@Ze-Stuart Great question! It actually reminds me of how my deep dive into the Suunto ecosystem began. For me, these watches aren’t just sports gear; they are a technological puzzle - kind of like Sudoku, but with more hex code.
After OpenSuuntoPlus launched, I was desperate for app examples to help me write my own. I decided to dig into the firmware, hoping to find the “real” apps hidden inside. I found several files, including a filesystem snapshot, but it wasn’t just a simple sequence of bytes. Instead, it was a massive pile of small blocks scrambled in a seemingly random order.
To better understand the algorithm, I decided to learn what Ghidra was all about (having never used it before). I naively thought watch firmware would be small, simple, and easy to analyze. I was… overconfident, to say the least!
However, it was incredibly productive. I spent time forming hypotheses, checking memory chip specs, and diving into SDKs for embedded OS memory management. I even managed to “de-fragment” a working disk image from those scattered blocks, though I’m pretty sure a few bugs are still lurking in there.
Around that time, I realized the role SDSApplicationServer plays in the system. I shifted my focus to the JSON-based REST API it uses. I figured if the VS Code extension could manipulate watch data through it, I could too. Ultimately, using the server was a much faster way to get data off the watch, and it largely confirmed what I’d spent days piecing together from raw blocks.
So, back to your question: Now that I have a better grasp of the architecture, I’ve been exploring how to communicate with the watch directly via the browser. Remembering my experience with Ghidra - and noting that the firmware is tiny compared to the SDSApplicationServer binary (which is dozens of megabytes!) - I haven’t quite summoned the courage to go back to this approach!
Analyzing serial interface dumps feels more manageable; control commands are usually just dozens of bytes. The more you work with them, moving from simple to complex, the clearer it gets. There are still plenty of mysteries left, but that’s exactly why I enjoy this “Sudoku” - it keeps things interesting!
