FreeStyle Libre 2 More Encryption Notes

Foreword: I know that I said I wouldn’t put reverse engineering projects as part of the Monday schedule, but I find myself having an unbalance between the two set of posts, and I wanted to get this out sooner rather than later, in the hope someone else can make progress.

You may remember I have been working on the FreeStyle Libre 2 encrypted communication protocol for a few months. I have actually taken a break from my Ghidra deep dive while I tried sorting my future out – and failing, thanks to the lockdown – but I got back to this a couple of weeks ago, since my art project completed, and I wanted to see if sleeping it over a bit meant getting a clearer view of it.

Unfortunately, I don’t think I’m any closer to figuring out how to speak to Libre 2 readers. I did manage to find some more information about the protocol, including renaming one of the commands to match the debug logs in the application. I do have some more information about the encoding though, which I thought I would share with the world, hoping it will help the next person trying to get more details on this — and hoping that they would share it with the world as well.

While I don’t have a final answer on what encryption they use on the Libre 2, I do have at least some visualization of what’s going on in the exchange sequence.

There’s 15 bytes sent from the Libre 2 reader to the software. The first eight are the challenge, while the other seven look like a nonce of some kind, possibly an initialization vector, which is used in the encryption phase only.

To build the challenge response, another eight bytes are filled with random returned by CryptGenRandom, which is a fairly low level, and deprecated, API. This is curious, given that the software itself is using Qt for the UI, but makes more sense when you realise that they use the same exact code in the driver used for uploading to the LibreView service, which is not Qt based. It also likely explains why the encryption is not using the QtCryptography framework at all.

This challenge response is then encrypted with a key — there are two sets of keys: Authorization keys are used only for this challenge phase, and Session keys are used to handle the rest of the communication. Each set includes an Encryption and a MAC key. The Authorization keys are both seeded with just the serial number of the device in ASCII form, and two literal strings, as pictured above: AuthrEnc and AuthrMAC. The session keys’ seeds include a pair of 8-bytes values as provided by the device after the authorization completes.

The encryption used is either a streaming cipher or a 64-bit block cipher. I know that, because I have multiple captures from the same device in which the challenge started with the same 8 bytes (probably because it lacked enough entropy to be properly random at initialization time), and they encrypted to exactly the same output bytes. Since the cleartext adds a random component, if it was a 128-bit block cipher, you would expect different ciphertext in the output — which kind of defeats the purpose of those 8 random bytes I guess?

The encrypted challenge response is then embedded in the response message, which includes four constant bytes (they define the message type, the length, and the subcommand, plus an extra constant byte thrown in), and then processed by the MAC algorithm (with the Authorization MAC key) to produce a 64-bit MAC, that is tackled at the end of the message. Then the whole thing is sent to the device, which will finally start answering.

As far as I can tell, the encryption algorithm is the same for Authorization and Session — with the exception of the different seed to the key generation. It also includes a different way to pass a nonce — the session encryption includes a sequence number, on both the device and the software, which is sent in clear text and fed into the encryption (shifted left by 18 bits, don’t ask me!) In addition to the sequence number, the encrypted packets have an unencrypted MAC. This is 4 bytes, but it’s actually done with the same algorithm as the authorization. The remaining 4 bytes are just dropped on the floor.

There’s a lot more that I need to figure out in the code, because not knowing anything about cryptography (and also not being that good with Ghidra). I know that the key generation and the encryption/decryption functions are parameterized with an algorithm value, which likely corresponds to an enum from the library they used. And that the parameterized functions dispatch via 21 objects (but likely not C++ objects, as they don’t seem to use vtables!), which can either point at a common function that returns an error (pretty much “not implemented”) or to an actual, implemented function — the functions check something: the enum in the case of key creation (which is, by the way, always 9), or some attribute of the object passed in for encryption and decryption.

These are clearly coming from a library linked in statically — I can tell because the code style for these is totally different from any other part of Abbott’s code, and makes otherwise no sense. It also is possibly meant to be obfuscated, or at least made it difficult — it’s not the same object out of the 21 that can answer the encrypt/decrypt function for the object, which makes it difficult to find which code is actually being executed.

I think at this point, the thing that is protecting their Libre 2 protocol the most is just the sheer amount of different styles of code in the binary: Qt, C++ with STL, C++ with C-style arrays, Windows APIs, this strange library, …

By the way, one thing that most likely would help with figuring this out would be if we could feed selected command streams into the software. While devices such as the Facedancer can help, given that most of this work is done in virtual machines, I would rather have my old idea implemented. I might look for time to work on this if I can’t find anyone interested, but if you find that this is an useful idea, I would much prefer being involved but not leading its implementation. Honestly, if I had more resources available, I would probably just pay someone to implement it, rather than buy a hardware Facedancer.

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