this post was submitted on 20 May 2024
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I used to go to a laundromat that used something like a smartcard to keep your balance. You'd refill it at the kiosk and swipe it at the washer/dryer.
I had a reader/writer around somewhere from a few years prior, when I was messing around with old Echostar boxes.
Wish I could have found it. Those machines didn't look to be connected to anything. I didn't see any wireless networks in the area and the equipment didn't have any data lines.
I'm almost willing to bet the balance was stored as an value on the card and gets read/rewritten with every swipe, and essentially just security-through-obscurity. Meaning I could either back up and rewrite a $20 card forever, or rewrite the balance to having FF credits or whatever.
It could be simply obscure like you say, but the absence of a network doesn't guarantee it's that easy to hack.
They could use a checksum and your trick would invalidate the card until you figured out the correct algorithm, which would require a new visit to the laundromat for every new attempt, so basically impractical.
That or the card is just simply encrypted, which would make it impossible to interpret. It would be easy to implement too because the shared secret is between machines that are all physically controlled by the laundromat.
If there's no central control or ledger, couldn't you just rewrite the card with the original values and the machines wouldn't know any difference?
Oh yeah, that's true, so you wouldn't have destroyed the card, but it's not a useful hack if they've done even the most basic security measures.
That said, I would be fascinated to know what was on that card. I'd give it pretty good odds of having absolutely no security measures whatsoever.
you could add a random number to the encrypted data on the card and require it to always be the same or larger than the last time that card was seen, and then increment it every time the card is used.
The problem with that is that if the machines don't talk to one another then there's no way to make that system work across machines. I guess if each machine enforced it then you would eventually run out of machines that work for your hacked card.
You could store a counter for every machine used on the card, realistically, given few Laundromats would have over 50 or so machines. That'd mean that as you say, restoring the cards initial state would break it for every machine you previously used.
Going way too far now for what would make sense for a Laundromat, but just to entertain the idea...
You could also use an OTP encryption scheme on the card, where the exchange encryption key is based on the laundry machine ID, card ID, and a current timestamp, and thus changes every time the card is used. It would then be quite hard to "restore" the initial state of the card without having the laundry machine's hidden ID. Everything you read off the card would be useless a second later.