>The msg that Sergey mentioned validates the card. I've never heard of an algorithm to validate the date and number on the back. I'm under the impression the 4 digit # on the back is random.

My 2 credit cards have a 3 number digit... does some card have more than 3 digits?

When I buy on internet and I enter a different number or date, I got an error... does my app have to connect somewhere on internet to validate the CC numbe, date and virification number? (in this case, i am affraid that it will be too slow for the user to wait that validation)