The Old Plastic Bag over the Credit Card Trick

We were low on kitty litter and soy milk, so I made a late night jaunt down to the Hadley Stop & Shop to pick up a few things. Going through the checkout line, I slid my Florence Savings Bank MasterCard through the scanner and was greeted with the message, “Invalid Magnetic Stripe Format.” I tried again – same result. The soy milk was getting warm and a queue was building up behind me. What to do?

The cashier noticed my consternation and handed me a plastic bag. When I shrugged stupidly she snatched my credit card, put it inside the bag and swiped it through the reader. Bingo! I saw the reassuring words, “Debit or Credit?”

I had just observed a workaround that is apparently fairly well known among the cash register community. When a card is scratched or somehow has bad chemistry with its reader, it can fail its parity check, a common software error detection technique. The addition of the layer of plastic above the magnetic strip dampens the magnetic noise that is responsible for the bad reading. A piece of scotch tape reportedly works well, too.

For 16-digit Visa and MasterCard numbers, the parity check algorithm looks like this:

1. Add up the digits in odd positions (i.e. 1st, 3rd, 5th, …, 15th) and multiply the sum by two. Call the result ODDSUMDOUBLED.


2. Add up all the digits in even positions (i.e. 2nd, 4th, 6th, …, 16th). Call this result EVENSUM.


3. Go back to the set of odd-position digits that you added up in Step One. Count how many of these digits are greater than 4 and call this BIGODDCOUNT.


4. Add up the numbers calculated in the first three steps. Call the sum CHECKSUM:
   CHECKSUM = ODDSUMDOUBLED + EVENSUM + BIGODDCOUNT.
   CHECKSUM must end in “0”. If it doesn’t the credit card number is invalid.

The card's final digit is called a check digit and is there is only one value for it that will enable the validation to work. Find a more detailed description at Anatomy of a Credit Card. If you are exceedingly curious or perhaps a sodoku fiend, pull out your wallet and validate your own card.