The Luhn Algorithm:
Take your credit card number (13 to 19 digits).
Drop the last digit and call it the checksum.
Now reverse the remaining digits.
Multiply the odd digits by 2.
Subtract 9 from all numbers over 9.
Add the new digits to get a sum.
Increase the sum by a one digit number to get to a number ending in zero.
The amount you increased it by will be the same as the checksum.
That means the card number is a valid one, but doesn't mean the account is still open or it hasn't expired.
If it doesn't match, it's a phony number and any online shopping cart will reject it before it tries to process it. It knows you typed it incorrectly.
Try it.
BTW, I use SafeShop for all purchases online. Once you have a grand total, get a SafeShop VISA number for that amount (online in another window). Looks just like a VISA credit card number with expiration date and security code. Works only once for only that amount. Can't be used again.
Neat (safe) idea.
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