Here are my scattered notes on how I got the Metrodroid app to work on my Android phones.
Sometimes I need to check my CharlieCard cash balance without a T kiosk.
I've personally confirmed the following phones work on GrapheneOS:
According to this 2023 hardwear.io '"Un-fare Advantage” - Hacking The MBTA CharlieCard From 2008 To Present' talk by Bobby Rauch, the MIFARE Classic keys for CharlieCards are included on the lists of well-known keys on Flipper Zero devices with the latest firmware.
As mentioned on the Security Notes page of the Metrodroid wiki, the MBTA really should allow ways for its customers to view the current balance of CharlieCards in an offline manner along with fixing fundamental issues with its ticketing system, instead of relying on on a system design entirely dependent on security through obscurity.
(Probably GPLv3, though I may change to some form of Creative Commons if I need to)