Leveling up my reverse engineering: time for Ghidra

In my quest to figure out how to download data from the Abbott FreeStyle Libre 2, I decided that figuring it out just by looking at the captures was a dead end. While my first few captures had me convinced the reader would keep sending the same challenge, and so I could at least replay that, it turned out to be wrong, and that excluded most of the simplest/silliest of encryption schemes.

As Pierre suggested me on Twitter, the easiest way to find the answer would be to analyze the binary itself. This sounded like a hugely daunting task for myself, as I don’t speak fluent Intel assembly, and particularly I don’t speak fluent Windows interfaces. The closest I got to this in the past has been the reverse engineering of the Verio, in which I ran the software on top of WinDbg and discovered, to my relief, that they not just kept some of the logging on, but also a whole lot of debug logs that made it actually fairly easy to reverse the whole protocol.

But when the options are learning enough about cryptography and cryptanalysis to break the encoding, or spend time learning how to reverse engineer a Windows binary — yeah I thought the latter would suit me better. It’s not like I have not dabbled in reversing my own binaries, or built my own (terrible) 8086 simulator in high school, because I wanted to be able to see how the registers would be affected, and didn’t want to wait for my turn to use the clunky EEPROM system we used in the lab (this whole thing is probably a story for another day).

Also, since the last time I considered reversing a binary (when I was looking at my laptop’s keyboard), there’s a huge development: the NSA released Ghidra. For those who have not heard, Ghidra is a tool to reverse engineer binaries that includes a decompiler, and a full blown UI. And it’s open source. Given that the previous best option for this was IDA Pro, with thousands of dollars of licenses expected, this opened a huge amount of doors.

So I spent a weekend in which I had some spare time to try my best on reversing the actual code coming from the official app for the Libre 2 — I’ll provide more detail of that once I understand it better, and I know which parts are critical to share, and which one would probably get me in trouble. In general, I did manage to find out quite a bit more about the software, the devices, and the protocol — if nothing else, because Abbott left a bunch of debug logging disabled, but built in (and no, this time the WinDbg trick didn’t work because they seems to have added an explicit anti-debugger exception (although, I guess I could learn to defeat that, while I’m at it).

Because I was at first a bit skeptical about my ability to do anything at all with this, I also have been running it in an Ubuntu VM, but honestly I’m considering switching back to my normal desktop because something on the Ubuntu default shell appears to mess with Java, and I can’t even run the VM at the right screen size. I have also considered running this in a Hyper-V virtual machine on my Gamestation, but the problem with that appears to be graphics acceleration: installing OpenSUSE onto it was very fast, but trying to use it was terribly sloppy. I guess the VM option is a bit nicer in the sense that I can just save it to power off the computer, as I did to add the second SSD to the NUC.

After spending the weekend on it, and making some kind of progress, and printing out some of the code to read it on paper in front of the TV with pen and marker, well… I think I’m liking the idea of this but it’ll take me quite a while, alone, to come up with enough of a description that it can be implemented cleanroom. I’ll share more details on that later. For the most part, I felt like I was for the first time cooking something that I’ve only seen made in the Great British Bake Off — because I kept reading the reports that other (much more experienced) people wrote and published, particularly reversing router firmwares.

I also, for once, found a good reason to use YouTube for tutorials. This video by MalwareTech (yes the guy who got arrested after shutting WannaCry down by chance) was a huge help to figure out features I didn’t even know I wanted, including the “Assume Register” option. Having someone who knows what he’s doing explore a tool I don’t know was very helpful, and indeed it felt like Adam Savage describing his workshop tools — a great way to learn about stuff you didn’t know you needed.

My hope is that by adding this tool to my toolbox – like Adam Savage indeed says in his Every Tool’s A Hammer (hugely recommended reading, by the way) – is that I’ll be able to use it not just to solve the mystery of the Libre 2’s encryption. But also that of the nebulous Libre 1 binary protocol, which I never figured out (there’s a few breadcrumbs I already found during my Ghidra weekend). And maybe even to figure out the protocol of one of the gaming mice I have at home, which I never completed either.

Of course all of this assumes I have a lot more free time than I have had for the past few years. But, you know, it’s something that I might have ideas about.

Also as a side note: while doing the work to figure out which address belongs to what, and particularly figure out the jumps through vtables and arrays of global objects (yeah that seems to be something they are doing), I found myself needing to do a lot of hexadecimal calculations. And while I can do conversions from decimal to binary in my head fairly easily, hex is a bit too much for me. I have been using the Python interactive interpreter for that, but that’s just too cumbersome. Instead, I decided to get myself a good old physical calculator — not least because the Android calculator is not able to do hex, and it seems like there’s a lack of “mid range” calculators: you get TI-80 emulators fairly easily, but most of the simplest calculators don’t have hex. Or they do, but they are terrible at it.

I looked up on Amazon for the cheapest scientific calculator that I could see the letters A-F on, and ordered a Casio fx-83GT X — that was a mistake. When it arrived, I realized that I didn’t pay attention to finding one with the hex key on it. The fx-83GT does indeed have the A-F inputs — but they are used for defining variables only, and the calculator does not appear to have any way to convert to hexadecimal nor to do hexadecimal-based operations. Oops.

Instead I ordered a Sharp WriteView EL-W531, which supports hex just fine. It has slightly smaller, but more satisfying, keys, but it’s yet another black gadget on my table (the Casio is light blue). I’ll probably end up looking out for a cute sticker to put on it to see it when I close it for storage.

And I decided to keep the Casio as well — not just because it’s handy to have a calculator at home when doing paperwork, even with all the computers around, but also because it might be interesting to see how much of the firmware is downloadable, and whether someone has tried flashing a different model’s firmware onto it, to expand its capabilities: I can’t believe the A-F keys are there just for the sake of variables, my guess is that they are there because the same board/case is used by a higher model that does support hex, and I’d expect that the only thing that makes it behave one way or the other is the firmware — or even just flags in it!

At any rate, expect more information about the Libre 2 later on this month or next. And if I decide to spend more time on the Casio as well, you’ll see the notes here on the blog. But for now I think I want to get at least some of my projects closer to completion.

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