it is (debit/credit, premiere or not, etc).

Number B: I have no idea. Might me my security code?

This is a security measure. These 4 digits must match the first 4 digits of your card number (the first 4 digits of the issuer code. The last 2 are card types for the issuer, though some different issuers may share the first 4 as well and only differ by the last 2). Amex cards don't have this (I'm not sure about Discover).

Number C is my security code.

Yes, this is called CVC or CVV2. It is used for card present transactions. The purpose of the code is to verify that a payment card is actually in the hands of the cardholder/merchant, for example when using the card over the internet or phone.

On American Express cards this is 4 digits, and they appear on the front of the card.

Number D is eventually a card code (GD is Giesecke & Devrient)

This is bank-specific, so I guess whatever you said...:-)

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On Discover, number B (the duplicated digits) appears on the back in the signature field. And it's the last four that are duplicated. In addition there's a number indicating which iteration of physical card this is, in case of reissuing a card (for loss or expiration of the old one). –  Ben Voigt Apr 7 at 17:00
1  
A little more information, the first 6 numbers are called the BIN number. The remaining 10 numbers are generated by an algorithm by the financial institution. A type of credit card fraud can result if a fraudster learns the algorithm allowing them to generate all the card numbers in a batch. To combat this most companies limit the number of cards generated with a certain algorithm. –  AxGryndr Apr 8 at 5:58

Number A is my credit card number. The 4 groups don't have a meaning.

There is a lot more structure to this number than you might think. There are digits used for linking the card to the issuer for easier routing of payments, as each issuer has a globally unique issuer identification number (IIN). Your unique account number with the issuer will be embedded in there somewhere too. This was described in another answer.

Another common property of the 16 digit number (for most cards) is use of the Luhn algorithm as a checksum, with the last digit computed from the other 15 as the check digit.

This helps provide an early warning in credit card number entry forms and other locations to differentiate between a card number which:

  • might be valid
  • definitely is not valid

A card number which fails the checksum can be automatically rejected without further ado as a mistype or a fake made-up sequence of random digits.

Of course, it isn't difficult with a little effort to invent a card number which successfully passes the check, meaning a successful checksum verification requires further confirmation of the card with the issuing authority to verify its authenticity and ensure sufficient funds are available.

Nevertheless, all the major card issuers use this algorithm for initial verification purposes of a supplied card number.

TL;DR: the long card number is not a sequence of random digits and certainly has structure to it.

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