I don’t know why Citi doesn’t make a bigger deal about publicizing this feature of their credit cards, but they actually have an excellent security option which helps protect customers making online purchases. They mention it only in one paragraph here.

If you have a Citibank credit card, you can use the Virtual Account Number feature to generate temporary “alias” numbers to give out in place of your actual number. Why would you want to do that? Well, there are lots of reasons… read on and I’ll give you a few.