About Security Digits
Security digits appear on the back of your credit card. They protect you against fraudulent use of your credit card.
By entering the security digits that appear on the back of your card, we can verify that you are in posession of the credit card that you're using to complete your purchase.
Where can I find the Security Code?
Visa / Mastercard / Discover
Flip your card over and look at the signature box. You should see either the
entire 16-digit credit card number or just the last four digits followed by a
special 3-digit code. This 3-digit code is your Card Security Code.