Suica, Pasmo and the like are all contactless SCs. The Wikipedia page is a good start, and you can go deeper from there: https://en.wikipedia.org/wiki/Suica
Every vending machine can verify signatures using a CA certificate.
Charging stations and a vending machines also have a signing key signed by that central certificate. Whatever is written on the card is signed by whoever wrote it.
Charging a card goes like this:
- Read everything from the card (data + signature)
- Verify signature + expiration dates, etc
- Extract amount on card
- Decrement said amount
- Write new amount to card
- Sign data on card using local signing key
It's a bit more complicated than that, but you get the idea.
In some systems the logs of all transactions are reconciled asynchronously (if and when internet connection is available)
so if a card is cloned it can eventually be detected and blocked.
The "local signing key" is the interesting part to me. I guess only vending machines in certain areas can get IC card support. I wonder how often those local keys need to be rotated.
That's a great answer though, thanks for writing it up!
