Doesn't the track data contain the card number? (Not looked into mag stripe cards in a while so my knowledge of them is rusty). If it does doesn't the card numb

Doesn't the track data contain the card number? (Not looked into mag stripe cards in a while so my knowledge of them is rusty). If it does doesn't the card number itself contain a check digit? If so if a skimmer recorded everything it saw passing though it the data could be recalculated like how rocqua stated. Sure the data wouldn't be immediately available and require some post processing but unless the skimmer only recorded a fixed length I can see that method of protection bypassed very quickly and easily. I know your posting about the skim detection tool but it just seems to me like a bad method of trying to defeat skimmers. I would guess such systems are used for trying to detect a "Lebanese loop" which traps the card when it tries to eject.