On my browser, I see the original post I made was very wide due to the long link, so I am reposting it here:

I don't know if this was mentioned before here or how widely it is known, but I used, for the first time yesterday, Citibank's neat Virtual Account Numbers. It is very easy to use (I used the "launch" method), and it is free. They provide you with randomly selected special account numbers that can be used just once, on-line!

It solves this problem/worry: we often want to pay for something on line with our credit card, but we know nothing about the company that runs the web site and are afraid to type in our regular credit card number there. Citibank has solved the problem. First, you have to have a regular credit card with them. Then you go to their web site and register there with a user ID and password -- things you would ordinarily do with any credit card. When you want to pay for something on-line with your credit card, you can first go to your Citibank web site and get a special (unique) Virtual Account Number (a one-time randomly selected number) that you use for the on-line purchase. That virtual number may only be used once, so if a crook learns it, it is useless to him. For the next on-line purchase, you go to your Citibank web site for another Virtual number. Even though the Virtual Account Number is, of course, different than your regular credit card number, the charge will appear on your regular account statement. (And, of course, you don't have to use the Virtual numbers; you could type in your regular account number if you wish.)

After making the post, I learned a little more about it. It seems that ordinarily the merchant can theoretically use the Virtual Account Number as many times as he wants for one month. But no other merchant can use that number. More important, there is an option that the cardholder can make when he gets the virtual number at the Citibank web site: he can limit the dollar amount that that number can be used for, a good safety check. So if he knows he is buying something for $34.78, he can put a $35.00 total limit on that Virtual Account Number.

Robert