this post was submitted on 20 May 2024
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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.