[phlip9]

I don't know much about the TKey, but it looks like they have some kind of remote attestation protocol available? (https://github.com/tillitis/tkey-verification/tree/main/cmd/...). That's usually how you avoid TOFU.

(1) the tillitis CA certifies your TKey device platform. You can now trust that it's running a specific firmware version with some platform pubkey.

(2) Your custom software is running and derives a keypair from it's derived secret + program binary hash.

(3) Somehow your custom software's pubkey gets locally certified by the platform's pubkey from (1). (not sure what this looks like w/ the TKey)

You now have a chain of trust from (1) the tillitis CA -> (3) the TKey device platform pubkey @ some specific firmware version -> (2) your custom software pubkey @ some specific version.

Now that we have a trusted pubkey for our service, I would open a secure channel to it via Noise IK or something (https://noiseexplorer.com/patterns/IK/). The TKey platform definitely looks a bit anemic so getting this working might be a challenge...

[cobratbq]

> I don't know much about the TKey, but it looks like they have some kind of remote attestation protocol available? (https://github.com/tillitis/tkey-verification/tree/main/cmd/...). That's usually how you avoid TOFU.

There is a tool to verify if the device is genuine by mechanism of a signature. You're outlining most of the process. The question is whether avoiding TOFU is the goal, right? I'm thinking, with the physical device in your hands and during first use, it's quite reasonable to establish the identity for your 'program' + 'user-secret'.

> You now have a chain of trust from (1) the tillitis CA -> (3) the TKey device platform pubkey @ some specific firmware version -> (2) your custom software pubkey @ some specific version.

This does mean you make this a global + centralized effort, right? (Also, it creates a dependency.)

> (3) Somehow your custom software's pubkey gets locally certified by the platform's pubkey from (1). (not sure what this looks like w/ the TKey)

With the specific firmware version: this requires a (possibly centralized) certification-process if only for a keypair, or qualification effort (if any) for the program?

To conclude: I am not convinced yet that TOFU is necessarily a bad thing. However, I do appreciate some ability to authenticate over many uses / longer stretches of time. (Hence the key-exchange + authn.) It seems there is a trade-off here, TOFU can be eradicated but requires other properties/effort. OTOH, the program-specific secret makes for a very strict form of trust. I'll take your comments into consideration, but it seems whichever way one chooses, there is a trade-off to be made.