I'm pretty sure it's a weird hybrid of a mostly centralized system and some on-device for the case where the user has momentary reception loss but the merchant

I'm pretty sure it's a weird hybrid of a mostly centralized system and some on-device for the case where the user has momentary reception loss but the merchant still has a solid internet connection. I'm pretty sure you can't be without reception for too long, it's not like an OTP key. The mobile app is very opaque and not decompiler-friendly, so I can't say with 100% certainty what it is. As far as a low-powered electronic device I think that's exactly what an RFID card is, and they work very well. The only real downside is lack of inter-city recognition of RFID systems, but I suppose the reality is most people only travel frequently between a consistent set of 1-3 cities and not usually more than that. My RFID "brick" of 20+ cards is quite the exception, and I'm more of a public transit enthusiast than most people, who usually just take taxis/rideshares when they travel to unfamiliar cities.