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The comment is meant to separate the hardware from the protections possible in software. TKey provides a unique secret per device, that transforms into a unique secret per (device + program-binary + user-secret). So, protection against swapped device is possible, just not if you simply check if the device is genuine.

Programs are loaded every time the device is connected by transmitting it to the device. So, you could swap the program binary on the computer, and have it send a malicious binary. Again, a unique secret is assigned to a byte-exact program-binary, so this is detected in the process.

(edit: I'm assuming the device contains the original firmware. I'm referring to programs loaded onto the TKey once programmed. Just to check/avoid confusion.)

You're right. I misremembered; read up on a lot of things in last months. Doesn't really matter, because we're discussing a protocol anyways.

Yeah, sorry, I realized that later. I forgot I posted the comment already.

> No protocol exists completely separate from its implementation.

That's a fair point. I am well aware of this.

I'd love to respond to this, but your comment "... that explicitly provides no security guarantees when someone has physical access to it, .." is too abstract for me. I'll make a few guesses.

- Is the device hackable? AFAIK not at this moment. The firmware is minimal. It is a relatively new device, so maybe I am not fully informed.

- Is the device stealable/swappable? Yes. However, it isn't possible/easy to access the internal device-secret (UDS) therefore, swapping it out leads to different secret for the program, cascading into the identity, therefore authentication would fail. (Also, if you steal it, then it's gone. :-P)

- There are protections against opening it up. I'm not an expert on this, so I cannot reliably reproduce from memory the ways it is resistant to this. However, it already means your destroying hardware in the process.

I'm not sure if we're talking about the same things. I am not confident that I understand your comment well enough to confirm/reject, so I'm going to clarify for that reason.

> I understand it that the above quoted text represents the device's long term private key?

The device owns an internal, inaccessible long-term 32-byte key called UDS. (I called it "device secret" in other comments.) The device contains fairly minimal firmware, primarily there to initialize the device and accept the bytes that are the program-binary.

When a program-binary is sent, it is written to memory. Then a secret is derived for the program, called CDI. Then device-firmware passes over control to the firmware-binary by jumping to the entry-point (IIRC).

`CDI = blake2s(UDS, blake2s(program-binary), 32-byte-user-secret)`. (Note: this is done in device-firmware and only the CDI is exposed to the program.

> You can do this:

I think here we're on the right track, but I'm guessing you're suggesting an rough idea. So, yeah, from the CDI one can derive a key for identity.

So the current protocol describes using that deterministically generated identity for authentication, and generate random bytes for deriving an ephemeral keypair for the key-exchange.

The client only needs an ephemeral keypair so relies on proper randomness only. (And it keeps a record of the identity public key, to verify its proof.)

> Next problems are replay attack prevention and forward secrecy. The two-birds-with-one-stone for that is for each side to:

I think these are not possible. Assuming (for this discussion) that your client is honest, you start off every session with proper randomness in your (unpredictable) ephemeral key. So from the first message on, replays are not feasible. Forward secrecy should be covered given that the whole session key, as it is established, is based off fresh input from randomness.

This, of course, leaves open the discussion on long-running sessions that keep using the same key without rotation. This is not addressed.

That's fine. It is indeed possible to perform a check for genuine hardware. (I'm not sure it qualifies as "attestation".) It does not protect you from malicious program-binaries and swapped devices.

> I think you're over-describing your use case, to the point that it's unclear what you're really saying. I read your "Introduction" section several times, and I don't understand if you're just saying "the use case is an authenticated key exchange" or something different. That makes it hard to judge the protocol.

Okay. I indeed failed to abstract away from the device properly. I'm guessing that the confusion comes from the fact that I had a specific device in mind, but failed to describe it.

> These two requirements are contradictory. How do you "authenticate" a server that has a different identity each time you interact with it?

Not necessarily. So how TKey works: the hardware contains a preprogrammed device secret. The hardware does not have storage. Upon each power-up it expects a program to be loaded. Upon loading that program, a secret is computed for that specific program: `blake2s(device-secret, program-binary, user-secret)` (user-secret is optional). The secret is generated deterministically, but unpredictable because we don't know the device-secret.

> Why not just use Diffie-Hellman? What else is this offering?

Given that there exists an "identity", which is the same every time the program loads, this identity can be used for authentication. The identity is different for every program, but after acquiring it once, a client application can perform a key-exchange that finishes with the signature proving the authenticity.

If 'device' or 'program-binary' or 'user-secret' changes, the secret changes. So if the secret is the same, you have quite strong guarantees that nobody screwed around with your device or program.

I will have a look. I checked quickly already, so if I understand the notation, I also leave out the last transaction. (2 messages vs 3 messages) Presumably because the authentication is one-sided. Will investigate further.

You're right. I wanted to abstract away from specific hardware and express that in the requirements. I definitely failed at that. See other comments for specifics and details on the device.

Yeah, thanks for reminding me. That is a nice suggestion.

I understand the "roll-your-own-crypto" comment. Note that I am taking the perspective of the protocol here, right? So, sure, vulnerabilities in the program are definitely a possibility. (See also other comments explaining more details about the device.) This is part of an experiment to see what can be accomplished with the TKey, which offers an interesting combo of security features.

Feel free to pour me an avalanche of missed attacks. I would be interested to know if your attacks are on the protocol level or in other ways. C programming risks are obviously there. RNG risks are obviously there. You are right that there are plenty of considerations in the program too. They are just not part of this post. (I also commented on the RNG remarks in other replies.)

To check: did you realize that you plug this device in your USB port, then send a program to it, then start using the device with that program loaded? (This is at run-time, every time, right?) Because the secret is generated for this specific combination, different programs will also have different secrets.

I get that you would want to authn the hardware itself. If that is your point, sure, you're right. However, that aims to address a slightly different problem, because then the certificate chain is tied to the hardware only. Note that part of the charm of the _identity_ generated in the program, is that the identity changes if only a single byte of program-binary is different. So it protects from malicious binaries too. (But not bugs in the program itself.)