No, NFC Readers Cannot Magically Read Opal Cards
A year or two ago, a friend showed me that a Flipper Zero can show recent activity of an Opal card via the NFC reading functionality. As someone who has no idea how NFC technology works, I assumed the data was stored in some standard format that all NFC readers could process.
Since then, I've made several feeble attempts to be able to do this with my phone because carrying a Flipper Zero everywhere really isn't the most practical thing. I've used most of the well known NFC apps like NFC Tools, NFSee and several others with none of them producing the result I wanted. I made all sorts of assumptions about why none of them could give me the data from legal considerations to simply not having the right implementation.
Well, this morning, I figured it out and I feel kind of dumb. I decided to look at the Flipper Zero docs and spotted the Reading what’s on your card section which reads:
For some cards, Flipper Zero can read and show data in a readable format. For example, you can get information such as the card validity date, balance, or remaining trips. This functionality is made possible thanks to parsers contributed by the community. You can view the full list of available parsers in the Flipper Zero GitHub repository.
After following the link to the repository, I almost immediately spot the opal.c file and have the realisation that has become the title of this blog post. Someone has specifically written a parser for the Flipper Zero... Conveniently, near the top of the opal.c file is a comment that reads Reference: https://github.com/metrodroid/metrodroid/wiki/Opal which is a link to the Opal wiki page for Metrodroid which is an open source Android app that's been specifically written to read transit cards which includes Opal.
Scanning the same Opal card with Metrodroid shows what I want (with the rest on the Info tab):
Eyy