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"I'm not really sure why they compare the last 4 digits to detect a clone, that seems wrong to me."

It catches lazy counterfeiting. Stamp a whole bunch of cards with the same number and info, while the magstripe is what gets read and is the real account you're stealing from, the theory being that no-one really cares about the number printed on the card.

Some retailers do this too, ask or enter the last four of the card to make sure the imprint matches the magstripe, as if they don't it's most likely a cloned card.




I particularly enjoy when a cashier asks me to read the last 4 digits of the card, rather than following the instructions on the terminal (which say something like "Give card to cashier...").


Oh. I had no idea why they did this. That makes sense. I guess I just figured it was so easy to PROPERLY clone and stamp a card, that it didn't cross my mind..


More than that, in the case of cards that use the credit card numbering system, instead of just ordinary bank cards, the number itself is algorithmically generated and the last 4 digits (specifically the very last one) act as a kind of hash check (https://en.wikipedia.org/wiki/Luhn_algorithm). Not that police or cashiers are likely to know that.


Oh huh! I had no idea.




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