Credit card transactions where the merchant is unable to physically see the credit card in your possession (such as with payments over the Internet or by phone) often require a short numerical code usually found on the back of the card. Have you ever stopped to wonder what that number is and why it is requested?
That number is known as the CID, or Card Identification Number. For Visa, MasterCard, and Discover credit cards, the CID is the three-digit number located on the signature line on reverse side of the card. For American Express credit cards, the CID is a four-digit number on the card’s front.
The purpose of providing the CID is to let the merchant know that you have the credit card in your physical possession when you pay for a product or service.
In that way, reading off or typing in the CID acts as a security precaution. Even if someone had your credit card number, they still would not know your CID. A thief who attempts to use your credit card number (perhaps off a receipt he or she found) would be unable to make purchases using your card at merchants that require the CID in order to complete a transaction.
Therefore, you should always keep your personal CID number closely guarded. A common technique used by thieves is to call and ask for your CID number, while pretending to be your credit card issuer. So, it's advisable to never provide your credit card’s Card Identification Number to any party that calls you making the request.
However, it can be expected that a reputable retailer will ask for the information should you initiate the call and request to make a purchase over the phone or perform a similar transaction online. In these instances the information can and should be provided to facilitate the transaction.